Principle of cold preservation n Hindering the growth or activity of microorganisms n Delay of enzymatic reactions n Delay of purely chemical reactions
Distinctions between refrigeration and freezing n Temperature: refrigeration (16 C to -2 C); freezing - generally -18 C or below; n storage time: refrigeration (days or weeks); freezing (months or years) n microbial activity. Below -9.5C no significant growth.
Effect of temperature on growth n Most spoilage organisms grow rapidly above 10C n Some poisoning organisms can grow slowly at 3.3 C n Psychotropic organisms can grow slowly (4.5 to -9.5 C). n Freezing does not destroy organisms completely. When food is thawed, there can be rapid multiplication and spoilage
Low temperature growth of some food borne pathogens ( C) n Bacillus cereus 7 n Clostridium botulinum 3.3 n Escherichia coli 4 n Listeria monocytogenes 3 n Salmonella 5.2 n Staphylococcus aureus 10 n Vibrio parachemolyticus 5
The high quality life (HQL), defined as the time of storage of the initially high quality product to the moment, when the first statistically significant (p<0.01 difference in quality appears. The practical storage life (PSL), defined as the period of storage, during which the product retains its characteristic properties and suitability for human consumption or intended process.
Changes in foods during cool storage are influenced by: n The growing conditions n Varieties of plants n Feeding practices of animals n Conditions of harvest and slaughter n Sanitation n Damage to tissues n Temperature of cool storage n Mixture of food in cool storage
Examples of changes in food n Too low temperature brings about a cold damage to fruits and vegetables i.e. weakened physiological state decrease resistance to microbial spoilage; delay in ripening; n loss in vitamins (C) n changes in carbohydrate composition
Benefits of cool storage other than preservation n Control the rates of chemical and enzymatic reactions n Control the rates of growth and metabolism of desirable microorganisms
Examples of benefits from cooling: n Aging of wines n Aging of meat n Ripening of cheese n Improvement in peeling and pitting of peaches n Reduction in changes in flavor during extraction and straining of citrus juices n Improvement in meat and bread cutting n Increase the solubility of CO 2 n precipitates waxes from edible oils
High-quality shelf life (months) of frozen foods at -12 C and -18 C n apple pie 9 12 n beef 2 6 n bacon 1 3 n bread 6 24 n chicken (raw) 7 14 n chicken (fried) 1 3 n fish (fatty) 1 week 2 n pork sausage 1 4
Undesirable Changes 1. Chemical reactions can occur in unfrozen water. A.Some foods blanched or sulfited before freezing. B.Vacuum packaging to keep out oxygen.
Undesirable Changes 2.Undesirable physical changes A.Fruits and vegetables lose crispness B.Drip loss in meats and colloidal type foods (starch, emulsions) Freeze product faster Control temperature fluctuations in storage. Modify starch, egg systems, etc.
Examples of changes in foods n exchange of flavors between foods n loss of firmness and crispiness n changes in color of red meat n oxidation of fats n staling of baked goods n softening of tissue and drippage from fish n lumping and caking of granular foods
Undesirable changes C.Freezer burn Package properly Control temperature fluctuations in storage. D.Oxidation Off-flavors Vitamin loss Browning E.Recrystallization
Freezing n Freezing curve for pure water n Freezing curve for frozen at moderately low temperature
Factors influencing the selection of temperature for frozen storage: n Textural changes n Enzymatic and no enzymatic reaction n Microbial changes n Costs
Freezing n Concentrations Effects n Ice crystal damage
Concentrations Effects n Precipitation of solute out of solution- gritty,sandy texture. n Solutes that do not precipitate will form concentrated solution- protein denaturation b/c of salting out effects. n Drop of pH below isoelectric point - coagulation of proteins. n Supersaturation of gases.
Ice crystal damage n Solid foods are of cellular structure n water is within and between the cells n When water freezes rapidly it forms minute ice crystals n When water freezes slowly it forms large crystals and clusters of crystals n Large crystals cause damage to cellular foods; disrupt emulsions, foams, gels.
Autoxidation of fats during frozen storage is affected by: n the degree of fatty acid unsaturation n the prefreezing holding time n duration and the temperature of frozen storage n the oxygen content of the enviroment n the presence of proxidants and antioxidants
Commercial Freezing Methods 1. Air Freezing -still-air sharp freezer, blast freezer, fluidized-bed freezer. Foods frozen by either "still" or "blast" forced air. cheapest "still" slowest more product changes "blast" faster, more commonly used. 2.Indirect Contact Freezing: single plate, double plate, pressure plate, slush freezer. Food placed in direct contact with cooled metal surface. relatively faster more expensive
Effect of Thawing on the Microbiology n In air at 27C 23 hrs 1000% n In air at 21C 36hrs 750% n In air at 7.2 C 63 hrs 225% n In running water, 16C 15hrs 250% n In running water, 21 C 12hrs 300% n Agitated water, 16C 9hrs 40% n Dielectric heat 15min negligible