Dairy and Food Engineering 3(2+1)

Lesson 19. Deterioration in Food Products and Physical deterioration.

Definitions:

Food Deterioration: A series of continuous degradative changes occurring in a food item which may affect the product's wholesomeness, result in a reduction of its quality, and/or alter its serviceability.

Wholesomeness: Wholesomeness is a term that refers to freedom from pathogenic or otherwise harmful microorganisms.

or

A characteristic possessed by a food product that is conducive to good health and well being of the consumer.

Spoilage:  Spoilage is a term which we often hear in conjunction with deterioration. It is often used as a synonym for deterioration. However, it needs to make distinction between these two terms. We define food spoilage as an arbitrary end point of the deterioration process which denotes that a food item is unwholesome and, therefore, is no longer suitable for human consumption.

(1) Spoilage is a benchmark--a signal to denote that a food item is unwholesome and is no longer suitable for human consumption.

(2) It can occur anywhere during the deterioration process. The point of spoilage depends upon such factors as type of food (milk vs beef), storage environment (low-temperature storage vs. High-temperature), and method of preservation (canning vs. Freeze dehydration).

Unwholesome: Unwholesome food is food procured, packed, or held under unsanitary conditions that renders it injurious to the health of the consumer, or food or food containers having naturally occurring or added harmful substances, or food found to be filthy, putrid, decomposed, or produced from a diseased animal or an animal that died other than by slaughter.

Off-condition: Off-condition is any variation from the expected appearance, feel, smell, or taste characteristics of a product when it was initially produced or processed for resale. (A product is considered unwholesome if any off condition affects it in such a way that the product may be injurious to the health of the consumer.)

Quality: Quality is a term that refers to the degree of excellence or grade of a Product.

Serviceability: Serviceability is a term that refers to the usefulness of a food item. Reduced serviceability in a product may result in the use of additional processing methods to return the food item to its original state.

 Food Deterioration

Major causes of food deterioration include:

1. Growth and activities of micro-organisms, mainly bacteria, yeasts and moulds;

2. Activities of natural food enzymes;

3. Insects, parasites and rodents;

4. Temperature, both heat and cold;

5. Moisture and dryness;

6. Air and in particular oxygen;

7. Light;

8. Time.

These causes are not isolated in nature. High temperature, moisture, and air will all affect the multiplication and activities of bacteria, as well as the chemical and enzymatic activities of the food. Bacteria, insects, and light can all be operating simultaneously to deteriorate food in the field or in a warehouse.

At any one time, many forms of deterioration may be in progress, depending upon the food and environmental conditions.

Preservation techniques are designed to counteract or slow the changes which cause deterioration by:

Physical Deterioration

Chemical Degradation

Biological Changes

19.1 Physical Deterioration

19.1.1 Moisture absorption

A physical change that causes food deterioration is excessive moisture. The gross changes in foods from excessive moisture are part of everyday experience. Dried, dehydrated, and freeze-dried foods are especially susceptible to this form of deterioration. These types of food are very hygroscopic (readily taking up and retaining moisture)

One of the major undesirable physical changes in food powders is caking due to absorption of moisture as a consequence of an inadequate barrier provided by the package. It may occur due to poor selection of packaging material or failure of the package integrity during storage. This may lead to other forms of deterioration, such as bacterial growth and chemical reactions such as oxidation, as well as surface defects like mottling, crystallization, and stickiness.

In a moisture-proof package, food materials such as fruits and vegetables can give off moisture from respiration and transpiration, which gets trapped within the package and supports the growth of microorganisms.

Anti-caking agents can be added to powders in order to inhibit caking and improve flowability. Studies in onion powders showed that at ambient temperature, caking does not occur at water activities of less than about 0.4. At higher activities, however, (a_w> 0.45) the anti-caking agents are completely ineffective. It appears that while they reduce inter-particle attraction and interfere with the continuity of liquid bridges, they are unable to cover moisture sorption sites.

19.1.2 Temperature

19.1.2.1. Low Temperature

(A) Freezing of many foods will cause undesirable changes, such as the destruction of emulsions and texture.

 Emulsified products, such as salad dressing and mustard, contain a fat/oil and water mixture which does not combine without special processing or additives. If these types of products are frozen, the emulsion will be destroyed and the fat and water will separate into distinct layers.

 Fruits and vegetables that are allowed to freeze and then thaw will have their texture disrupted. Skin’s cracks leave the food susceptible to attack by microorganisms. The texture of canned fruits and vegetables becomes softened and mushy due to uncontrolled freezing, also called as chilling injury.

(B)  Many fruits and vegetables, like other living systems, have optimum temperature requirements after harvest. Several fruits and vegetables when held at common refrigeration temperatures of about 41ºF (5ºC) are weakened or killed and various deteriorative processes like off-color development, surface pitting, and various forms of decay occur.

Bananas, lemons, squash, and tomatoes are some products that need to be held at temperatures no lower than 50ºF (10ºC) for maximum quality retention. This provides an exception to the inaccurate generalization that cold storage preserves all foods, and the colder the better.

19.1.2.2. High Temperature

There is a moderate temperature range over which much of the food is handled, i.e 50º-100ºF (10º-38ºC)Within this range, for every 18ºF (10ºC) rise in temperature, the rate of chemical reaction is approximately doubled, in terms of many enzymatic as well as non enzymatic reactions. Excessive heat treatment results into denaturation of proteins, breakage of emulsions, drying out of foods by moisture removal, and destruction of vitamins.

(A)  Excessive heat treatment in green vegetables causes loss of integrity of cell walls and membranes with release of acids and enzymes. This results in the softening of texture as well as the development of off-colors and off-flavors.

(B) The consequences of excessive heat on muscle tissue leads to denaturation and clumping of proteins, and enzyme inactivation. This results in a toughening of the texture, loss of water holding capacity, cooked or caramel flavors, and development of off-colors.

 19.1.3. Dehydration

Dehydration, is another form of physical change that causes food deterioration. It can be defined as the loss of water from the food product.

Foods, especially fresh, chilled, and frozen, are subject to dehydration. Foods contain a substantial amount of water. Meat products contain 70 to 75 % water, whereas fresh fruits and vegetables contain from 80 to 95 % water. Since water vapor is continually seeking to go from an area of high concentration to an area of low concentration, improper storage conditions and improper packaging will lead to dehydration. Dehydration results when the humidity is too low in a storage area.

The signs of dehydration include dryness or shriveling on the surface of the food item, development of off-colors, with usually a darkening effect.

In frozen foods, the dehydration is known as freezer burn, whereas in fresh fruits and vegetables, it is known as wilt.

19.1.4. Mechanical Damage

When any food item receives mechanical damage, not only is the appearance of the item affected but the damaged food tissue also becomes more susceptible to other forms of deterioration. Such foods become more susceptible to invasion by microorganisms, for the damaged area serves as a port of entry. The cell walls of foods are also destroyed by mechanical abrasion, and the inherent enzymes in the food product are liberated from the cells. Once liberated, the enzymes begin the process of deterioration or, more specifically, autolysis. The notable changes would be a softening in the texture, development of off-colors, and development of off-flavors.

19.1.4.1 Freezer Burn

Freezer burn is a deteriorative condition commonly found in frozen foods, especially poultry, water foods, and red meats. This condition is a physical change in the food item that results in no loss of wholesomeness.

Abnormal Characteristics

The abnormal characteristics that are exhibited by a freezer-burned product include an abnormal color, usually white or pale amber. The surface of the product will be dry and shriveled, and usually a buildup of frost on the surface of the product, the texture similar to that of a dry sponge.

Cause of Freezer Burn

Freezer burn is caused by the evaporation (sublimation) of moisture from the surface of the food item which results in the product having a bleached, unattractive appearance and adverse effects on the palatability.

(A) Dehydration

In a freezer, the relative humidity is low because the cold dense air is not capable of holding much moisture. The moisture content of the food may vary from 65 to 90%, depending upon the type of food. This uneven concentration of moisture makes frozen foods very susceptible to moisture loss, hence the products which are improperly wrapped and packaged will be susceptible to dehydration resulting in a freezer burn.

(B) Colour change

The frost formed on the outside of the product leads to excessive loss of moisture hence resulting in a colour change in the product. The small holes formed on the product are more susceptible to the oxidative process; there is an increased area of exposure to oxygen.

Control of Freezer Burn

Freezer burn may be prevented by a skintight covering or an ice glaze for the food product, which may be obtained by the use of water impermeable films, dip coatings, or spray coatings.

19.1.5. Light

Light, causes food deterioration, by fading of color in many food items. Some vitamins are destroyed by light, notably riboflavin, vitamin A, and vitamin C. Milk in bottles exposed to the sun develops "sunlight" flavor due to light induced fat oxidation and changes in the protein.

Sensitive foods often can be protected from light by impervious packaging or by incorporating compounds into glass and transparent films that screen out specific wavelengths.

One of the most common problem due to light is the greening of potatoes. This condition is also referred to as sunburn. It is the result of the exposure of the potatoes to sunlight during growth or after digging, and to artificial light during display for sale. After exposing for two days or longer to either natural or artificial light, a green pigment develops. The skin and the flesh are affected. Chlorophyll and an alkaloid called solanine are produced due to exposure to light. Chlorophyll is tasteless and harmless, whereas the green tubers acquire a bitter, pungent taste due to solanine and if eaten in quantity, they may be poisonous. 

19.1.6. Time

Another major cause of food deterioration is that of time or the aging process. After slaughter, harvest, or food manufacture, there is a time when the quality of food is at its peak, but this is only a transitory period. The growth of microorganisms, destruction by insects, action of food enzymes, non-enzymatic interaction of food constituents, loss of flavor, effects of heat, cold, moisture, oxygen, and light, all progress with time. The longer the time, the greater the destructive influences.

It is true that certain cheeses, sausages, wines, and other fermented foods are improved with aging up to a point. However, for the vast majority of foods, quality decreases with time, and the major goal of food handling and preservation practices is to capture and maintain freshness. The storage life of foods is determined to a great extent by type of food, method of processing, method of packaging, and storage environment. This is the rationale behind the frequencies of cyclic inspections of food items.