III Shelf Life of Dairy Products
Technological Options to Prolong Shelf Life
Freezing of Dairy Products
Sebnem Tavman1 and Tuncay Yilmaz2
- 1 Ege University, Engineering Faculty, Food Engineering Department, Izmir, Turkey
- 2 Manisa Celal Bayar University Engineering Faculty, Food Engineering Department, Manisa, Turkey
One of the best ways of shelf-life extension is freezing for all kind of food products and additives. It provides significantly long-term shelf life and easy to apply for every kind of foods. Although new preservation techniques such as high-pressure, infrared irradiation, pulsed electric field, and ultrasound, are increasingly popular, freezing method is also a generally preferred method regarding its simplicity and conventionally applicability.
However, freezing also affects the physical properties of components because of the conversion of water in it. This phenomenon occurs during energy removal of the product from its initial temperature through below freezing temperature. General application for freezing requires temperature decrease up to storage level around -18°C. Cooling curves and phase diagrams give information about precooling, supercooling, freezing, tempering, eutectic, ice nucleation, and glass transition points of the products. All procedures and technologies obey and can be corrected due to explained phase points of the products.
The other crucial point of freezing is microbiological and chemical concern. Freezing inhibits or slows down reactions covering physicochemical and biochemical reactions in food matrix but does not stops and remove them permanently. Thus, during storage, some of the deteriorative activities continue.
Storage temperature, time, and thawing procedures are all quality factors for preventing quality loss of frozen products. Research indicates that although microbial activities stop under -18°C, other reactions continue, such as enzymatic and nonenzymatic changes, even though motion of molecules is inhibited due to freezing of water molecules (Kennedy, 2000; Rahman, 2007; Sun, 2012) .
Freezing process means removal of both sensible and latent heat of the products. During freezing, three phases—precooling/chilling, phase change, subcooling/temper- ing—are observed. In precooling, only sensible heat is removed and the temperature of the product is diminished to facilitate ice crystallization of free water. Then phase change is observed and latent heat of fusion is removed. The phase change step is
Advances in Dairy Products, First Edition.
Edited by Francesco Conto, Matteo A. Del Nobile, Michele Faccia, Angelo V. Zambrini, and Amalia Conte. © 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd.
crucial due to formation of ice crystals so as to get high-quality frozen foods. After water is converted to ice, then subcooling starts. Having polymers such as fat, protein, and carbohydrates makes food freezing more complicated, and the freezing curve is different compared to pure substances.
During and after freezing, temperature has important effects on stability of frozen foods. These effects can be grouped as follows: normal stability, which slows reactions; neutral stability, which has no effect; and reversed stability, which eases reactions. Concentration during freezing creates unfrozen intact water that might change such properties as ionic strength, pH, water activity especially change in oxidation reduction potential is crucial for shelf life. Oxygen removal from ice crystals also problematic and it was noted that during freezing some damages occur, such as osmotic, solute induced, and structural, regardless of aqueous system in food.
For slow cooling, crystallization occurs in the outer cells slowly, and if time is sufficient, water moves due to osmotic pressure. Slow freezing is responsible for cell shrinkage and membrane damage. Additionally, this water may not turn back after thawing because of cell structure damage.
Membrane damage causes internal cell materials (potassium ion, galaktosidase, low molecular solutes, amino acids, RMA, DNA) leakage, which results in cell death. Osmotic dehydration is the other type of damage. The occurrence of repulsive forces gives tendency to large anisotropic stresses in the membranes, concluding in deformation, phase separation, and formation of a nonlamellar phase. Furthermore, salt addition and lowered pH also play a role in the complex nature of freeze injury and cell death. On the other hand, regardless of freezing speed, salt accumulation in unfrozen water may kill the cell. To baffle or prohibit cell damage due to salt concentration, cryo- protectants such as sugar are added to the water phase.
The quality of the frozen food can be improved by controlling freezing process, as well as careful pre- and post-freezing storage and preparation. Freezing rate is vital, of course. In general, fast freezing is preferable to slower freezing. Especially for plant material, freezing speed is important compared to other food components such as meat and dairy products. Note, however, that some products will crack or be damaged if they exposed directly to extremely low temperature for a long time.
Freeze cracking damage can be explained by volume expansion and contraction expansion. Volume expansion occurs during ice formation, and the amount of empty space in microstructure is the major factors. During rapid cooling, nonuniform contraction causes internal stress. And due to fast freezing, crust formation occurs at the surface, which is capable of cracking. It was found that size, moisture content, density, elasticity modulus, Poisson's ratio, and porosity are the major factors for freeze cracking. Although very high freezing speed may totally damage the food product, no single property can be expressed for cause of crack (Rahman, 2007; Sun, 2012).
In current applications, fast freezing is chosen for process cost and space. Also, fast freezing is invariably lethal for live organisms such as yeast, depending on producing intracellular ice crystals (Rahman, 2007). This lethality may be desired or not, depending on the type of food material. Care must be taken for foods such as culture-added ones. Freezing decreases the viable microbe population by 10% to 60%, which can be increased gradually in storage period. Fragility of microorganisms differs considerably, but actually population diminishes in freezing and then increases exponentially during the thawing period. Although the maximum advised storage for preventing microbial spoilage is between -9oC and -12oC, enzymes can cause deterioration.
In general, Gram-negatives are more sensitive to frozen death than Gram-positives. Nonsporulating rods and sphericals are resistant, while Clostridium and Bacillus remain stable by freezing. Stationary-phase bacteria are more stable than growing- phase bacteria.
Water is everything in the freezing process and has different phases during and at the end of freezing. Broadly, it divides into two groups, called free (freezable) water and bound (unfreezable) water. Unfreezable water keeps its liquid phase at very low temperature. The biggest problem and product damage comes from that unfreezable water in frozen food matrix, and it is responsible for enzymatic deteriorative reactions during storage. The concept of glassy state should be applied to ensure frozen food stability. In the glassy state, molecular movement is reduced as much as possible. In general, unfreezable water molecules are mobilized by solutes, and this immobilized amount of water can be estimated for different foods by mathematical calculations and experimentally.