Polyphenol oxidases and peroxidases are among the most studied enzymes in fruits and vegetables. Owing to the deleterious effects of discoloration and off-flavor formation induced by their actions, these enzymes have not ceased to be a matter of concern to food technologists, while their versatility as catalyst and their diversity as protein present a challenge to the biochemist. This article gives an account on the present state of knowledge in this field. The occurrence of polyphenol oxidases and peroxidases in food and food raw materials, and their role and importance in food processing are briefly outlined. Results of biochemical research including catalytic properties, substrate specificity, susceptibility towards pH and temperature, action of inhibitors, isolation, purification, and characteristics of the enzymes are given, with special emphasis on recent achievements based on high resolution separation and isoenzyme techniques. Finally, the behavior of polyphenol oxidase and peroxidase in selected major groups of fruits and vegetables is discussed. Some contradictions found in the literature are pointed out and some questions that have not been given the necessary attention by researchers so far are mentioned.

Polyphenol oxidase and peroxidase in fruits and vegetables.

Interest in food colorants as shown by the number of patents has doubled in recent years with natural pigments outnumbering synthetics by five to one The natural colorant area can be subdivided into anthocyanins, betalains, chlorophylls, carotenoids, flavonoids, polyphenols, Monascus, hemes, quinones, biliproteins, safflower, turmeric, and miscellaneous All involve different groups of chemical compounds which may be used directly as colorants, or may be chemically modified to produce different hues or increased stability All usually involve a method of collection, extraction, purification, possibly stabilization, and formulation A variety of hues can be obtained ranging from green through yellow, orange, red, blue, and violet, depending on the source of colorant Similarly, water or oil‐soluble formulations can be prepared depending on the type of colorant

Food colorants: Anthocyanins

Polyphenol oxidase (PPO)-catalyzed browning reactions are of significant importance in the fruit and vegetable industry. These reactions proceeding in many foods of plant origin cause deterioration and loss of food quality. A better knowledge of the factors that influence the action of PPO is imperative in order to control and manipulate its detrimental activity in plant products. This paper presents an overview of the current understanding of the reaction properties, biochemical characteristics and potential physiological roles of PPO in plants. Reaction properties will include general PPO reactions, specificities and molecular mechanisms of these reactions, and methods available to assess PPO activity. Physicochemical properties will evaluate substrate specificity, environmental influences such as pH and temperature, multiplicity, latency, and activators and inhibitors of PPO. The discussion will conclude with potential physiological roles of PPO in plants.

Physicochemical properties and function of plant polyphenol oxidase: a review'

A number of factors affecting anthocyanin stability and color are discussed in this review. The anthocyanins are probably the most spectacular of plant pigments since they are responsible for most of the red, purple and blue pigmentation of flowers, fruits and vegetables. However, because of their highly reactive nature, anthocyanins readily degrade, or react with other constituents in the media, to form colorless or brown colored compounds. The presence of an ox-onium ion adjacent to carbon 2 makes the anthocyanins particularly susceptible to nucleophilic attack by such compounds as sulfur dioxide, ascorbic acid, hydrogen peroxide and even water. Loss of anthocyanin pigmentation also occurs in the presence of oxygen and various enzymes, and as a result of high temperature processing. A certain degree of pigment stabilization may be conferred by acylation with various organic acids, copigmentation, self-association and/or metal chelation. In addition, pH has a marked effect on anthocyanin stability, and on the color of media containing these pigments. A number of anthocyanin-rich sources have been investigated for their potential as commercial pigment extracts. Although their application is primarily limited to acidic media, continued research on the chemistry of anthocyanins may lead to application and stabilization of these pigments in a wider variety of food products.

Anthocyanins as food colorants —a review

Anthocyanins occur widely in plants, being responsible for their blue, purple, violet, magenta, red and orange coloration; while betalains, consisting of red-violet betacyanins and yellow-orange betaxanthins, occur exclusively in families of the order Caryophyllales. The occurrence of these two classes of pigments is mutually exclusive. Their stability is markedly influenced by environmental and processing factors such as pH, temperature, O2, enzymes and condensation reactions. Due to their inherent instability these pigments, and betalains in particular, have not been widely used as food colorants. However, structural variants of the anthocyanins, i.e. polyacylated and copigmented species, have demonstrated remarkable stability and have great potential for use as stable natural colorants. Anthocyanin and betalain preparations can be used to colour a variety of foods and pharmaceuticals with compatible physicochemical properties, yielding highly coloured and high quality products. Their application could be enhanced, however, with new sources and stable structural variants, modification of current processes and foods, and technological advances (e.g. industry-scale extractions/purifications, microbial purifications, biotechnology) that would make purer and more stable preparations available. In this chapter the chemistry and biochemistry of anthocyanins and betalains—their structure and distribution, functions, biosynthesis, factors influencing their stability, methods of extraction and analysis, and current and potential sources and uses—are reviewed.

Anthocyanins and betalains

The factors affecting postharvest color losses in Concord grapes were investigated, with emphasis on the extraction, partial purification and characterization of naturally occurring enzymes that may be involved. Color losses were accelerated in Concord juice by the crude enzyme extracted from the grapes. Catechol was oxidized more rapidly than other o-dihydroxyphenols, although pyrogallol, caffeic acid and dl-DOPA were good substrates for the crude polyphenoloxidase (PPO). The optimum activity was in the pH range 5.9–6.3 and at temperatures 25–30°C. Kinetic data for the crude enzyme showed a Michaelis constant of 67 mM of catechol at pH 6.0. Effective inhibitors were Na2S2 O5, dithiothreitol, phenylhydrazine, cysteine and others. Peroxidase (PRO) activity of the crude extract was very low in ripe Concord grapes. Anthocyanin pigment degradation by the crude enzyme extract was dependent on maturity of the grapes. The most rapid degradation of pigment occurred when crude enzyme from ripe grapes was added.

Characteristics of concord grape polyphenoloxidase involved in juice color loss

Separation by GLC indicated the major nonvolatile acids in Concord grapes to be tartaric and malic with lesser quantities of succinic, citric, quinic and ascorbic acids present. In the less mature fruit, malic acid was present in greater quantities than tartaric acid; however, as the grape berries ripened malic acid was apparently metabolized more rapidly than was tartaric acid so that tartaric became the dominant acid present in the fully ripe fruit.

J. N. Cash

Papers

Characteristics of concord grape polyphenoloxidase involved in juice color loss

Changes in nonvaolatile acids of concord grapes during maturation

Nonvolatile acids of strawberries