The freezing of fruits slows down, but does not stop, the phys- ical, chemical, and biochemical reactions that cause quality deterioration. There is a slow progressive change in sensory and nutritional quality during frozen storage that becomes noticeable after a period of time. Safe, high-quality frozen fruits with maximum nutritional values can be produced if diligent controls are maintained at all times. These include temperature control, extended quality shelf life, microbiolog- ical safety, and the retention of nutrients.
Two principles dominate the control of quality and safety in frozen foods: product–process–package (PPP) factors and time–temperature–tolerance (TTT) factors. PPP factors need to be considered at an early stage in the production of frozen fruits, and they are the bases of commercial success of the product. The PPP factors are as follows:
r Product: High-quality frozen food requires high-quality raw materials and ingredients.
r Process: The speed and effectiveness of the freezing op- erations and the use of additional processes (blanching, etc.).
r Package: Packaging offering physical and chemical bar- riers.
TTT factors maintain the quality and safety during stor- age. TTT concepts refer to the relationship between storage temperature and storage life. For different foods, different mechanisms govern the rate of quality degradation, and the most successful way of determining practical storage life is to subject the food to long-term storage at different temper-
atures. TTT relationships predict the effects of changing or fluctuating temperatures on quality shelf life (IIR 1996).
Safe, high-quality frozen fruits with maximum nutritional values can be produced if the directions given below are followed:
r Selection of suitable product for freezing r PPP factors
r Knowledge of the effect of freezing, frozen storage, and thawing on the fruit tissues that causes physical, chemi- cal, and biochemical changes
r Stability of frozen fruits (TTT factors) r Thawing
r Microbiological quality and safety of frozen fruits.
Selection of Suitable Product for Freezing High-quality frozen fruit requires high-quality raw material.
Generally, quality cannot be gained from processing, but it certainly can be lost. Fruits are best when frozen fully ripe but still firm and at the peak of quality, with a pleasing color, texture, flavor, and maximum nutritional value. Large differ- ences in frozen fruit quality exist between fruit varieties and cultivars based on chemical, biochemical, and physical char- acteristics that determine the sensory and nutritional quality.
Differences in cell wall structure, enzyme activity, amounts of pigments, sugars, organic acids, volatile compounds, vi- tamins C, A, and E, and other components are factors that affect the differences in sensory and nutritional quality of raw fruits. Freezing potential of fruit varieties or cultivars are evaluated with practical trials after freezing, frozen storage, and thawing of the fruit products. The suitability of varieties or cultivars for freezing can be studied on the basis of phys- ical (texture and color), physical–chemical (pH, acidity, and soluble solids), chemical (volatile, pigments, and polyphenol compounds), nutritional (vitamins and dietary fiber content), and sensory aspects (firmness, color, and taste). These stud- ies have been done with different fruits such as kiwi (Cano and Mar´ın 1992, Cano et al. 1993a), mango (Mar´ın et al.
1992, Cano and Mar´ın 1995), pineapple (Bartolom´e et al.
1996a, 1996b, 1996c), papaya (Cano et al. 1996a, Lobo and Cano 1998), raspberry (De Ancos et al. 1999, 2000a, 2000b, Gonz´alez et al. 2002), and strawberry (Castro et al. 2002).
Another criterion for selection of suitable variety or cultivar can be the enzymatic systems activity (polyphenoloxidase (PPO), peroxidase (POD), lipoxygenase (LOX), etc.) in raw fruit and during freezing and frozen storage. Employing va- rieties with low enzymatic activities could reduce the devel- opment of browning, off-flavors and off-odors, and color and textural changes (Cano et al. 1990b, 1996b, 1998, Gonz´alez et al. 2000).
Harvesting fruits at optimum level for freezing purposes is difficult. The need for efficient production often implies the use of mechanical harvesting at a time when the fruit has reached an acceptable maturity level to avoid mechanical
damage. Postharvest techniques allow the storage of unripe climateric fruits at specific atmosphere, temperature, and hu- midity conditions until they reach proper maturity levels to be frozen (Cano et al. 1990a, 1990b, Mar´ın et al. 1992). Noncli- materic fruits (strawberries, raspberries, etc.) are harvested, preferably when fully ripe but still firm, cooled immediately after picking, and frozen as soon as possible (Gonz´alez et al.
2002). However, the quality advantages of immediate freez- ing could not be detected after a long frozen storage period (6–12 months; Plocharski 1989).
Preparing, Pretreatments, and Packaging Successful freezing should retain the initial quality present in the raw fruit selected for freeze processing.
Preparing
Fruits must be prepared before freezing according to the frozen fruit end-use. Washing, rinsing, sorting, peeling, and cutting the fruits are not specific steps for frozen fruits; these are preparatory operations similar to other types of processing but must be carried out quickly and with great care to avoid damaging the fragile fruit tissue. Peeling, stone removal, and cutting in cubes, slices, or halves are usually mechanical operations. Size reduction before freezing results in a faster freezing and consequently a better frozen fruit quality. For economical factors, certain fruits like peaches, apricots, and plums are frozen whole immediately after harvesting and peeling; stone removal and cutting is done after a partial thawing.
Consumption of fruit juices and nectars has increased in the world due to recommendations for better nutrition and healthier diets. Fruits and fruit juices meet these recommen- dations. Nectars and fruit juices can be manufactured with fresh fruit, but with frozen fruit, higher yields are obtained.
At present, frozen juices represent an important segment of the international drink industry. Preparing fruit for frozen juice requires different steps: pressing, clarification, heat treatment, and concentration. Also, purees and pulps repre- sent an important ingredient for the manufacturing industries for dairy products, cakes, ice creams, jellies, and jams (Chen 1993).
Pretreatments
The importance of enzyme content to fruit quality has been extensively reviewed (Philippon and Rouet-Mayer 1984, Robinson and Eskin 1991, Friedman 1996, Browleader et al.
1999). Enzymes, namely PPO, POD, LOX, catalase (CAT), and pectinmethylesterase, are involved in the fast deteri- oration of fruit during postharvest handling and process- ing. Enzymes not inactivated before freezing can produce off-flavors, off-odors, color changes, development of brown color, and loss of vitamin C and softness during frozen storage
and thawing. Water blanching is the most common method for inactivating vegetable enzymes (Fellows 2000). It causes denaturation and, therefore, inactivation of the enzymes that also causes destruction of thermo-sensitive nutrients and losses of water-soluble compounds such as sugar, minerals, and water-soluble vitamins. Blanching is rarely used for fruits because they are usually consumed raw and heat treatment causes important textural changes. An alternative to blanch- ing fruit is to use ingredients and chemical compounds that have the same effect as blanching.
Blanching Heat treatment to inactivate vegetable enzymes can be applied by immersion in hot water, by steam blanching or by microwave blanching. Hot water blanching is usually done between 75◦C and 95◦C for 1–10 minutes, depending on the size of the produce. Hot water blanching also removes tissue air and reduces the occurrence of undesirable oxidation reactions during freezing and frozen storage. Steam blanch- ing reduces the water-soluble compounds losses and is more energy efficient than water blanching. Of all the enzymes in- volved in producing quality losses during processing, POD and CAT seem to be the more heat stable, and thus could be used as an index of adequate blanching. Generally, a qual- ity blanched produce permits some POD and CAT activity.
Complete POD inactivation indicates overblanching. Blanch- ing also helps to destroy microorganisms on the surface of the produce. Blanching destroys semipermeability of cell mem- branes and removes cell turgor. Reduced turgor is perceived as softness and lack of crispness and juiciness. These are some of the most important sensory characteristics of fruit.
Although loss of tissue firmness in blanched frozen fruits af- ter thawing indicates that blanching is not a good pretreatment for the majority of the fruits, some results have been interest- ing (Reid 1996). Hot water blanching peeled bananas prior to slicing, freezing, and frozen storage produced complete PPO and POD inactivation and a product with acceptable sensory quality (Cano et al. 1990a). Microwave blanching has not been an effective pretreatment for banana slices (Cano et al.
1990b) but interesting results have been obtained with frozen banana purees (Cano et al. 1997).
Addition of Chemical Compounds Substitutes for ther- mal blanching have been tested with different enzymatic inhibitors. They are mainly antibrowning additives such as sulfiting agents (sulfur dioxide or inorganic sulfites salts) and ascorbic acid, which are applied by dipping or soak- ing the fruit in different solutions before freezing (Skrede 1996). Enzymatic browning involving the enzyme PPO is the principal cause of fruit quality losses during postharvest and processing. PPO catalyzes the oxidation of mono- and orthodiphenols to quinones, which can cyclize, undergo fur- ther oxidation, and polymerize to form brown pigments or react with amino acids and proteins that enhance the brown color produced (Fig. 7.3).
R
OH
R
OH
OH
Reducing agent
Amino acids proteins
Complex brown polymers O
O
R PPO + O2 PPO + O2
Figure 7.3. Enzyme-catalyzed initiation of browning by PPO showing the point of attack by reducing agents.
The proposed mechanisms of antibrowning additives that inhibit enzymatic browning are (1) direct inhibition of the enzyme, (2) interaction with intermediates in the browning process to prevent the reaction leading to the formation of brown pigments, or (3) to act as reducing agents, promot- ing the reverse reaction of the quinone back to the origi- nal phenols (Fig. 7.3; Friedman 1996, Ashie et al. 1996).
Other acid treatments such as dipping in citric acid or hy- drochloric acid solution (1%) could be a commercial pre- treatment for browning control and quality maintenance of frozen litchi fruit (Yueming-Jiang et al. 2004). Although all the fruits contain polyphenolic compounds, some fruits as peaches, apricots, plums, prunes, cherries, bananas, apples, and pears show a greater tendency to develop browning very quickly during processing. Research efforts have been done to develop new natural antibrowning agents in order to re- place sulfites, the most powerful and cheapest product until now, but they cause adverse health effects in some asthmat- ics. In this framework, Maillard reaction products have been recognized as a strong apple PPO inhibitor (Billaud et al.
2004). Also, some frozen fruits like apples and cherimoya are pretreated by dipping its slices in sodium chloride solutions (0.1–0.5%) in combination with ascorbic or citric acid, in or- der to remove intracellular air and reduce oxidative reactions (Reid 1996, Mastrocola et al. 1998).
Fruit texture is significantly altered by freezing, frozen storage, and thawing. Fruits have thin-walled cells rich in pectin substances, in particular in the middle lamella between cells, and with a large proportion of intracellular water, which can freeze, resulting in cell damage. Freezing–thawing also accelerates the release of pectin, producing de-esterification of pectins and softens the fruit tissue. Optimum freezing rate reduces tissue softening and drip loss, and the addition of calcium ions prior to freezing increases the firmness of fruit after thawing. These ions fortify the fruit by changing the pectin structure. Calcium maintains the cell wall structure in fruits by interacting with the pectic acid in the cell walls to
form calcium pectate. Dipping in calcium chloride solution (0.18% Ca) or pectin solution (0.3%) improves the quality of frozen and thawed strawberries (Suutarinen et al. 2000).
Osmotic Dehydration: Addition of Sugars and Syrups Dipping fruits in dry sugar or syrups is a traditional pretreat- ment to preserve color, flavor, texture, and vitamin C content and to prevent the browning of frozen–thaw fruits. Sugar or syrups are used as cryoprotectants by taking out the fruit cell water by osmosis and excluding oxygen from the tis- sues. Partial removal of water before freezing might reduce the freezable water content and decrease ice crystal damage, making the frozen fruit stable. Therefore, minor damage to cellular membranes occurs and oxidative reactions and en- zymatic degradation reactions are minimized. The process of dehydration before freezing is known as dehydrofreez- ing(Fito and Chiralt 1995, Robbers et al. 1997, Bing and Da-Wen 2002). During osmotic dehydration, the water flows from the fruit to the osmotic solution, while osmotic solute is transferred from the solution into the product, providing an important tool to impregnate the fruit with protective solutes or functional additives. Syrup is considered a better protect- ing agent than dry sugar. Dry sugars are recommended for fruits, such as sliced peaches, strawberries, figs, grapes, cher- ries, etc., that produce enough fruit juice to dissolve the sugar.
Dipping fruit, whole or cut, in syrup allows a better protec- tion than dry sugar because the sugar solution is introduced inside the fruit. Syrup concentrations between 20% and 65%
are generally employed, although 40% syrup is enough for the majority of the fruits. Sucrose is the osmotic agent most suitable for fruits although other substances, including su- crose, glucose, fructose, lactose,l-lysine, glycerol, polyols, maltodextrin, starch syrup, or combinations of these solutes can be used (Bing and Da-Wen 2002, Zhao and Xie 2004).
Osmotic dehydration is carried out at atmospheric pressure or under vacuum. Among developments in osmotic treatments, vacuum impregnation may be the newest. The exchange of partial freezable water for an external solution is promoted by pressure, producing different structural changes and lower treatment time than osmotic dehydration at atmospheric pres- sure. Successful applications of dehydrofreezing and vacuum impregnation on fruits have been recently reviewed (Zhao and Xie 2004). Great color, flavor, and vitamin C retention have been achieved in frozen–thawed strawberries, raspber- ries, and other types of berries treated with a 20% or 40%
syrup concentration before freezing and long-term frozen storage between 6 months and 3 years (Skrede 1996). The effects of dehydrofreezing process on the quality of kiwi, strawberry, melon, and apples have been reported (Garrote and Bertone 1989, Tregunno and Goff 1996, Spiazzi et al.
1998, Talens et al. 2002, 2003). The quality and texture of dehydrofrozen and thawed fruit has been improved by using osmotic solutions in combination with ascorbic acid solution (antibrowning treatment) and/or calcium chloride or pectin solutions (Skrede 1996, Suutarinen et al. 2000, Talens et al.
2002, 2003, Zhao and Xie 2004). Another important factor contributing to fruit quality improvement is vacuum impreg- nation, which is useful in introducing functional ingredients into the fruit tissue structure, conveniently modifying their original composition for development of new frozen products enriched with minerals, vitamins, or other physiologically ac- tive nutritional components (Zhao and Xie 2004).
Packaging
Packaging of frozen fruits plays a key role in protecting the product from air and oxygen that produce oxidative degrada- tion, contamination by external sources, and damage during shipping. Package barrier properties protect the frozen fruit from oxygen, light, and water vapor, each of which can result in deterioration of colors, oxidation of lipids and unsatu- rated fats, denaturation of proteins, degradation of ascorbic acid, and a general loss of characteristic sensory and nutri- tional qualities. Similarly, barrier properties protect against the loss of moisture from the frozen food to the external en- vironment to avoid external dehydration or “freezer burn”
and weight loss. The primary function of food packaging is to protect the food from external hazards. In addition, packaging materials should have a high heat transfer rate to facilitate rapid freezing. Also, the package material should not affect the food in any way, as indicated by European Di- rectives on food contact materials, including migration limits (EC Directives 1990, 2002) and the Code of Federal Regula- tions in the United States regarding food contact substances (CFR 2004). A wide range of materials has been used for packaging of frozen fruits, including plastic, metals, and pa- per/cardboard, or polyethylene bags. Laminates can provide a combination of “ideal” package properties. Board and pa- per packages are often laminated with synthetic plastics to improve the barrier properties. Table 7.1 shows some compar- isons of barrier properties for a range of common package materials. Fruit products can be packaged before freezing
(fruits with sugar or syrup, purees and juices concentrated or not) or after freezing (whole or cut fruits). The importance of packaging material to the stability of frozen fruits has been reviewed (Skrede 1996). In general, quality differences (pig- ment content, ascorbic acid retention, color, and consistency) between frozen products packaged in different types of pack- ages are mainly detected after a long period of frozen storage (⬎3 months) and at temperatures over –18◦C.
Effect of Freezing, Frozen Storage, and Thawing on Fruit Tissues: Physical, Chemical, and Biochemical Changes
Plant Cell Structure
Understanding the effect of freezing on fruit requires a short review of plant cell structure. A relationship between cell structure properties and freezing cell damage has been ex- tensively reviewed (Reid 1996, Skrede 1996). Plant cells are surrounded by a membrane and interspersed with extensive membrane systems that structure the interior of the cell into numerous compartments. The plasmalemma or plasma mem- brane encloses the plasma of the cell and is the interface be- tween the cell and the extracellular surroundings. Contrary to animal cells, plant cells are almost always surrounded by a cell wall and many of them contain a special group of organelles inside: the plastids (chloroplasts, leucoplasts, amyloplasts, or chromoplasts; Fig. 7.4).
An important property of the plant cell is its extensive vacuole. It is located in the center of the cell and makes up the largest part of the cells volume and is responsible for the turgor. It helps to maintain the high osmotic pressure of the cell and the content of different compounds in the cell, among which are inorganic ions, organic acids, sugars, amino acids, lipids, oligosaccharides, tannins, anthocyanins, flavonoids, and more. Vacuoles are surrounded by a special type of membrane, the tonoplast. The cell wall of plants con- sists of several stacked cellulose microfibrils embedded in a
Table 7.1. Relative Oxygen and Water Vapor Permeabilities of Some Food Packaging Materials (References Values Measured at 23◦C and 85% RH)
Relative Permeability
Package Material Oxygen (mL m−2day−1atm−1) Water Vapor (g m−2day−1)
Aluminum ⬍50 (very high barrier) ⬍10 (very high barrier) variable
Ethylene vinyl acetate (EVOH) ⬍50 (very high barrier)
Polyester (PET) 50–200 (high barrier) 10–30 (high barrier)
Polycarbonate (PC) 200–5000 (low barrier) 100–200 (medium barrier)
Polyethylene (PE)
High density (HDPE) 200–5000 (low barrier) ⬍10 (very high barrier) Low density (LDPE) 5000–10,000 (very low barrier) 10–30 (high barrier)
Polypropylene (PP) 200–5000 (low barrier) 10–30 (high barrier)
Source:Atmosphere Controle 2000 (http://atmosphere-controle.fr/permeability.html).
Vacuole
Mitochondrion Cytoplasm
Nucleus
Cell wall Cell membrane Chloroplast Endoplasmic
reticulum
Figure 7.4. Cross-section of a plant cell.
polysaccharide matrix able to store water, thereby increas- ing the cell volume (hydration and absorbtion). According to their capacity to bind or store water, the polysaccharides involved in the matrix can be classified as follows: pectin⬎ hemicellulose⬎cellulose⬎lignin.
Pectins are mainly polygalacturonic acids with differing degrees ofg-galactosyl,l-arabinosyl, orl-rhanmosyl residue and are predominant in the middle lamella, the layer between cells. The de-esterification process of pectin is related to the softness of fruit tissues during ripening and processing.
Physical Changes and Quality
Volume Expansion The first factor that produces mechan- ical damage to the cell is the volume expansion due to the formation of ice that affects the integrity of cell membrane.
Recrystallization Ice crystals can change the quality of frozen fruits in different ways. First, the speed of freezing af- fects frozen–thaw fruit quality. Slow speed freezing produces large and sharp ice crystals that can produce mechanical dam- age to the fragile plants cell membranes, causing the cell or- ganelles to collapse and lose their contents (sugars, vitamins, pigments, volatile compounds, phenol, enzymes, etc.) and a breakdown of the pectin fraction in the cell wall, which af- fects fruit tissue texture. During frozen storage, retail-display, or the carry-home period, fluctuations in product temperature produces ice recrystallization that affects the number, size, form, and position of the ice crystal formed during freezing.
Frequent large fluctuation produces partial fusion of ice and the reforming of large and irregular ice crystals that can dam- age cellular membranes and produce a freeze-dried product, allowing sublimed or evaporated water to escape.
Sublimation: Freezer Burn The sublimation of the ice may occur during frozen storage if the packaging product is unsuitable. Moisture loss by evaporation from the surface of the product leads to “freezer burn,” which is recognized as a light-colored zone on the surface of the product. Dehy- dration of the product can be avoided by improving the type of package, increasing humidity, and decreasing the storage temperature.
The recrystallization and freezer burn dehydration increase with temperature fluctuations, but the harmful effect of these two processes on frozen fruit quality can be decreased by lowering the storage temperature below –18◦C (IIR 1996).
Chemical and Biochemical Changes and Quality
The chemical and biochemical reactions related to sensory and nutritional quality changes of fruits are delayed but not completely stopped at subzero temperature. Quality changes, such as loss of the original fruit color or browning, develop- ment of off-odor and off-taste, texture changes, and oxidation of ascorbic acid, are the main changes caused by chemical and biochemical mechanisms that affect fruit quality. Also, pH changes in fruit tissues detected during freezing and frozen storage can be a consequence of these degradation reactions.
Color Changes Color is the most important quality char- acteristic of fruits because it is the first attribute perceived by the consumers and is the basis for judging the product accept- ability. The most important color changes in fruits are related to chemical, biochemical, and physicochemical mechanisms:
(a) breakdown of cellular chloroplasts and chromoplasts, (b) changes in natural pigments (chlorophylls, carotenoids, and anthocyanins), and (c) development of enzymatic browning.
Mechanical damage (ice crystals and volume expansion) caused by the freezing process can disintegrate the frag- ile membrane of chloroplasts and chromoplasts, releasing chlorophylls and carotenoids, and facilitating their oxidative or enzymatic degradation. Also, volume expansion increases the loss of anthocyanins by lixiviation due to disruption of cell vacuoles.
Chlorophylls Chlorophylls are the green pigment of vegetables and fruits, and their structures are composed of tetrapyrroles with a magnesium ion at their center. Freezing and frozen storage of green vegetables and fruits cause a green color loss due to degradation of chlorophylls (a and b) and transformation in pheophytins, which transfers a brown- ish color to the plant product (Cano 1996). One example is kiwi fruit slices that show a decrease in chlorophyll concen- tration between 40% and 60%, depending on cultivar, after freezing and frozen storage at –20◦C for 300 days (Cano et al. 1993a). Different mechanisms can cause chlorophyll degradation: loss of Mg due to heat and/or acid, which trans- forms chlorophylls into pheophytins, or loss of the phytol group through the action of the enzyme chlorophyllase (EC 3.1.1.14), which transforms chlorophyll into pheophorbide.