Alternatives sought to inhibit enzymatic browning in fruits and vegetables.

Controlling enzymatic browning in minimally processed vegetables and fruits has received a great deal of attention. Browning reactions in vegetables and fruits become evident when a food material is processed or subjected to mechanical injury. In the past, enzymatic browning has been controlled by using sulphites.

But there is a need to substitute sulphites with other approaches. The most frequently studied alternative to sulphite is probably ascorbic acid. Scientists at VTT Biotechnology and Food Research (PO Box 1501, FIN-02044-VTT, Espoo, Finland) have investigated chemical, enzymatic and physical methods to prevent the browning of vegetables and fruits.

Ascorbic acid is a highly effective inhibitor of enzymatic browning, primarily because of its ability to reduce quinones back to phenolic compounds before they can undergo further reaction to form pigments. Unfortunately, once the ascorbic acid has been completely oxidized to DHAA (dehydroascorbic acid), quinones can accumulate and undergo browning. Ascorbic acid derivatives, like AAP and AATP, have been used as browning inhibitors alone or in combinations with other inhibitors for potatoes and apples.

A biological preservation method may enjoy better consumer acceptance than preservation techniques that use traditional chemical preservatives. Lactic acid bacteria (LAB) produce a variety of low-molecular-weight mass compounds including acids, alcohols, carbon dioxide, diacetyl, hydrogen peroxide and other metabolites. Bacteriocin-producing LAB show potential for use in minimally processed foods. Hurdle technology using natural preservatives, such as inhibitors produced by LAB, may also work as browning inhibitors.

Sulphites are multifunctional agents. They prevent enzymatic and non-enzymatic browning. They control the growth of microorganisms, act as bleaching agents, antioxidants or reducing agents, and carry out various other functions. However, the use of sulphites has some disadvantages. In addition to being corrosive to machinery and destructive to nutrients, sulphites may produce tissue softening and off-flavors in products. In the United States, sulphite compounds are considered GRAS materials when used in accordance with good manufacturing practice. But they cannot be used on fruits and vegetables served or sold raw to consumers.

Sulphite treatments can also be used in conjunction with vacuum packaging. But here anaerobic conditions can be created that are conducive to anaerobic fermentation and the growth of pathogenic organisms. Adverse health effects associated with sulphite usage, and an increase in consumer preference for fresh, natural foods, have also driven the search for a practical and functional alternative to sulphite agents.

Many alternatives include compounds that are effective substitutes for only one or two of the functionalities obtained with sulphites. It is unlikely that a multifunctional sulphite substitute can be developed. Instead, combinations of several active ingredients, formulated to meet the needs of specific commodities and product types, probably will be developed.

Another alternative involves the use of edible coatings. Antioxidants can be added to edible coatings to protect against oxidative rancidity, degradation and discoloration. Some researchers have found that edible coatings reduced enzymatic browning in whole and sliced mushrooms. Further improvement in the antibrowning property of the coating was accomplished with the incorporation of an antioxidant and a chelator (1% ascorbic acid, 0.2% calcium disodium ethylene diaminetetraacetic acid). Various sulphated polysaccharides, including carrageenans, amylose sulphate and xylan sulphate, were found to be effective as browning inhibitors with diced apple. Further information. R. Ahvenainen; phone: [phone]; fax: [phone]; URL: http://www.vtt.fi.