4Principle of cold preservation Hindering the growth or activity of microorganismsDelay of enzymatic reactionsDelay of purely chemical reactions
5Distinctions between refrigeration and freezing Temperature: refrigeration (16 C to C); freezing - generally -18 C or below;storage time: refrigeration (days or weeks); freezing (months or years)microbial activity. Below -9.5C no significant growth.
7Effect of temperature on growth Most spoilage organisms grow rapidly above 10CSome poisoning organisms can grow slowly at 3.3 CPsychotropic organisms can grow slowly (4.5 to -9.5 C).Freezing does not destroy organisms completely. When food is thawed, there can be rapid multiplication and spoilage
9Low temperature growth of some food borne pathogens ( C) Bacillus cereusClostridium botulinumEscherichia coliListeria monocytogenesSalmonellaStaphylococcus aureusVibrio parachemolyticus
10The 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.
15Changes in foods during cool storage are influenced by: The growing conditionsVarieties of plantsFeeding practices of animalsConditions of harvest and slaughterSanitationDamage to tissuesTemperature of cool storageMixture of food in cool storage
16Examples of changes in food 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;loss in vitamins (C)changes in carbohydrate composition
18Benefits of cool storage other than preservation Control the rates of chemical and enzymatic reactionsControl the rates of growth and metabolism of desirable microorganisms
19Examples of benefits from cooling: Aging of winesAging of meatRipening of cheeseImprovement in peeling and pitting of peachesReduction in changes in flavor during extraction and straining of citrus juicesImprovement in meat and bread cuttingIncrease the solubility of CO2precipitates waxes from edible oils
21High-quality shelf life (months) of frozen foods at -12 C and -18 C apple piebeefbaconbreadchicken (raw)chicken (fried)fish (fatty) weekpork sausage
22Undesirable Changes1. Chemical reactions can occur in unfrozen water.Some foods blanched or sulfited before freezing.Vacuum packaging to keep out oxygen.
232. Undesirable physical changes Fruits and vegetables lose crispness Undesirable Changes2. Undesirable physical changesFruits and vegetables lose crispnessDrip loss in meats and colloidal type foods (starch, emulsions)Freeze product fasterControl temperature fluctuations in storage.Modify starch, egg systems, etc.
24Examples of changes in foods exchange of flavors between foodsloss of firmness and crispinesschanges in color of red meatoxidation of fatsstaling of baked goodssoftening of tissue and drippage from fishlumping and caking of granular foods
25Undesirable changes C. Freezer burn D. Oxidation E. Recrystallization Package properlyControl temperature fluctuations in storage.D. OxidationOff-flavorsVitamin lossBrowningE. Recrystallization
26Freezing Freezing curve for pure water Freezing curve for frozen at moderately low temperature
30Concentrations Effects Precipitation of solute out of solution- gritty ,sandy texture.Solutes that do not precipitate will form concentrated solution- protein denaturation b/c of salting out effects.Drop of pH below isoelectric point -coagulation of proteins.Supersaturation of gases.
31Ice crystal damage Solid foods are of cellular structure water is within and between the cellsWhen water freezes rapidly it forms minute ice crystalsWhen water freezes slowly it forms large crystals and clusters of crystalsLarge crystals cause damage to cellular foods; disrupt emulsions, foams, gels.
32Autoxidation of fats during frozen storage is affected by: the degree of fatty acid unsaturationthe prefreezing holding timeduration and the temperature of frozen storagethe oxygen content of the enviromentthe presence of proxidants and antioxidants
33Commercial 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
40Effect of Thawing on the Microbiology In air at 27C hrs %In air at 21C hrs %In air at 7.2 C hrs %In running water, 16C 15hrs %In running water, 21 C 12hrs %Agitated water, 16C hrs %Dielectric heat min negligible