Scientific Emotions of Freezing

Introduction

Food preservation is defined as the process of addressing and dealing with food in a way that maintains its taste and preserves its nutritive value as long as possible via stopping or slowing spoilage with the aim of preventing food borne illnesses. The process of food preservation include inhibiting the growth of bacteria, fungi and other microorganisms besides slowing down the process of oxidation of fats, which causes food rancidity. The most common food preservation methods include drying, freezing, vacuum packing, canning, preservation in syrup, sugar crystallization, salting, irradiation, addition of preservatives and inert gases.

Food Spoilage

There are plenty of factors that cause food deterioration and spoilage. Chief among these are 

Microorganisms, which consist of bacteria, yeasts and fungi (molds.)

Enzymes, which occur naturally in plants, will cause foods to deteriorate in time resulting in the loss of nutritional value, flavor and palatability.

Exposure to light will cause the destruction of some vitamins, and the rate of all chemical and biological reactions, including the actions of enzymes and the growth of microbes, will increase as the temperature increases.

So all preserved foods will keep better and longer when protected from light and stored at cooler temperatures.

For any type of food preservation method, the retention of nutritional components is a concern, but Freezing is probably the least destructive when properly done.

Freezing

Freeing is method of food preservation whereby:

• The heat is removed
• Temperature of the food is reduced below its freezing point
• And a portion of water in food undergoes a change in state to form ice crystals (aw lowered)

In freezing preservation is achieved by:

• Low temperature
• Reduced water activity due to ice formation and high concentration of solutes in unfrozen water
• Blanching of some foods

Chemical Effects of Freezing

Concentration of chemicals in liquid phase

Increased acidity

Low pH- Protein denaturation

Effect more pronounced during storage and slow freezing

Types of changes:

• Flavour and odor deterioration
• Pigment degradation
• Enzymatic brownng
• Auto oxidation of ascorbic acid
• Protein insolubilization
• Lipid oxidation

The above changes are dependant on initial substrate concentration, pH,Aw,O, handling and processing, and time and temperature.

Prevetion of Chemical Changes can be done by:

Inactivation of enzymes

Low temperature storage

Alternation of pH

Exclusion of oxygen

Freezing slows down, but does not destroy, enzymes in fruits and vegetables. That is why it is important to stop enzyme activity before freezing. The two methods you can use are blanching and adding chemical compounds such as ascorbic acid.

(Enzymes are proteins present in plants and animals. While the plant or animal lives, enzymes help speed up the ripening and maturing processes. Even after we harvest plants or slaughter animals, enzyme reactions can continue and result in undesirable color, flavor and texture changes in the food).

Enzyme activity does not harm frozen meats or fish and is neutralized by the acids in frozen fruits

(Plant products contain enzymes which give off ethylene gas, so exposure to anaerobic conditions (e.g. freezer storage) can lead to off colours and flavours. That's why pre-frozen fruits and vegetables have generally been blanched to kill the enzymes)

Fats in meat, fish and poultry become rancid during freezer storage. This is caused by contact with air left in the package or air that enters the package because proper storage materials were not used. Control rancidity by trimming excess fat from meat before freezing, using a wrapping material that prevents air from reaching the product, and by storing foods for the recommended length of time.

Slow Freezing & Rapid Freezing

Slow freezing causes a larger loss of drip on thawing, a larger loss of nitrogenous constituents and nucleic acid derivatives to the drip, and a larger loss of water-holding capacity of meat, than fast freezing.

In addition, slow freezing, as compared to fast freezing, increases protein breakdown. Rapid freezing preserves the integrity of muscle proteins to a greater extent than slow freezing.

Rapid freezing prevents undesirable large ice crystals from forming throughout the product because the molecules don't have time to form into the characteristic six-sided snowflake. Slow freezing creates large, disruptive ice crystals. During thawing, they damage the cells and dissolve emulsions. This causes meat to "drip" and lose juiciness. Ideally, food 2-inches thick should freeze completely in about 2 hours

Slow Freezing

• Rates of cooling is less than 1 degree C/ minute
• Ice crystal form in extracellular locations
• Large ice crystal formation
• Maximum dislocation of water
• Shrinkage
• Less than maximum attainable food quality

Rapid Freezing

• Produces both extracellular and intracellular (mostly) locations of ice crystals
• Small ice crystals
• Numerous ice crystals
• Minimum dislocation of ice crystals
• Frozen appearance similar to the unfrozen state
• Food qulity usually superior to that attained by slow freezing.

The location of ice crystal in tissue is dependant on:

• Freezing rate

Slow

Rapid

• Specimen temperature
• Nature of the cell

Biological Effects of Freezing:

• Growth of mos is temperature dependant
• No pathogens can grow around 5 degree C
• No mos growth <-5degree C

Freezing cannot kill pathogens if food is already contaminated. However, some mos are killed, some are injured and some are stable.

Controlling initial microbial load inaddition to rapid freezing, and storage at -18 degree C (constant) will reduce the detrimental effects of microorganisms.Frozen food should be thawed rapidly (low temperature) and to be used immedialtely or store at 5 degree C or cook for safety.

Quality Concerns in frozen foods includes:

• Freezer Burn
• Recrystallization
• Drip loss
• Loss of functionality
• Chemical reactions
• Color changes.

The bright red color of meat as purchased usually turns dark or pale brown depending on its variety. This may be due to lack of oxygen, freezer burn or abnormally long storage.

Freezing doesn't usually cause color changes in poultry. However, the bones and the meat near them can become dark. Bone darkening results when pigment seeps through the porous bones of young poultry into the surrounding tissues when the poultry meat is frozen and thawed.

The dulling of color in frozen vegetables and cooked foods is usually the result of excessive drying due to improper packaging or over-lengthy storage.

Best Practices

All foods have a limited shelf-life, no matter how they are stored or preserved. It is imperative that you have a plan for rotating your food stash to keep it fresh.

Proper packaging helps maintain quality and prevent freezer burn.  Freeze unopened vacuum packages as is. If a package has accidentally been torn or has opened while food is in the freezer, the food is still safe to use; merely overwrap or rewrap it. Freezer burn does not make food unsafe, merely dry in spots. It appears as grayish-brown leathery spots and is caused by air coming in contact with the surface of the food. Cut freezer-burned portions away either before or after cooking the food. Heavily freezer-burned foods may have to be discarded for quality reasons.

Never stack packages to be frozen. Instead, spread them out in one layer on various shelves, stacking them only after frozen solid.

For best results, freeze foods at -18 degrees Celsius or lower as soon as they are packaged and sealed. Set the temperature control at -10 degrees Fahrenheit or lower about 24 hours in advance to help food freeze rapidly. Do not overload your freezer with unfrozen food. Add only the amount that will freeze within 24 hours.

Place packages in contact with refrigerated surfaces in the coldest part of the freezer. Leave a little space between packages so that air can circulate freely. When the food is frozen, packages can be restacked close together.