Dairy and Food Engineering

Lesson 21. Control of Deteriorations in Food Products

21.2Control by Freezing

Many foods are preserved by freezing. However, freezing does not destroy most enzymes. Many frozen foods can deteriorate enzymatically, even though the rates of the reactions may be slow. Vegetables are the worst offenders, hence in order to preserve peas, green beans, corn, and so forth, by freezing, it is first necessary to heat them briefly to almost 100ºC (212ºF) before they are frozen to prevent deterioration in frozen storage. Cold storage can be combined with storage in an environment with added of carbon dioxide, sulphur dioxide, etc. according to the nature of product to be preserved.

21.3 Control Techniques for Fruit

Generally, fruits do not require heat treatment such as blanching, as many of them are adversely altered in flavor when heated. However, enzymatic darkening often occurs in frozen fruits such as sliced peaches. To counteract this, they are often packed with sugar syrups containing ascorbic acid or similar oxidation inhibitors (antioxidants). Along with proper packaging, this diminishes greatly the amount of atmospheric oxygen reaching the fruit, which is necessary for the darkening reactions.

Once fruit is harvested, any natural resistance to the action of spoiling micro-organisms is lost. Changes in enzymatic systems of the fruit also occur on harvest which may also accelerate the activity of spoilage organisms.

        Means that are commonly used to prevent spoilage of fruits must include:

• Care to prevent cutting or bruising of the fruit during picking or handling;

• Refrigeration to minimize growth of micro-organisms and reduce enzyme activity;

• Packaging or storage to control respiration rate and ripening;

• Use of preservatives to kill micro-organisms on the fruit.

A principal economic loss occurring during transportation and/or storage of produce such as fresh fruit is the degradation which occurs between the field and the ultimate destination due to the effect of respiration. Methods to reduce such degradation are       as follows:

• Refrigerate the produce to reduce the rate of respiration;

• Vacuum cooling;

• Reduce the oxygen content of the environment in which the produce is kept to a value not above 5% of the atmosphere but above the value at which anaerobic respiration would begin. When the oxygen concentration is reduced within 60 minutes the deterioration is in practice negligible.

• Storage in controlled atmosphere where carbon dioxide and oxygen levels are monitored, increasing concentration of CO2 and lowering that of oxygen according to fruit species. Excellent results can be obtained for pomace fruit; in particular the storage period for apples has been extended. Application of this combined procedure requires airtight storage rooms.

21.4 Chemical preservation is combined with acidification of food medium (lowering pH); and using combined chemical preservatives.

21.5 Deterioration control by drying/dehydration

Fresh fruits and vegetables are preserved with the procedure (freeze-drying) that combines the advantages of drying (reduction of volume and weight by 50%) with those of freezing (maintaining vitamins and to a large extent organoleptic properties).

A significant advantage of this process is the short drying time and the finished products after defreezing and rehydration/reconstitution are of a better quality compared with products obtained by dehydration alone. Vitamin C content in the product is maintained in the cold storage of dried/dehydrated vegetables at -8° C for more than one year, at a relative humidity of 70-75 %.

21.6 Preservation by lactic fermentation

     Natural acidification can be combined with cold storage for pickles in order to prolong storage time or shelf-life.

21.7 Combined preservation procedures

Even though a lot of progress has been made in the area of preservation techniques, no single procedure when applied alone can be considered wholly satisfactory from a microbiological, physico-chemical and organoleptic point of view; even if to a great extent the food value is assured.

Starting with this consideration, the actual tendency in food preservation is to study the application of combined preservation procedures, aiming at the realization of maximum efficiency from a microbiological and biological point of view, with reduction to a minimum of organoleptical degradation and decrease in food value.

       The principles of combined preservation procedures for microbiological preservation are:

• Avoid or reduce secondary (undesirable) effects in efficient procedures

• Avoid qualitative degradation appearing during storage of products preserved

• Increase efficiency of preservation procedures by supplementary means;

• Combine preservation procedures in order to obtain maximum efficiency, by specific action on various types of micro-organisms present;

• Establish combined factors that act simultaneously on bacterial cells.

Other than these, there are lot many methods to control deterioration of food, such as, salting, sugaring, pickling, concentration, irradiation, sterilization, solar drying, high pressure processing, control of water activity and pH, smoking, fermentation, use of chemicals such as acidulants, edible films and coating, etc.

These preservation procedures have two main characteristics as far as being applied to all food products is concerned:

• Some of them are applied only to one or some categories of foods; others can be used across the board a wider application (cold storage, freezing, drying/dehydration, sterilization, etc.);

• Some guarantee food preservation on their own while others require combination with other procedures, either as principal or as auxiliary processes in order to assure preservation (for example smoking has to be preceded by salting).

 References:

• U.S. Army medical department center and school Fort sam houston, Texas 8234-6100

Suggested Reading:

• Mark Sewald and Dr. Jon DeVries. Food Product Shelf Life. Medallion Laboratories. Analytical Progress.