Lesson 11 Food Freezing PDF

Lesson 11 Food Freezing __ ROLENZ DERICK R. CRUZ Food Technology Department College of Industrial Technology Bicol University © 2021 Rolenz Cruz All rights reserved. This work is an...

Lesson 11 Food Freezing __ ROLENZ DERICK R. CRUZ Food Technology Department College of Industrial Technology Bicol University © 2021 Rolenz Cruz All rights reserved. This work is an intellectual property of the author. Redistribution or sale of any part of this presentation 1 © 2021 and lecture without prior consent of the author Rolenz is not Cruz allowed. © 2021 Rolenz Cruz All rights reserved. This work is an intellectual property of the author. Redistribution or sale of any part of this presentation and lecture without prior consent of the author is not allowed. 2 © 2021 Rolenz Cruz  LESSON OUTCOME At the end of the lesson, you should be able to: Explain the principles of food freezing, including the process and its effects on food quality, such as texture, flavor, and nutritional value. Apply the principles of freeze concentration to demonstrate a process for concentrating foods, showcasing the ability to control variables and assess the impact on food characteristics. 3 © 2021 Rolenz Cruz Freezing a process of removing heat from a product, to bring down its temperature below its freezing point, accompanied by a phase change from water to ice. Storage of food at T between −18°C to −30°C. Two main factors that make freezing an appropriate preservation method: 1. The reducing temperature leads to reduced rates of degradation phenomena and quality changes, thus stabilizing product quality 2. The change of water to ice, which implies the decrease of the concentration of the unfrozen phase 4 © 2021 Rolenz Cruz Freezing Liquid water notably promotes the microbiological, enzymatic, and chemical activity in the stored foods, reducing its storage life. – Thus, the freezing of water to ice basically puts a stop to most of these activities. When compared to refrigerated foods, it is the formation of ice crystals that cause frozen foods to keep much longer than refrigerated foods. – Thus, a frozen system with ice has significantly reduced liquid moisture content (and much lower water activity), leading to a food matrix of enhanced stability. 5 © 2021 Rolenz Cruz Freezing Soft fruits may be stored For between 3 and 6 months at −12°C For 24 months and beyond at −24°C Most meat has a shelf life of 6 to 9 months at −12°C 15 to 24 months at −24°C. The quality and nutrient content of any frozen food at the point of consumption are dependent upon the quality of the original raw material, the length of storage and the storage conditions and the extent and nature of the freezing process. 6 © 2021 Rolenz Cruz Freezing Process During the freezing process, food undergoes three successive stages: 1. the pre-freezing period, 2. the freezing stage, and 3. the temperature decrease to the final storage temperature. 7 © 2021 Rolenz Cruz Freezing Process 1. In the first pre-freezing process, there will be an initial drop in the T of the product until it reaches its initial FP, where the sensible heat of the product is removed. At point S the water remains liquid, although the T is below the FP – known as supercooling. Typical Freezing Curve of High Moisture Food Matrix 8 © 2021 Rolenz Cruz Freezing Process The T rises rapidly to the FP as ice crystals begin to form. 2. In the freezing process, the T of the product remains relatively steady as the latent heat is removed. The FP is gradually depressed by the increase in [solute] in the unfrozen liquid, and the T therefore falls slightly. Typical Freezing Curve of High Moisture Food Matrix 9 © 2021 Rolenz Cruz Freezing Process 3. In the last stage of deep freezing, the sensible heat of ice and that of the nonfrozen phase are removed until the final storage T is reached. At the same time, an amount of the latent heat is removed due to the freezing of additional water as a result of the decreasing temperature. Typical Freezing Curve of High Moisture Food Matrix 10 © 2021 Rolenz Cruz  11 © 2021 Rolenz Cruz Ice Crystal Formation The FP of a food may be described as ‘the temperature at which a small amount of ice crystal exists in equilibrium with the surrounding water’. However, before an ice crystal can form, a nucleus of water molecules must be present. Nucleation therefore precedes ice crystal formation. 12 © 2021 Rolenz Cruz Ice Crystal Formation There are two types of nucleation: 1. Homogeneous Nucleation – the chance orientation and combination of water molecules. 2. Heterogeneous Nucleation – the formation of a nucleus around suspended particles or at a cell wall. Heterogeneous nucleation is more likely to occur in foods and takes place during supercooling. The length of the supercooling period depends on the type of food and the rate at which heat is removed. 13 © 2021 Rolenz Cruz Ice Crystal Formation High rates of heat transfer produce large numbers of nuclei. – Fast freezing, therefore, produces a large number of small ice crystals. – However, large differences in crystal size are found with similar freezing rates due to different types of food and even in similar foods that have received different pre-freezing treatments. The rate of ice crystal growth is controlled by the rate of heat transfer for the majority of the freezing curve of foods. 14 © 2021 Rolenz Cruz Freezing Rate The location, number, and size of the ice crystals formed determine the resulting texture of the frozen–thawed product. When the rate of heat removal is low, water can transfer from the interior of the cell, and the cell dehydrates, with water being incorporated into the external ice crystals. Therefore, SLOW FREEZING results in large ice crystal formation exclusively in the extracellular locations, which actually tend to squeeze the cell structures as they grow. This means that, upon thawing, they leave a product with severe textural breakdown. 15 © 2021 Rolenz Cruz Freezing Rate Besides the freezing rate, there are other factors that influence the quality of the final frozen–thawed product. – Even though we prefer fast freezing procedures to attain the formation of numerous tiny nuclei, if the storage conditions are not the proper (T fluctuations), these tiny ice crystals undergo recrystallization, and tend to merge, resulting in larger crystals with the result being that the advantage of fast freezing is lost. – Therefore, appropriate T conditions during storage are as important as the freezing process itself. 16 © 2021 Rolenz Cruz Volume Changes The volume of ice is 9% greater than that of pure water, and an expansion of foods after freezing would therefore be expected. However, the degree of expansion varies considerably owing to the following factors: 1. Moisture Content Higher MC produces greater changes in volume. 2. Cell Arrangement Plant materials have intercellular air spaces which absorb internal moisture, however increases in V are minimal – e.g. whole strawberries increase in V by 3.0% whereas coarsely ground strawberries increase by 8.2% when both are frozen to - 20°C. 17 © 2021 Rolenz Cruz Volume Changes However, the degree of expansion varies considerably owing to the following factors: 3. Solute Concentrations High concentrations reduce the FP and do not freeze 4. Crystallized Components, including ice, fats and solutes These components contract when they are cooled and this reduces the volume of the food. 18 © 2021 Rolenz Cruz Effects of Freezing on Foods The main effect of freezing on food quality is damage caused to cells by ice crystal growth. Freezing causes negligible changes to pigments, flavors or nutritionally important components, although these may be lost in preparation procedures or deteriorate later during frozen storage. – In food emulsions, they can be destabilized by freezing, and proteins are sometimes precipitated from solution, which prevents the widespread use of frozen milk. – In baked goods a high proportion of amylopectin is needed in the starch to prevent retrogradation and staling during slow freezing and frozen storage. 19 © 2021 Rolenz Cruz Effects of Freezing on Foods There are important differences in resistance to freezing damage between animal and plant tissues. Meats have a more flexible fibrous structure which separates during freezing instead of breaking, and the texture is not seriously damaged. 20 © 2021 Rolenz Cruz Effects of Freezing on Foods In fruits and vegetables, the more rigid cell structure may be damaged by ice crystals. The extent of damage depends on the size of the crystals and hence on the rate of heat transfer. However, differences in the quality of raw materials and the pre-freezing treatments both have an effect on food product quality than changes caused by correctly-operated freezing, frozen storage, and thawing procedures. 21 © 2021 Rolenz Cruz Effects of Freezing on Foods Slow Freezing Ice crystals grow in intercellular spaces and deform and rupture adjacent cell walls. Water therefore moves from the cells to the growing crystals. Cells become dehydrated and permanently damaged by the increased solute concentration and a collapsed and deformed cell structure. On thawing, cells do not regain their original shape and turgidity. The food is softened and cellular material leaks out from ruptured cells (termed ‘drip loss’). 22 © 2021 Rolenz Cruz Effects of Freezing on Foods Fast Freezing Smaller ice crystals form within both cells and intercellular spaces. There is little physical damage to cells, hence there is minimal dehydration of the cells. The texture of the food is thus retained to a greater extent. However, very high freezing rates may cause stresses within some foods that result in splitting or cracking of the tissues. 23 © 2021 Rolenz Cruz Effects of Freezing on Foods Fast Freezing Slow Freezing 24 © 2021 Rolenz Cruz Effects of Freezing on Foods 25 © 2021 Rolenz Cruz Effects of Freezing on Foods In general, the lower the temperature of frozen storage, the lower is the rate of microbiological and biochemical changes. However, freezing and frozen storage do not inactivate enzymes. The main changes to frozen foods during storage are as follows: 1. Degradation of pigments Chloroplasts and chromoplasts are broken down and chlorophyll is slowly degraded to brown pheophytin even in blanched vegetables. In fruits, changes in pH due to precipitation of salts in concentrated solutions change the color of anthocyanins. 26 © 2021 Rolenz Cruz Effects of Freezing on Foods The main changes to frozen foods during storage are as follows: 2. Loss of vitamins Water-soluble vitamins (e.g. vit. C and pantothenic acid) are lost at sub-freezing T. Vitamin C losses are highly T dependent; a 10°C increase in T causes a six-fold to twentyfold increase in the rate of vit. C degradation in vegetables and a thirtyfold to seventyfold increase in fruits. Losses of other vitamins are mainly due to drip losses, particularly in meat and fish (if the drip loss is not consumed). 27 © 2021 Rolenz Cruz Effects of Freezing on Foods The main changes to frozen foods during storage are as follows: 3. Residual enzyme activity In vegetables which are inadequately blanched or in fruits, the most important loss of quality is due to polyphenol oxidase activity which causes browning, and lipoxygenases activity which produces off-flavors and off-odors from lipids and causes degradation of carotene. Proteolytic and lipolytic activity in meats may alter the texture and flavor over long storage periods. 4. Oxidation of lipids This reaction takes place slowly at -18°C and causes off-odors and off-flavors. 28 © 2021 Rolenz Cruz Recrystallization Physical changes to ice crystals are collectively known as recrystallisation and are an important cause of quality loss in some foods. There are three types of recrystallisation in foods as follows: 1. Isomass recrystallisation. This is a change in surface shape or internal structure, usually resulting in a lower surface-area-to-volume ratio. 29 © 2021 Rolenz Cruz Recrystallization 2. Accretive recrystallisation. Two adjacent ice crystals join together to form a larger crystal and cause an overall reduction in the number of crystals in the food. 3. Migratory recrystallisation. This is an increase in the average size and a reduction in the average number of crystals, caused by the growth of larger crystals at the expense of smaller crystals. 30 © 2021 Rolenz Cruz Thawing When food is thawed in air or water, surface ice melts to form a layer of water. Water has a lower thermal conductivity and a lower thermal diffusivity than ice and the surface layer of water therefore reduces the rate at which heat is conducted to the frozen interior. This insulating effect increases as the layer of thawed food grows thicker. (In contrast, during freezing, the increase in thickness of ice causes heat transfer to accelerate.) Thawing is therefore a longer process than freezing when temperature differences and other conditions are similar. 31 © 2021 Rolenz Cruz Thawing During thawing, the initial rapid rise in T (AB) is due to the absence of a significant layer of water around the food. 32 © 2021 Rolenz Cruz Thawing There is then a long period when the T of the food is near to that of melting ice (BC). During this period any cellular damage caused by slow freezing or recrystallisation, results in the release of cell constituents to form drip losses – causes loss of water- soluble nutrients. 33 © 2021 Rolenz Cruz Thawing Drip Loss – It is the fluid loss after thawing which causes dehydration and nutrient loss of frozen foods. Slow freezing can produce drip losses of 20%. Fast freezing without surface cracking can reduce losses to approximately 5%. 34 © 2021 Rolenz Cruz Thawing In addition, drip losses form substrates for enzyme activity and microbial growth. – Microbial contamination of foods, caused by inadequate cleaning or blanching has a pronounced effect during this period. 35 © 2021 Rolenz Cruz Thawing Some foods are cooked immediately and are therefore heated rapidly to a temperature which is sufficient to destroy microorganisms. Others (e.g. ice cream, cream and frozen cakes) are not cooked and should therefore be consumed within a short time of thawing. When food is thawed by microwave heaters, heat is generated within the food, and the changes described do not take place. The main considerations in thawing are: – to avoid overheating – to minimize thawing times – to avoid excessive dehydration of the food. 36 © 2021 Rolenz Cruz Overview Food Freezing We were able to explain the principles of food freezing, including the process and its effects on food quality, such as texture, flavor, and nutritional value. Principles of Freezing Freezing Process Ice Crystal Formation Freezing Rate Volume Changes Effects of Freezing on Foods Effects of Frozen Storage Recrystallization & Thawing 37 © 2021 Rolenz Cruz That’s all. Thank you for listening! __ ROLENZ DERICK R. CRUZ Food Technology Department College of Industrial Technology Bicol University 38 © 2021 Rolenz Cruz