Freezing of fruits and vegetables (2024)

Freezing is one of the oldest and most widely used methods offood preservation, which allows preservation of taste, texture, and nutritionalvalue in foods better than any other method. The freezing process is acombination of the beneficial effects of low temperatures at whichmicroorganisms cannot grow, chemical reactions are reduced, and cellularmetabolic reactions are delayed (Delgado and Sun, 2000).

1.1 The importance of freezing as apreservation method

Freezing preservation retains the quality of agriculturalproducts over long storage periods. As a method of long-term preservation forfruits and vegetables, freezing is generally regarded as superior to canning anddehydration, with respect to retention in sensory attributes and nutritiveproperties (Fennema, 1977). The safety and nutrition quality of frozen productsare emphasized when high quality raw materials are used, good manufacturingpractices are employed in the preservation process, and the products are kept inaccordance with specified temperatures.

The need for freezing and frozen storage

Freezing has been successfully employed for the long-termpreservation of many foods, providing a significantly extended shelf life. Theprocess involves lowering the product temperature generally to -18 °C orbelow (Fennema et al., 1973). The physical state of food material ischanged when energy is removed by cooling below freezing temperature. Theextreme cold simply retards the growth of microorganisms and slows down thechemical changes that affect quality or cause food to spoil (George,1993).

Competing with new technologies of minimal processing offoods, industrial freezing is the most satisfactory method for preservingquality during long storage periods (Arthey, 1993). When compared in terms ofenergy use, cost, and product quality, freezing requires the shortest processingtime. Any other conventional method of preservation focused on fruits andvegetables, including dehydration and canning, requires less energy whencompared with energy consumption in the freezing process and storage. However,when the overall cost is estimated, freezing costs can be kept as low (or lower)as any other method of food preservation (Harris and Kramer, 1975).

Current status of frozen food industry in U.S. and othercountries

The frozen food market is one of the largest and most dynamicsectors of the food industry. In spite of considerable competition between thefrozen food industry and other sectors, extensive quantities of frozen foods arebeing consumed all over the world. The industry has recently grown to a value ofover US$ 75 billion in the U.S. and Europe combined. This number has reached US$27.3 billion in 2001 for total retail sales of frozen foods in the U.S. alone(AFFI, 2003). In Europe, based on U.S. currency, frozen food consumption alsoreached 11.1 million tons in 13 countries in the year 2000 (Quick Frozen FoodsInternational, 2000). Table 1 represents the division of frozen food industry interms of annual sales in 2001.

Advantages of freezing technology in developingcountries

Developed countries, mostly the U.S., dominate theinternational trade of fruits and vegetables. The U.S. is ranked number one asboth importer and exporter, accounting for the highest percent of fresh producein world trade. However, many developing countries still lead in the export offresh exotic fruits and vegetables to developed countries (Mallett,1993).

For developing countries, the application of freezingpreservation is favorable with several main considerations. From a technicalpoint of view, the freezing process is one of the most convenient and easiest offood preservation methods, compared with other commercial preservationtechniques. The availability of different types of equipment for severaldifferent food products results in a flexible process in which degradation ofinitial food quality is minimal with proper application procedures. As mentionedearlier, the high capital investment of the freezing industry usually plays animportant role in terms of economic feasibility of the process in developingcountries. As for cost distribution, the freezing process and storage in termsof energy consumption constitute approximately 10 percent of the total cost(Person and Lohndal, 1993). Depending on the government regulations, especiallyin developing countries, energy cost for producers can be subsidized by means oflowering the unit price or reducing the tax percentage in order to enhanceproduction. Therefore, in determining the economical convenience of the process,the cost related to energy consumption (according to energy tariffs) should beconsidered. Electricity prices for some countries are given in Table2.

Table 1. Frozen food industry in terms of annual sales in2001
(Source: Information Resources)

Increasing consumer demand in developing countries due tomodernization

The proportion of fresh food preserved by freezing is highlyrelated to the degree of economic development in a society. As countries becomewealthier, their demand for high-valued commodities increases, primarily due tothe effect of income on the consumption of high-valued commodities in developingcountries. The commodities preserved by freezing are usually the most perishableones, which also have the highest price. Therefore, the demand for thesecommodities is less in developing areas. Besides, the need for adequatetechnology for freezing process is the major drawback of developing countries incompeting with industrialized countries. The frozen food industry requiresaccompanying developments and facilities for transporting, storing, andmarketing their products from the processing plant to the consumer (Mallett,1993). Thus, a large amount of capital investment is needed for these types offacilities. For developing countries, especially in rural or semi-rural areas,the frozen food industry has therefore not been developed significantly comparedto other countries.

In recent years, due to the changing consumer profile, thefrozen food industry has changed significantly. The major trend in consumerbehavior documented over the last half century has been the increase in thenumber of working women and the decline in the family size. These two factorsresulted in a reduction in time spent preparing food. The entry of more womeninto the workforce also led to improvements in kitchen appliances and increasedthe variability of ready-to-eat or frozen foods available in the market.Besides, the increased usage of microwave ovens, affecting food habits ingeneral and the frozen food market in particular, as well as allowing rapidpreparation of meals and greater flexibility in meal preparation. The frozenfood industry is now only limited by imagination, an output of which increasescontinuously to supply the increasing demand for frozen products andvariability.

Table 2. Unit electricity prices for industry¹ (U.S. Dollars per Kilowatt-hour)
Source: United States - Energy Information Administration, Monthly Energy Review, July 2003.

n.a. = Not Available.
¹ Energyend-use prices including taxes converted using exchange rates.
²Electricity prices in the United States, including income taxes, environmentalcharges, and other charges.

Market share of frozen fruits and vegetables

Today in modern society, frozen fruits and vegetablesconstitute a large and important food group among other frozen food products(Arthey, 1993). The historical development of commercial freezing systemsdesigned for special food commodities helped shape the frozen food market.Technological innovations as early as 1869 led to the commercial development andmarketing of some frozen foods. Early products saw limited distribution throughretail establishments due to insufficient supply of mechanical refrigeration.Retail distribution of frozen foods gained importance with the development ofcommercially frozen vegetables in 1929.

The frozen vegetable industry mostly grew after thedevelopment of scientific methods for blanching and processing in the 1940s.Only after the achievement of success in stopping enzymatic degradation, didfrozen vegetables gain a strong retail and institutional appeal. Today, marketstudies indicate that considering overall consumption of frozen foods, frozenvegetables constitute a very significant proportion of world frozen-foodcategories (excluding ice cream) in Austria, Denmark, Finland, France, Germany,Italy, Netherlands, Norway, Sweden, Switzerland, UK, and the USA. The divisionof frozen vegetables in terms of annual sales in 2001 is shown in Table3.

Commercialization history of frozen fruits is older thanfrozen vegetables. The commercial freezing of small fruits and berries began inthe eastern part of the U.S. in about 1905 (Desrosier and Tressler, 1977). Themain advantage of freezing preservation of fruits is the extended usage offrozen fruits during off-season. Additionally, frozen fruits can be transportedto remote markets that could not be accessed with fresh fruit. Also, freezingpreservation makes year-round further processing of fruit products possible,such as jams, juice, and syrups from frozen whole fruit, slices, or pulps. Insummary, the preservation of fruits by freezing has clearly become one the mostimportant preservation methods.

Future trends in freezing technology

The frozen food industry is highly based in modern science andtechnology. Starting with the first historical development in freezingpreservation of foods, today, a combination of several factors influences thecommercialization and usage of freezing technology. The future growth of frozenfoods will mostly be affected by economical and technological factors. Growth inpopulation, personal incomes, relative cost of other forms of foods, changes intastes and preferences, and technological advances in freezing methods are someof the factors concerned with the future of freezing technology (Enochian andWoolrich, 1977).

Population growth and increasing demand for food has generatedthe need for commercial production of food commodities in large-scaleoperations. Thus, availability of proper equipment suitable for continuousprocessing would be valuable for freezing preservation methods. In additiondepending on personal incomes, relative cost of frozen products is one of themost important of economical factors. Producing the highest quality at thelowest cost possible is highly dependent on the technology used. As a result,developments in freezing technology in recent years have mostly beencharacterized by the improvements in mechanical handling and process control toincrease freezing rate and reduce cost (George, 1993).

Today an increasing demand for frozen foods already exits andfurther expansion of the industry is primarily dependent on the ability of foodprocessors to develop higher qualities in both process techniques and products.Improvements can only be achieved by focusing on new technologies andinvestigating poorly understood factors that influence the quality of frozenfood products. Improvements in new and convenient forms of foods, as well asmore information on relative cost and nutritive values of frozen foods, willcontribute toward continued growth of the industry (Desrosier and Tressler,1977).

Table 3. Frozen vegetables in terms of annual sales in2001
(Source: Information Resources)

1.2 General recommendations on thefreezing process

Freezing is a widely used method of food preservation based onseveral advantages in terms of retention of food quality and ease of process.Beginning with the earliest history of freezing, the technology has been highlyaffected over the years by the developments and improvements in freezingtechniques. In order to understand and handle the concepts associated withfreezing of foods, it is necessary to examine the fundamental factors governingthe freezing process.

1.2.1 Freezingtechnology

Freezing has long been used as a method of preservation, andhistory reveals it was mostly shaped by the technological developments in theprocess. A small quantity of ice produced without using a "natural cold" in 1755was regarded as the first milestone in the freezing process. Firstly, ice-saltsystems were used to preserve fish and later on, by the late 1800’s,freezing was introduced into large-scale operations as a method of commercialpreservation. Meat, fish, and butter, the main products preserved in this earlyexample, were frozen in storage chambers and handled as bulk commodities(Persson and Lohndal, 1993).

In the following years, scientists and researcherscontinuously worked to achieve success with commercial freezing trials onseveral food commodities. Among these commodities, fruits were one of the mostimportant since freezing during the peak growing season had the advantage ofpreserving fruit for later processing into jams, jellies, ice cream, pies, andother bakery foods. Although commercial freezing of small fruits and berriesfirst began around 1905 in the eastern part of the United States, the commercialfreezing of vegetables is much more recent. Starting from 1917, only privatefirms conducted trials on freezing vegetables, but achieving good quality infrozen vegetables was not possible without pre-treatments due to the enzymaticdeterioration. In 1929, the necessity of blanching to inactivate enzymes beforefreezing was concluded by several researchers to avoid deterioration andoff-flavours caused by enzymatic degradation.

The modern freezing industry began in 1928 with thedevelopment of double-belt contact freezers by a technologist named ClarenceBirdseye. After the revolution in the quick freezing process and equipment, theindustry became more flexible, especially with the usage of multi-platefreezers. The earlier methods achieved successful freezing of fish and poultry,however with the new quick freezing system, packaged foods could be frozenbetween two metal belts as they moved through a freezing tunnel. Thisimprovement was a great advantage in the commercial large-scale freezing offruits and vegetables. Furthermore, quick-freezing of consumer-size packageshelped frozen vegetables to be accepted rapidly in late 1930s.

Today, freezing is the only large-scale method that bridgesthe seasons, as well as variations in supply and demand of raw materials such asmeat, fish, butter, fruits, and vegetables. Besides, it makes possible movementof large quantities of food over geographical distances (Persson and Londahl,1993). It is important to control the freezing process, including thepre-freezing preparation and post-freezing storage of the product, in order toachieve high-quality products (George, 1993). Therefore, the theory of thefreezing process and the parameters involved should be understoodclearly.

1.2.2 Freezingprocess

The freezing process mainly consists of thermodynamic andkinetic factors, which can dominate each other at a particular stage in thefreezing process (Franks, 1985). Major thermal events are accompanied byreduction in heat content of the material during the freezing process as isshown in Figure 1. The material to be frozen first cools down to the temperatureat which nucleation starts. Before ice can form, a nucleus, or a seed, isrequired upon which the crystal can grow; the process of producing this seed isdefined as nucleation. Once the first crystal appears in the solution, a phasechange occurs from liquid to solid with further crystal growth. Therefore,nucleation serves as the initial process of freezing, and can be considered asthe critical step that results in a complete phase change (Sahagian and Goff,1996).

Freezing point of foods

Freezing point is defined as the temperature at which thefirst ice crystal appears and the liquid at that temperature is in equilibriumwith the solid. If the freezing point of pure water is considered, thistemperature will correspond to 0 °C (273°K). However, when foodsystems are frozen, the process becomes more complex due to the existence ofboth free and bound water. Bound water does not freeze even at very lowtemperatures. Unfreezable water contains soluble solids, which cause a decreasein the freezing point of water lower than 0 °C. During the freezingprocess, the concentration of soluble solids increases in the unfrozen water,resulting in a variation in freezing temperature. Therefore, the temperature atwhich the first ice crystal appears is commonly regarded as the initial freezingtemperature. There are empirical equations in literature that can calculate theinitial freezing temperature of certain foods as a function of their moisturecontent (Levy, 1979).

Figure 1. A schematic illustration of overall freezingprocess.

There are several methods of food freezing, and depending onthe method used, the quality of the frozen food may vary. However, regardless ofthe method chosen, the main principle behind all freezing processes is the samein terms of process parameters. The International Institute of Refrigeration(IIR) has provided definitions to establish a basis for the freezing process.According to their definition, the freezing process is basically divided intothree stages based on major temperature changes in a particular location in theproduct, as shown in Figures 2 and 3 for pure water and foodrespectively.

Beginning with the prefreezing stage, the food is subjected tothe freezing process until the appearance of the first crystal. If the materialfrozen is pure water, the freezing temperature will be 0 °C and, up to thistemperature, there will be a subcooling until the ice formation begins. In thecase of foods during this stage, the temperature decreases to below freezingtemperature and, with the formation of the first ice crystal, increases tofreezing temperature. The second stage is the freezing period; a phase changeoccurs, transforming water into ice. For pure water, temperature at this stageis constant; however, it decreases slightly in foods, due to the increasingconcentration of solutes in the unfrozen water portion. The last stage startswhen the product temperature reaches the point where most freezable water hasbeen converted to ice, and ends when the temperature is reduced to storagetemperature (Persson and Lohndal, 1993).

The freezing time and freezing rate are the most importantparameters in designing freezing systems. The quality of the frozen product ismostly affected by the rate of freezing, while time of freezing is calculatedaccording to the rate of freezing. For industrial applications, they are themost essential parameters in the process when comparing different types offreezing systems and equipment (Persson and Lohndal, 1993).

Figure 2. Practical definition of the freezing process forpure water (Mallett, 1993).

Figure 3. Practical definition of the freezing process forfoods (Mallett, 1993).

Freezing time

Again, freezing time is one of the most important parametersin the freezing process, defined as time required to lower product temperaturefrom its initial temperature to a given temperature at its thermal center. Sincethe temperature distribution within the product varies during freezing process,the thermal center is generally taken as reference. Thus, when the geometricalcenter of the product reaches the given final temperature, this ensures theaverage product temperature has been reduced to a storage value. Freezing timedepends on several factors, including the initial and final temperatures of theproduct and the quantity of heat removed, as well as dimensions (especiallythickness) and shape of product, heat transfer process, and temperature. TheInternational Institute of Refrigeration (1986) defines various factors offreezing time in relation to both the product frozen and freezing equipment(Persson and Lohndal, 1993). The most important are:

• Dimensions andshape of product, particularly thickness

• Initial and finaltemperatures

  See Also

  Savor the Freshness: Discover Various Food Preservation Methodsis the oldest method of food preservation in which it reduces water content in food and thus prevents the bacterial growth.The Brief History of Canning Food6 Simple Ways to Preserve Foods From Your Garden - YES! Magazine Solutions Journalism

• Temperature of refrigeratingmedium

• Surface heat transfercoefficient of product

• Change in enthalpy

• Thermal conductivity ofproduct

Calculation of freezing time in food systems is difficult incomparison to pure systems since the freezing temperature changes continuouslyduring the process. Using a simplified approach, time elapsed between initialfreezing until when the entire product is frozen can be regarded as the freezingtime. Plank’s equation (Eq.1) is commonly used to estimate freezing time,however due to assumptions involved in the calculation it is only useful forobtaining an approximation of freezing time. The derivation of the equationstarts with the assumption the product being frozen is initially at freezingtemperature. Therefore, the calculated freezing time represents only thefreezing period. The equation can be further modified for different geometriesincluding slab, cylinder, and sphere, where for each geometry, the coefficientsare arranged in relation to the dimensions (Plank, 1980).

Table 4. Coefficients P and R of Equation 1

(1)

where l₁ is thelatent heat of frozen fraction, k and r are thethermal conductivity and density of the frozen layer, while h is the coefficientof heat transfer by convection to the exterior. T_f denotes the bodytemperature of the product when introduced into a freezer in wich the externaltemperature is T_e The coefficients R and P are given in Table 4 andarranged according to the geometry of the product frozen. where the letter edenotes the dimension (i.e. for infinite slab geometry, e is thickness of theslab and for infinite cylinder or sphere e is replaced by r which denotes theradius of the clylinder or sphere).

As mentioned earlier, the equation of Plank assumes the foodis at a freezing temperature at the beginning of the freezing process. However,the food is usually at a temperature higher than freezing temperature. The realfreezing time should therefore be the sum of time calculated from the equationof Plank and the time needed for the product’s surface to decrease frominitial temperature to freezing temperature (Barbosa-Canovas and Ibarz,2002).

Several works have attempted to calculate real freezing time,as in one presented by Nagaoka et al., (1955). Nagaoka’s equation(Eq. 2) calculates the amount of heat elimination required to decrease aproduct’s temperature from initial temperature to freezing temperature, aswell as the amount of heat released during the phase change and the amount ofheat eliminated to reach freezing temperature. Further empirical equations canbe found in literature in detail (Chen, 1985; Levy, 1979; Succar and Hakayawa,1983).

(2)

where Ti is the temperature of the food at the initiation offreezing, DH is the difference between the enthalpyof the food at initial temperature and end of freezing. Re and Pl are thedimensionless numbers, while k and h are the thermal conductivity and thecoefficient of heat transfer, respectively.

For calculating freezing time of products with irregularshape, a common property of most food products - especially fruits andvegetables - a dimensionless factor has been employed in equations (Clelandet al., 1987a,b).

Freezing rate

The freezing rate (°C/h) for a product or package isdefined as the ratio of difference between initial and final temperature ofproduct to freezing time. At a particular location within the product, a localfreezing rate can be defined as the ratio of the difference between the initialtemperature and desired temperature to the time elapsed in reaching the givenfinal temperature (Persson and Lohndal, 1993). The quality of frozen products islargely dependent on the rate of freezing (Ramaswamy and Tung, 1984). Generally,rapid freezing results in better quality frozen products when compared with slowfreezing. If freezing is instantaneous, there will be more locations within thefood where crystallization begins. In contrast, if freezing is slow, the crystalgrowth will be slower with few nucleation sites resulting in larger icecrystals. Large ice crystals are known to cause mechanical damage to cell wallsin addition to cell dehydration. Thus, the rate of freezing for plant tissues isextremely important due to the effect of freezing rate on the size of icecrystals, cell hydration, and damage to cell walls (Rahman, 1999). The figure 4shows a general behavior of the dynamics curve of freezingpreservation.

Rapid freezing is advantageous for freezing of many foods,however some products are susceptible to cracking when exposed to extremely lowtemperature for long periods. Several mechanisms, including volume expansion,contraction and expansion, and building of internal pressure, are proposed inliterature explaining the mechanisms of product damage during freezing (Hung andKim, 1996).

Figure 4. Freezing preservation dynamicscurve.

Energy requirements

For fruits and vegetables, the amount of energy required forfreezing is calculated based on the enthalpy change and the amount of product tobe frozen. The following equation is reported by Riedel (1949) for calculationof refrigeration requirements for fruits and vegetables.

(3)

X_SNJ: Percentage of the product solids differentfrom juice (Dry matter fraction of the juice)
DHj: Enthalpy change during freezing of the juicefraction
DT: Temperature difference betweeninitial and final temperature of the product

1.2.3Refrigeration

Refrigeration is defined as the elimination of heat from amaterial at a temperature higher than the temperature of its surroundings. Themechanism of refrigeration is a part of the freezing process and freezingstorage involved in the thermodynamic aspects of freezing. According to thesecond law of thermodynamics, heat only flows from higher to lower temperatures.Therefore, in order to raise the heat from a lower to a higher temperaturelevel, expenditure of work is needed. The aim of industrial refrigerationprocesses is to eliminate heat from low temperature points towards points withhigher temperature. For this reason, either closed mechanical refrigerationcycles in which refrigeration fluids circulate, or open cryogenic systems withliquid nitrogen (LIN) or carbon dioxide (CO₂), are commonly used bythe food industry.

The main elements in a closed mechanical refrigeration systemare the condenser, compressor, evaporator, and the expansion valve. Therefrigerants hydrochlorofluorocarbon (HCFC) and ammonia are examples of therefrigerants circulated in these types of mechanical refrigeration systems. Asimple scheme for the closed mechanical refrigeration system is shown in Figure5.

Figure 5. A simple scheme for a one-stage closed mechanicalrefrigeration system.
(Adapted from Stoecker, W.F. and Jones J.W.,Refrigeration and Air Conditioning, McGraw-Hill, New York, 1982)

Starting at the suction point of the compressor, fluid in avapor state is compressed into the compressor where an increase in temperatureand pressure takes place. The fluid then flows through the condenser where itdecreases in energy by giving off heat and converting to a liquid state. Afterthe phase, a change occurs inside the condenser, the fluid flows through theexpansion valve where the pressure decreases to convert liquid into a form ofliquid-gas mixture. Finally, the liquid-gas mixture flows through the evaporatorwhere it is converted into a saturated vapor state and removes heat from theenvironment in the process of cooling. With this last stage the loop restartsagain.

The other refrigeration system employed by the food industryis the cryogenic system with carbon dioxide or liquid nitrogen. The refrigerantin this system is consumed differently from the circulating fluid in closedmechanical systems.

Refrigerants

There are several refrigerants available for refrigerationsystems. The selection of a proper refrigerant is based on physical,thermodynamic, and chemical properties of the fluid. Environmentalconsiderations are also important in refrigerant selection, since leaks withinthe system produce deleterious effects on the atmospheric ozone layer. Somerefrigerants, including halocarbons, have been banned to avoid potentialhazardous effects (Stoecker and Jones, 1982). For industrial applications,ammonia is commonly used, while chlorofluoromethane and tetrafluoroethane arealso recommended as refrigerants (Persson and Lohndal, 1993).

1.2.4 Freezingcapacity

Freezing equipment selection is based on the requirements forfreezing a certain quantity of food per hour. For any type of freezer, freezingcapacity (expressed in tonnes per hour) is defined as the ratio of the quantityof the product that can be loaded into the freezer to the holding time of theproduct in that particular freezer. The first parameter, the amount of foodproduct loaded into the freezer, is affected by both the dimensions of theproduct and the mechanical constraints of the freezer. The denominator (holdingtime) has an important role in freezing systems and is based on the calculationof the amount of heat removed from the product per hour, which varies dependingon the type of product frozen (Persson and Lohndal, 1993).

1.2.5 Freezingsystems

There is a variety of freezing systems available for freezing,and for most products, more than one type of freezer can be used. Therefore, inselecting a freezing system initially, a cost-benefit analysis should beconducted based on three important factors: economics, functionality, andfeasibility. Financial considerations mainly involve capital investment and theproduction cost of selected equipment. Product losses during freezing operationshould be included in cost estimation since generating higher cost freezers mayhave other benefits in terms of reducing product losses. Functional factors aremostly based on the suitability of the selected freezer for particular products.The mode of process, either in-line or batch, should be considered based on thefact that computerized systems are becoming more important for ease of handlingand lowering production costs. Mechanical constraints for the freezer shouldalso be considered since some types of freezers are not physically suitable forfreezing certain products. Lastly, the feasibility of the process should beconsidered in terms of plant location or location of the processing area, aswell as cleanability and hygienic design, and desired product quality (Johnstonet al., 1994).

These factors and initial considerations can help eliminateseveral choices in freezer selection, but the relative importance of factors maychange depending on the process. For developing countries where the freezingapplication is relatively new, the cost factor becomes more important than otherfactors due to the decreased production rates and need for lower capitalinvestment costs.

1.2.6 FreezingEquipment

The industrial equipment for freezing can be categorized inmany ways, namely as equipment used for batch or in-line operation, heattransfer systems (air, contact, cryogenic), and product stability. The rate ofheat transfer from the freezing medium to the product is important in definingthe freezing time of the product. Therefore, the equipment selected for freezingprocess characterizes the rate of freezing.

Air-blast freezers

The air blast freezer is one the oldest and commonly usedfreezing equipment due to its temperature stability and versatility for severalproduct types. In general, air is used as the freezing medium in the freezingdesign, either as still air or forced air. Freezing is accomplished by placingthe food in freezing rooms called sharp freezers. Still, air freezing is thecheapest way of freezing and has the added advantage of a constant temperatureduring frozen storage, which allows usage for unprocessed bulk products likebeef quarters and fish. However, it is the slowest method of freezing due to thelow surface heat transfer coefficient of circulating air inside the room.Freezing time in sharp freezers is largely dependent on the temperature of thefreezing chamber and the type, initial temperature, and size of product(Desrosier and Desrosier 1977). An improved version of the still air freezer isthe forced air freezer, which consists of air circulation by convection insidethe freezing room. However, even modification of the sharp freezer with extrarefrigeration capacity and fans for increased air circulation does not helpcontrol the air flow over the products during slow freezing. A typical designfor air blast freezers is shown in Figure 6.

There are a considerable number of designs and arrangementsfor air blast freezers, primarily grouped in two categories depending on themode of process, as either inline or batch. Continuous freezers are the mostsuitable systems for mass production of packaged products with similar freezingtimes, in which the product is carried through on trucks or on conveyors. Thesystem works on a semi-batch principle when trucks are used, since they remainstationary during the process except when a new truck enters one end of thetunnel, thus moving the others along to release a finished one at the exit. Thebatch freezers are more flexible since a variety of products can be frozen atthe same time on individual trolleys. Over-loading may be a problem for thesetypes of freezers, thus the process requires closer supervision than continuoussystems.

Figure 6. Air blast freezer.

Tunnel freezers

In tunnel freezers, the products on trays are placed in racksor trolleys and frozen with cold air circulation inside the tunnel. In order toallow air circulation, optimum space is provided between layers of trolley,which can be moved continuously in and out of the freezer manually or byforklift trucks. This freezing system is suitable for all types of products,although there are some mechanical constraints including the requirement of highmanpower for handling, cleaning, and transportation of trays (Mallett, 1993). Atrolley for a tunnel freezer is shown in Figure 7.

Figure 7. Trolley in a tunnel freezer.

Belt freezers

Belt freezers were first designed to provide continuousproduct flow with the help of a wire mesh conveyor inside the blast rooms. Apoor heat transfer mechanism and the mechanical problems were solved in modernbelt freezers by providing a vertical airflow to force air through the productlayer. Airflow has good contact with the product only when the entire product isevenly distributed over the conveyor belt. In order to decrease required floorspace, the belts can be arranged in a multi-tier belt freezer or a spiral beltfreezer. Spiral belt freezers consist of a belt that can be bent laterallyaround a rotating drum to maximize belt surface area in a given floor space.This type of design has the advantage of eliminating product damage in transferpoints, especially for products that require gentle handling (Mallett, 1993).Both packed and unpacked products with long freezing times (10 min to 3 hr) canbe frozen in spiral belt freezers due to the flexibility of the equipment(ASHRAE, 1994). A typical spiral belt freezer is shown in Figure 8.

Fluidized bed freezers

The fluidized bed freezer, a fairly recent modified type ofair-blast freezer for particular product types, consists of a bed with aperforated bottom through which cold air is blown vertically upwards (Rahman,1999). The system relies on forced cold air from beneath the conveyor belt,causing the products to suspend or float in the cold air stream (George, 1993).The use of high air velocity is very effective for freezing unpacked foods,especially when they can be completely surrounded by flowing air, as in the caseof fluidized bed freezers.

Figure 8. The cross-section view of a spiral beltfreezer.
(Courtesy of Frigoscandia Equipment Ltd., UK)

Figure 9a. Cross-sectional view of a fluidized bedfreezer.
(Courtesy of Frigoscandia Equipment Ltd., UK)

The use of fluidization has several advantages compared withother methods of freezing since the product is individually quick frozen (IQF),which is convenient for particles with a tendency to stick together (Persson andLohndal, 1993). The idea of individually quick frozen foods (IQF) started withthe first technological developments aimed at quick freezing. The need for aneffective means of freezing small particles with the potential for lumpingduring the process is the objective of IQF freezing. Small vegetables, prawns,shrimp, french-fried potatoes, diced meat, and fruits are some of the productsnow frozen with this technology. A typical fluidized-bed freezer is shown inFigures 9a and 9b.

Figure 9b. Simple working principle of a fluidized bedfreezer.
(Saravacos, G.D., Kostaropoulos, A.E., 2002)

Contact freezers

Contact freezing is the one of the most efficient ways offreezing in terms of heat transfer mechanism. In the process of freezing, theproduct can be in direct or indirect contact with the freezing medium. Fordirect contact freezers, the product being frozen is fully surrounded by thefreezing medium, the refrigerant, maximizing the heat transfer efficiency. Aschematic illustration is given in Figure10. For indirect contact freezers, theproduct is indirectly exposed to the freezing medium while in contact with thebelt or plate, which is in contact with the freezing medium (Mallett,1993).

Figure 10. Direct contact freezer.

Immersion freezers

The immersion freezer consists of a tank with a cooledfreezing media, such as glycol, glycerol, sodium chloride, calcium chloride, andmixtures of salt and sugar. The product is immersed in this solution or sprayedwhile being conveyed through the freezer, resulting in fast temperaturereduction through direct heat exchange (Hung and Kim, 1996). Direct immersion ofa product into a liquid refrigerant is the most rapid way of freezing sinceliquids have better heat conducting properties than air. The solute used in thefreezing system should be safe without taste, odour, colour, or flavour, and forsuccessful freezing, products should be greater in density than the solution.Immersion freezing systems have been commonly used for shell freezing of largeparticles due to the reducing ability of product dehydration when the outerlayer is frozen quickly. A commonly seen problem in these freezing systems isthe dilution of solution with the product, which can change the concentrationand process parameters. Thus, in order to avoid product contact with the liquidrefrigerant, flexible membranes can be used (George, 1993). A simpleillustration of the immersion freezer is shown in Figure 11.

Figure 11. Simple illustration of a typical immersionfreezer (Fellows, 2000).

Figure 12. Indirect contact freezer.

Indirect contact freezers

In this type of freezer, materials being frozen are separatedfrom the refrigerant by a conducting material, usually a steel plate. Themechanism of indirect contact freezer is shown in Figure 12. Indirect contactfreezers generally provide an efficient medium for heat transfer, although thesystem has some limitations, especially when used for packaged foods due toresistance of package to heat transfer. Additionally, corrosive effects mayoccur due to interaction of metal packages with heat transfersurfaces.

Figure 13a. Pressure application in a platefreezer.

Plate freezers

The most common type of contact freezer is the plate freezer.In this case, the product is pressed between hallow metal plates, eitherhorizontally or vertically, with a refrigerant circulating inside the plates.Pressure is applied for good contact as schematically shown inFigure13a.

Figure 13b. Plate freezer with a two-stage compressor andsea water condenser
(Courtesy of DSI Samifi FreezersS.r.I.)

This type of freezing system is only limited to regular-shapedmaterials or blocks like beef patties or block-shaped packaged products. Atypical plate freezer is shown in Figure 13b.

Contact belt freezers

This type of freezer is designed with single-band ordouble-band for freezing of thin product layers as shown in Figure 14. Thedesign can be either straight forward or drum. Typical products frozen in beltfreezers are, fruit pulps, egg yolk, sauces and soups (Persson and Lohndall,1993).

Cryogenic freezers

Cryogenic freezing is a relatively new method of freezing inwhich the food is exposed to an atmosphere below -60 °C through directcontact with liquefied gases such as nitrogen or carbon dioxide (Hung and Kim,1996). This type of system differs from other freezing systems since it is notconnected to a refrigeration plant; the refrigerants used are liquefied in largeindustrial installations and shipped to the food-freezing factory in pressurevessels. Thus, the small size and mobility of cryogenic freezers allow forflexibility in design and efficiency of the freezing application. Low initialinvestment and rather high operating costs are typical for cryogenic freezers(Persson and Lohndal, 1993).

Figure 14. Contact belt freezer.
(Courtesy ofFrigoscandia Equipment Ltd., UK)

Liquid Nitrogen freezers

Liquid nitrogen, with a boiling temperature of -196 °C atatmospheric pressure, is a by-product of oxygen manufacture. The refrigerant issprayed into the freezer and evaporates both on leaving the spray nozzles and oncontact with the products. The system is designed in a way that the refrigerantpasses in counter current to the movement of the products on the belt givinghigh transfer efficiency. The refrigerant consumption is in the range of 1.2-kgrefrigerant per kg of the product. Typical food products used in this systemare, fish fillets, seafood, fruits, berries (Persson and Lohndal,1993).

Liquid carbon dioxide freezers

Liquid carbon dioxide exists as either a solid or gas whenstored at atmospheric pressure. When the gas is released to the atmosphere at-70 °C, half of the gas becomes dry-ice snow and the other half stays inthe form of vapor. This unusual property of liquid carbon dioxide is used in avariety of freezing systems, one of which is a pre-freezing treatment before theproduct is exposed to nitrogen spray (George, 1993).

1.2.7 Packaging

Proper packaging of frozen food is important to protect theproduct from contamination and damage while in transit from the manufacturer tothe consumer, as well as to preserve food value, flavour, colour, and texture.There are several factors considered in designing a suitable package for afrozen food. The package should be attractive to the consumer, protected fromexternal contamination, and effective in terms of processing, handling, and cost(Rahman, 1999). Proper selection is based on the type of package and material.There are typically three types of packaging used for frozen foods: primary,secondary, and tertiary. The primary package is in direct contact with the foodand the food is kept inside the package up to the time of use. Secondarypackaging is a form of multiple packaging used to handle packages together forsale. Tertiary packaging is used for bulk transportation of products (Harrisonand Croucher, 1993).

Packaging materials should be moisture-vapor-proof to preventevaporation, thus retaining the highest quality in frozen foods. Oxygen shouldalso be completely evacuated from the package using a vacuum or gas-flush systemto prevent migration of moisture and oxygen (ASHRAE, 1994; Sebranek, 1996).Glass and rigid plastic are examples of moisture-vapor-proof packagingmaterials. Many packaging materials, however, are not moisture-vapor-proof, butare sufficiently moisture-vapor-resistant to retain satisfactory quality infoods. Most bags, wrapping materials, and waxed cartons used in freezingpackaging are moisture-vapor-resistant. In general, the containers should beleakage free while easy to seal. Durability of the material is another importantfactor to consider, since the packaging material must not become brittle at lowtemperatures and crack (MSU, 1999).

A range of different packaging materials, mainly grouped asrigid and non-rigid containers, can be used for primary packaging. Glass,plastic, tin, and heavily waxed cardboard materials are in the rigid containergroup and usually used for packaging of liquid food products. Glass containersare mostly used for fruits and vegetables if they are not water-packed. Plasticsare the derivatives of the oil-cracking industry (Brydson, 1982). Non-rigidcontainers include bags and sheets made of moisture-vapor-resistant heavyaluminum foil, polyethylene or laminated papers. Bags are the most commonly usedpackaging materials for frozen fruits and vegetables due to their flexibilityduring processing and handling (Harrison and Croucher, 1993). They can be usedwith or without outer cardboard cartons to protect against tearing.

Shape and size of the container are also important factors infreezing products. Serving size may vary depending on the type of product andselection should be based on the amount of food determined for one meal. Forshape of the container, freezer space must be considered since rigid containerswith flat tops and bottoms stack well in the freezer, while round containerswaste freezer space.

1.2.8 Frozen storage anddistribution

The quality of the final product depends on the history of theraw material. Using the lowest possible temperature is essential for frozenstorage, transport, and distribution in achieving a high-quality product, sincedeteriorative processes are mainly temperature dependent. The lower the producttemperature is, the slower the speed of reaction is leading to loss of quality.The temperatures of supply chains in freezing applications from the factory tothe retail cabinet should be carefully monitored. The temperature regimecovering the freezing process, the cold-store temperatures (£ -18 °C), distribution temperatures (£ -15 °C), and retail display (£ -12 °C) are given as legal standards (Harrisonand Croucher, 1993).

1.3 Freezing fruits and vegetables insmall and medium scale operations and its potential applications in warmclimates

The preservation of fruits and vegetables by freezing is oneof the most important methods for retaining high quality in agriculturalproducts over long-term storage. In particular, the freshness qualities of rawfruits and vegetables can be retained for long periods, extending well beyondthe normal season of most horticultural crops (Arthey, 1993). The potentialapplication of freezing preservation of fruits and vegetables, includingtropical products, has been increasing recently in parallel with developments indeveloping countries. Freezing of fruits and vegetables in small and mediumscale operations is detailed in the following sections and a general flowchartis shown in Figure 15.

Figure 15. A general flow chart of frozen fruits andvegetables (Mallett, 1993).

1.3.1 Freezingfruits

The effect of freezing, frozen storage, and thawing on fruitquality has been investigated over several decades. Today frozen fruitsconstitute a large and important food group (Skrede, 1996). The quality demandedin frozen fruit products is mostly based on the intended use of the product. Ifthe fruit is to be eaten without any further processing after thawing, texturecharacteristics are more important when compared to use as a raw material inother industries. In general, conventional methods of freezing tend to destroythe turgidity of living cells in fruit tissue. Different from vegetables, fruitsdo not have a fibrous structure that can resist this destructive effect.Additionally, fruits to be frozen are harvested in a fully ripe state and aresoft in texture. On the contrary, a great number of vegetables are frozen in animmature state (Boyle et al., 1977). Fruits have delicate flavours thatare easily damaged or changed by heat, indicating they are best eaten when rawand decrease in quality with processing. In the same way, attractive colour isimportant for frozen fruits. Chemical treatments or additives are often used toinactivate the deteriorative enzymes in fruits. Therefore, proper processing isessential for all steps involved, from harvesting to packaging and distribution.A freezing guide for freezing fruits is shown in table 5.

Production andharvesting

The characteristics of raw materials are of primary importancein determining the quality of the frozen product. These characteristics includeseveral factors such as genetic makeup, climate of the growing area, type offertilization, and maturity of harvest (Boyle et al., 1977).

The ability to withstand rough handling, resistance to virusdiseases, molds, uniformity in ripening, and yield are some of the importantcharacteristics of fruits in terms of economical aspects considered inproduction. The use of mechanical harvesting generally causes bruising of fruitsand results in a wide range of maturity levels for fruits. In contrast,hand-picking provides gentler handling and maturity sorting of fruits. Howeverin most cases, it is non-economical compared to mechanical harvesting due tohigh labor cost (Boyle et al., 1977).

As a rule, harvesting of fruits at an optimum level forcommercial use is difficult. Simple tests like pressure tests are applied todetermine when a fruit has reached optimum maturity for harvest. Colour is alsoone of the characteristics used in determining maturity since increasedmaturation causes a darker colour in fruits. A combination of colour andpressure tests is a better way to assess maturity level for harvesting (Skrede,1996).

Controlled atmosphere storage is a common method of storagefor some fruits prior to freezing. In principle, a controlled atmosphere high incarbon dioxide and low in oxygen content slows down the rate of respiration,which may extend shelf life of any respiring fruit during storage. Due to thefact that these fruits do not ripen appreciably after picking, most fruits arepicked as near to eating-ripe maturity as possible.

Pre-process handling andoperations

Freezing preservation of fruits can only help retain theinherent quality present initially in a product since the process does notimprove the quality characteristics of raw materials. Therefore, quality levelof the raw materials prior to freezing is the major consideration for successfulfreezing. Washing and cutting generally results in losses when applied afterthawing. Thus, fruits should be prepared prior to the freezing process in termsof peeling, slicing or cutting. Freezing preservation does not require specificunit operations for cleaning, rinsing, sorting, peeling, and cutting of fruits(Spiess, 1984).

Fruits that require peeling before consumption should bepeeled prior to freezing. Peeling is done by scalding the fruit in hot water,steam or hot lye solutions (Boyle and Wolford, 1968). The effect of peeling onthe quality of frozen products has been studied for several fruits, includingkiwi (Robertson, 1985), banana (Cano et al., 1990), and mango (Cano andMarín, 1992). The rate of freezing can be increased by decreasing thesize of products frozen, especially for large fruits. An increase in thefreezing rate results in smaller ice crystals, which decreases cellular damagein fruit tissue. Banana, tomato, mango, and kiwi are some examples of largefruits commonly cut into smaller cubes or slices prior to freezing (Skrede,1996).

The objective of blanching is to inactivate the enzymescausing detrimental changes in colour, odour, flavour, and nutritive value, butheat treatment causes loss of such characteristics in fruits (Gutschmidt, 1968).Therefore, only a few types of fruits are blanched for inactivation of enzymesprior to freezing. The loss of water-soluble minerals and vitamins duringblanching should also be minimized by keeping blanching time and temperature atan optimum combination (Spiess, 1984).

Effect of ingredients

Addition of sugars is an extremely important pretreatment forfruits prior to freezing since the treatment has the effect of excluding oxygenfrom the fruit, which helps to retain colour and appearance. Sugars whendissolved in solutions act by withdrawing water from cells by osmosis, resultingin very concentrated solutions inside the cells. The high concentration ofsolutes depresses the freezing point and therefore reduces the freezing withinthe cells, which inhibits excessive structural damage (Munoz-Delgado, 1978).Sugar syrups in the range of 30-60 percent sugar content are commonly used tocover the fruit completely, acting as a barrier to oxygen transmission andbrowning. Several experiments have shown the protective effect of sugar onflavour, odour, colour, and nutritive value during freezing, especially forfrozen berries (Gutschmidt, 1968).

Packaging

Fruits exposed to oxygen are susceptible to oxidativedegradation, resulting in browning and reduced storage life of products(Munoz-Delgado, 1978; Tomassicchio et al., 1986). Therefore, packaging offrozen fruits is based on excluding air from the fruit tissue. Replacement ofoxygen with sugar solution or inert gas, consuming the oxygen by glucose-oxidaseand/or the use of vacuum and oxygen-impermeable films are some of the methodscurrently employed for packaging frozen fruits. Plastic bags, plastic pots,paper bags, and cans are some of the most commonly used packaging materials(with or without oxygen removal) selected, based on penetration properties andthickness (Gradziel, 1988).

There are several types of fruit packs suitable for freezing:syrup pack, sugar pack, unsweetened pack, and tray pack and sugar replacementpack. The type of pack is usually selected according to the intended use for thefruit. Syrup-packed fruits are generally used for cooking purposes, whiledry-packed and tray-packed fruits are good for serving raw in salads andgarnishes.

Syrup pack

The proportion of sugar to water used in a syrup pack dependson the sweetness of the fruit and the taste preference of the consumer. For mostfruits, 40 percent sugar syrup is recommended. Lighter syrups are lower incalories and mostly desirable for mild-flavoured fruits to prevent masking theflavour, while heavier syrups may be used for very sour fruits (Kendall,2002).

Syrup is prepared by dissolving the sugar in warm water andcooling the solution down before usage. Just enough cooled syrup is used tocover the prepared fruit after it has been settled by jarring the container. Inorder to keep the fruit under the syrup, a small piece of crumpled waxed paperor other water resistant wrapping material is placed on top; the fruit ispressed down into the syrup before closing, then sealed and frozen (Beck,1996).

Pectin can be used to reduce sugar content in syrups whenfreezing berries, cherries, and peaches. Pectin syrups are prepared bydissolving 1 box of powdered pectin with 1 cup of water. The solution is stirredand boiled for 1 minute; 1/2 cup of sugar is added and dissolved; the solutionis then cooled down with the addition of cold water. Previously prepared fruitis put into a 4 to 6 quart bowl and enough pectin syrup is added to cover thefruit with a thin film. The pack is sealed and promptly frozen (Brady,2002).

Sugar packs

In preparing a sugar pack, sugar is first sprinkled over thefruit. Then the container is agitated gently until the juice is drawn out andthe sugar is dissolved. This type of pack is generally used for soft slicedfruits such as peaches, strawberries, plums, and cherries, by using sufficientsyrup to cover the fruit. Some whole fruits may also be coated with sugar priorto freezing (Beck, 1996).

Unsweetened packs

Unsweetened packs can be prepared in several ways, eitherdry-packed, covered with water containing ascorbic acid, or packed inunsweetened juice. When water or juice is used in syrup and sugar packs, fruitis submerged by using a small piece of crumpled water-resistant material.Generally, unsweetened packs yield a lower quality product when compared withsugar packs, with the exception, some fruits such as raspberries, blueberries,scalded apples, gooseberries, currants, and cranberries maintain good qualitywithout sugar (Beck, 1996).

Tray packs

Unsweetened packs are generally prepared by using tray packsin which a single layer of prepared fruit is spread on shallow trays, frozen,and packaged in freezer bags promptly. The fruit sections remain loose withoutclumping together, which offers the advantage of using frozen fruit piece bypiece.

Sugar replacement packs

Artificial sweeteners can be used instead of sugar in the formof sugar substitutes. The sweet taste of sugar can be replaced by using thesekinds of sweeteners, however the beneficial effects of sugar like colourprotection and thick syrup can not be replaced. Fruits frozen with sugarsubstitutes will freeze harder and thaw more slowly than fruits preserved withsugar (Beck, 1996).

1.3.2 Freezingvegetables

Freezing is often considered the simplest and most natural wayof preservation for vegetables (Cano, 1996). Frozen vegetables and potatoes forma significant proportion of the market in terms of frozen food consumption(Mallett, 1993). The quality of frozen vegetables depends on the quality offresh products, since freezing does not improve product quality. Pre-processhandling, from the time vegetables are picked until ready to eat, is one of themajor concerns in quality retention.

Table 5. Fruit freezing guide
(Kendall, ColoradoState University, Cooperative Extension, 2002)

Crop cultivar, production, andmaturity

The choice of the right cultivar and maturity before crop isharvested are the two most important factors affecting raw material quality. Rawmaterial characteristics are usually related to the vegetable cultivar, cropproduction, crop maturity, harvesting practices, crop storage, transport, andfactory reception.

The choice of crop cultivars is mostly based on theirsuitability for frozen preservation in terms of factory yield and productquality. Some of the characteristics used as selection criteria are as follows(Cano, 1996):

• Suitability formechanical harvesting

• Uniform maturity

• Exceptional flavour anduniform colour and desirable texture

• Resistance todiseases

• High yield

Although cultivar selection is a major factor affecting thequality of the final product, many practices in the field and factors duringgrowth of crop can also have a significant effect on quality. Those practicesinclude site selection for growth, nutrition of crop, and use of agriculturalchemicals to control pests or diseases. The maturity assessment for harvestingis one of the most difficult parts of the production. In addition toconventional methods, new instruments and tests have been developed to predictthe maturity of crops that help determining the optimum harvest time, althoughthe maturity assessment differs according to crop variety (Hui et al.,2004).

Harvesting

At optimum maturity, physiological changes in severalvegetables take place very rapidly. Thus, the determination of optimumharvesting time is critical (Arthey, 1993). Some vegetables such as green peasand sweet corn only have a short period during which they are of prime quality.If harvesting is delayed beyond this point, quality deteriorates and the cropmay quickly become unacceptable (Lee, 1989). Most of the vegetables aresubjected to bruising during harvesting.

Pre-process handling

Vegetables at peak flavour and texture are used for freezing.Postharvest delays in handling vegetables are known to produce deterioration inflavour, texture, colour, and nutrients (Lee, 1989). Therefore, the delaysbetween harvest and processing should be reduced to retain fresh quality priorto freezing. Cooling vegetables by cold water, air blasting, or ice will oftenreduce the rate of post-harvest losses sufficiently, providing extra hours ofhigh quality retention for transporting raw material to considerable distancesfrom the field to the processing plant (Deitrich et al., 1977).

Blanching

Blanching is the exposure of the vegetables to boiling wateror steam for a brief period of time to inactivate enzymes. Practically everyvegetable (except herbs and green peppers) needs to be blanched and promptlycooled prior to freezing, since heating slows or stops the enzyme action, whichcauses vegetables to grow and mature. After maturation, however, enzymes cancause loss in quality, flavour, colour, texture, and nutrients. If vegetablesare not heated sufficiently, the enzymes will continue to be active duringfrozen storage and may cause the vegetables to toughen or develop off-flavoursand colours. Blanching also causes wilting or softening of vegetables, makingthem easier to pack. It destroys some bacteria and helps remove any surface dirt(Desrosier and Tressler, 1977).

Blanching in hot water at 70 to 105 °C has beenassociated with the destruction of enzyme activity. Blanching is usually carriedout between 75 and 95 °C for 1 to 10 minutes, depending on the size ofindividual vegetable pieces (Holdsworth, 1983). Blanched vegetables should bepromptly cooled down to control and minimize the degradation of soluble andheat-labile nutrients (Deitrich et al., 1977).

The enzymes used as indicators of effectiveness of theblanching treatment are peroxidase, catalase, and more recently lipoxygenase.Peroxidase inactivation is commonly used in vegetable processing, sinceperoxidase is easily detected and is the most heat stable of these enzymes(Arthey, 1993).

Vegetables can be blanched in hot water, steam, and in themicrowave. Hot water blanching is the most common way of processing vegetables.Blanching times recommended for various vegetables are given in Table 6, whichindicates that the operation time can vary depending on the intended productuse. For water blanching, vegetables are put in a basket and then placed in akettle of boiling water covered with a lid. Timing begins immediately(Archuleta, 2003). Steam blanching takes longer than the water method, but helpsretain water-soluble nutrients such as water-soluble vitamins. For steamblanching, a single layer of vegetables is placed on a rack or in a basket at3-5 cm above water boiling in a kettle. A tightly fitted lid is placed on thekettle and timing is started. Microwave blanching is usually recommended forsmall quantities of vegetables prior to freezing. Due to the non-uniform heatingdisadvantage of microwaves, research is still being conducted to obtain betterresults with microwave blanching.

Table 6. Vegetable freezing guide (Archuleta,2003)

Packaging

There are several factors to consider in packaging frozenvegetables, which include protection from atmospheric oxygen, prevention ofmoisture loss, retention of flavour, and rate of heat transfer through thepackage (Arthey, 1996). There are two basic packing methods recommended forfrozen vegetables: dry pack and tray pack.

In the dry pack method, the blanched and drained vegetablesare put into meal-sized freezer bags and packed tightly to cut down on theamount of air in the package. Proper headspace (approximately 2 cm) is left atthe top of rigid containers before closing. For freezer bags, the headspace islarger. Provision for headspace is not necessary for foods such as broccoli,asparagus, and brussels sprouts, as they do not pack tightly in containers(Kendall, 2002).

In the tray pack method, chilled, well-drained vegetables areplaced in a single layer on shallow trays or pans. Trays are placed in a freezeruntil the vegetables become firm, then removed. Vegetables are filled intocontainers. Tray-packed foods do not freeze in a block but remain looselydistributed so that the amount needed can be poured from the container and thepackage reclosed (Kendall, 2002).

The technical approach of chapter 1 focuses mainly on thelarge scale freezing industry. However, it is important to highlight that allpreliminary steps before freezing food products are quite similar whether on alarge or small scale. Furthermore, chapter 3 focuses more on the more suitableapproach for small food freezing industry.