Effect of Preservation Temperatures

Heating

Most preservation methods involve a heat treatment.  Canning (ranges from mild to severe heat treatments) is the final step in preserving high acid foods, low acid foods, pickles, and jams and jellies.  Blanching, which is a mild heat treatment, is used to prepare foods for freezing and drying.  Heating affects the overall flavor and texture of the food.  Heating also increases nutrient loss and kills microorganisms.

Flavor and texture.  Heating changes both the flavor and texture of foods.  The degree of change is related to how sensitive the food is to heat treatment.  Delicate foods, such as berries, tend to be more adversely affected than is meat.

High-temperature, short time exposures to heat are less destructive on flavor and texture than are high or low temperature, long-time processes.  Pasteurization and blanching are examples of heat treatments that have a minimal effect on flavor and texture.  Canning has a significant effect on these characteristics because it is a high-temperature, long time process.

Appearance.  Heating also affects the appearance of foods.  Sugars and starches undergo browning when heated to high temperatures.  An example of a browning reaction is Maillard browning (as opposed to the browning that occurs when an apple slice is exposed to air, which is enzymatic browning).  Maillard browning, which is non-enzymatic browning, results in desirable color, flavor, odor, and sometimes texture changes.  It occurs primarily during the roasting, baking, grilling, and frying of some foods.   The brown-colored compounds that are formed are characteristic of bread crust, potatoes, baked cakes, biscuits, and caramelized candy.

Nutrient Loss.  The effect of heating on nutrient content depends on the sensitivity of the nutrient to the various conditions during the process, such as heat, oxygen, pH, and light. 

Effect of Processing on Nutrients in Foods

Nutrient	Effect of Processing

Read Also

Fat

Oxidation accelerated by light

Protein

Denatured by heat (improves digestion)

Vitamin C (ascorbic acid)

Decreases during storage, drying, heating, oxidation, cell damage (chopping and slicing)

Losses due to oxidation catalyzed by copper and iron

Stable to heat under acidic conditions (canning tomatoes)

Vitamin B₁ (Thiamine)

Destroyed by high temperatures, neutral and alkaline conditions

Lost in cooking water

Vitamin B₂ (Riboflavin)

Sensitive to light at neutral and alkaline conditions

Moderately heat stable under neutral conditions

Sensitive to heat under alkaline conditions

Vitamin B₃ (Niacin)

The most stable vitamin -- stable to heat and light

Lost in cooking water

Folate

Decreases with storage or prolonged heating

Lost in cooking water

Destroyed by use of copper utensils

Vitamin B₆ (Pyridoxine)

Heat stable in alkaline and acidic conditions

Pyridoxal is heat labile

Vitamin B₁₂

Destroyed by light and high pH

Carotenes

Easily destroyed by heat

Oxidizes and isomerizes when exposed to heat and light

Vitamin A

Very heat labile – esasily destroyed by heat; easily oxidized

Vitamin D

Oxidizes when exposed to heat and light

Vitamin E

Oxidizes readily

SOURCE:  Morris, A., A. Barnett, and O. Burrows.  2004.  Effects of Processing on Nutrient Content of Foods.   Available at:  www.paho.org/English/CFNI/cfni-caj37No304-art-3.pdf

Freezing

Protein. While there is little change in the nutritive value of protein after freezing, protein is denatured.  Denatured protein is undesirable because it results in curdled proteinaceous materials.  This is especially problematic during repeated freezing and thawing, which results in mushy foods and increased drip or water loss.

Fats.  Deterioration of fats and oils occurs in frozen foods over time due to oxidation.  The greater the amount of unsaturated fat in a food, the more the potential for oxidative rancidity of the fat.  Fats in frozen fish tissue tend to become rancid more quickly than the fats in other frozen animal tissue because fish fat is more highly unsaturated.   In the case of meats, pork fat becomes rancid after six months storage at 0^oF, while beef fat retains good quality after two years of storage at that temperature.  Plant tissues are least susceptible because they have the least amount of fat.                

Vitamins.  Freezing does not destroy vitamins.  In fact, the lower the food temperature, the better the retention of nutrients.  However, frozen foods undergo some processing prior to freezing.  It is during this processing that vitamin losses occur.  This happens, for example, during washing or soaking, blanching, trimming, and grinding.  Exposure of tissues to air results in vitamin losses due to oxidation. 

For example, vitamin C losses occur when tissues are ruptured and exposed to air.  During storage in the frozen state, vitamin C losses continue.  The higher the storage temperature, the greater the loss of vitamin C.  Greater losses of vitamin C occur with frozen foods than with any other vitamins. 

Thiamin is heat-sensitive.  Some is destroyed during blanching.  Further losses occur during freezing.  Riboflavin in frozen foods decreases during preparation for freezing, but little or no destruction of thiamin occurs during frozen storage.

Of the fat-soluble vitamins, freezing alters the carotenoids little, although some loss occurs during storage.  Blanching of plant tissues improves the storage stability of carotenoids.  Not packing frozen foods (in moisture-vapor-resistant packaging) leads to oxidation and destruction of fat-soluble vitamins (A and E) as well as vitamin C.