Quality losses are the major concern in frozen food industry. Ice formation involves a serious of modifications that deteriorate food quality. Although these modifications slow down at low temperatures, they continue during frozen storage conditions.
During the freezing of food, water is transferred into ice crystals and solutes concentrate in the unfrozen matrix. Slow freezing results in a maximum ice crystal purity and maximum concentration of solutes in the unfrozen phase, leading to equilibrium conditions.
In contrast, rapid freezing results in a considerable entrapment of solutes by growing crystals and a lower concentration of solutes in the unfrozen phase. The increasing concentration of solutes in the unfrozen matrix increases the ionic strength and can produce changes affecting the biopolymer structure. Water structure and water-solute interactions may be altered and interactions between macromolecules such as proteins may increase. Formation of ice crystals can produce the release of contents of food issues; reactions that normally do not occur in intact cells may occur as a consequence of the freezing process. The possibility that enzymes come in contact with different substrates increases, leading to quality deterioration during frozen storage. Most enzymes exhibit substantial activity after freezing and thawing and many enzymes show significant activity in partially frozen systems. Freezing can give unusual effects on chemical reactions; temperature and concentration of the reactants in the unfrozen phase (freeze concentration effects) are the main factors responsible for changes in the reaction rates in frozen products. In many frozen systems, reaction rates as a function of temperature go through a maximum at some temperature below the initial freezing point. This is a consequence of opposing factors: low temperatures that bring reaction rates down, and increasing solute concentration in the unfrozen phase that may increase these reaction rates. For example, oxidation of myoglobin (meat pigment) was accelerated at temperatures close to -5℃.
The most important chemical changes that can proceed during freezing and frozen storage are enzymatic reactions, protein denaturation, lipid oxidation, degradation of pigments and vitamins, and flavor deterioration.
Storage at low temperatures can decrease the activity of enzymes in tissues but cannot inactivate them. In raw products, hydrolases (enzymes catalyzing hydrolytic cleavage) such as lipases (carboxylic ester hydrolases), phospholipases (phosphatide cleaving enzymes), proteases (hydrolases cleaving peptide bonds) etc., may remain active during frozen storage.
Hydrolytic enzymes can produce quality deterioration in products that are not submitted to thermal treatments before freezing: however, blanching of vegetables or cooking of meat inactivates these enzymes.
Lipases and phospholipases, hydrolyze ester linkages of triacylglycerol and phospholipids, respectively; the hydrolysis of lipids can lead to undesirable flavor and textural changes. Certain lipases can remain active in frozen food systems stored even at -29 ℃. Lipase activity is evident in the accumulation of free fatty acids. Freezing may accentuate lipolysis by disrupting the lysosomal membrane that releases hydrolytic enzymes, the release of short-chain free fatty acids can lead to hydrolytic rancidity, producing off-flavors and may interact with proteins forming complexes that affect texture.
Proteases catalyze the hydrolysis of proteins to peptides and amino acids; in meat these endogenous enzymes are considered beneficial, tenderizing the muscles during rigor mortis.
Conditioned meat on freezing not only retains the texture quality but also has a smaller tendency to drip on thawing.
The browning of plant tissue is caused by enzymatic oxidation of phenolic compounds in the presence of oxygen. Disruption of cells by ice crystals can start enzymatic browning by facilitating contact between o-diphenol oxidase and its substrate.
The oxido-reductases are of primary importance because their action leads to off-flavor and pigment bleaching in vegetables, and browning in some fruits.
In vegetable and fruit tissues, endogenous pectin methyl esterases catalyze the removal of the methoxyl groups from pectines. In the case of frozen strawberries, these enzymes produce gelation during storage. Hydrolytic enzymes, such as chlorophylases and anthocyanases present in plants, may catalyze a destruction of pigments in frozen tissues affecting the color, if they are not inactivated by blanching.
The main causes of freeze-induced damage to proteins are ice formation and recrystallization, dehydration, salt concentration, oxidation, changes in lipid groups and the release of certain cellular metabolites. Freeze-induced protein denaturation and related functionality losses are commonly observed in frozen fish, meat, poultry, egg products and dough.
During freezing, proteins are exposed to an increased concentration of slats in the unfrozen phase; the high ionic strength can produce competition with existing electrostatic bonds modifying the native protein structure. Losses in functional properties of proteins are commonly analyzed by comparing water-holding capacity, viscosity, gelation, emulsification, foaming and whipping properties. Freezing has an important effect in decreasing water-holding capacity of muscle systems on thawing, producing changes also in protein solubility. This decrease occurs during freezing because water-protein associations are replaced by protein-protein associations or other interactions. Proteins exposed to the aqueous medium of the biological tissues have a hydrophobic interior, and charges (or polar) side chains in the surface. The migration of water molecules from the interior of the tissue during extracellular freezing leads to a more dehydrated state disrupting protein-solvent interactions. Protein molecules exposed to a less polar medium have a greater exposure of hydrophobic chains, modifying protein conformation. To maintain the minimum free energy, protein-protein interactions via hydrophobic and ionic interactions occur, resulting in protein denaturation and the formation of aggregates.
Oxidative process during frozen storage can also contribute to protein denaturation; oxidizing agents (enzymes, haem, and transition metals) can react with proteins.
Lipid oxidation is another reaction that severely limits the shelf life of a frozen product, leading to loss of quality (flavor, appearance, nutritional value, and protein functionality). Lipid oxidation is a complex process that proceeds upon a free radical process. During the initiation stage, a hydrogen atom is removed from a fatty acid, leaving a fatty acid alkyl that is converted in the presence of oxygen to a fatty acid peroxyl radical. In the next step, the peroxyl radical subtracts a hydrogen from an adjacent fatty acid forming a hydroperoxide molecule and a new fatty acid alkyl radical process. Decomposition of hydroperoxides of fatty acids to aldehydes and ketones is responsible for the characteristic flavors and aromas (randicity).
Redox-active transition metals are major factors catalyzing lipid oxidation in biological systems; iron, in particular, is a well-known catalyst.
Both enzymatic and non-enzymatic pathways can initiate lipid oxidation. One of the enzymes considered important in lipid oxidation is lipoxygenase that is present in many plants and animals and can generate offensive flavors and also a loss of pigment colors. Lipid oxidation is particularly important in meats (including poultry) and seafood. Fatty meats and fish, in particular, suffer from this adverse reaction during long-term frozen storage.
Oxidative flavor deterioration is produced in both plant and animal products. It is identified more with frozen muscle than with frozen vegetable products because blanching is typically applied to vegetables prior to freezing.
Pigment degradation and color quality deterioration are related to lipid oxidation. Haem pigments in red meats, and carotenoid-fading in a salmonid flesh are subjected to oxidative degradation during storage. Chlorophyll is also capable of serving as a secondary substrate in lipid oxidation.