Browning in fruit products

Stadtman, E. R. 1948. Nonenzymatic browning in fruit products. Advaruxs in Food... 

Earl R. Stadtman, Nonenzymatic Browning in Fruit Products. 325... 

UF is used to clarify various fruit juices (apple, grape, pear, pineapple, cranberry, orange, lemon) which are recovered as the permeate [Blanch et al., AlChE Symp. Ser. 82, 59 (1986)]. UF has also been used to remove pigments and reducing browning in wine production [Kosikowski in Membrane Separations in Biotechnology, McGregor (ed.), Marcel Dekker, New York, 1986]. 

L-ascorbic acid (AA) and its isomer D-erythorbic acid (EA) (also called D-isoascorbic acid) have been used as inhibitors of enzymatic browning in fruit and vegetable products for at least 50 years, (15-17). These compounds prevent quinone accumulation and subsequent pigment formation by reducing the 0-quinones generated from the phenolic substrates of PPO back to O-dihydroxyphenolic compounds (17-18). AA also can act as a PPO inhibitor (19-20). AA and EA are used interchangeably although there are indications that AA is more effective in some systems (21-22). 

Studies reviewed herein have demonstrated the potential ability of several new approaches to control enzymatic browning in fruit and vegetable products the use of novel AA derivatives, polyphosphates and (5-cyclodextrin as browning inhibitors the application of browning inhibitors to cut fruits and vegetables by pressure infiltration and the use of cvs. having a low tendency to brown. It is likely that one or more of these approaches will be useful as an alternative to sulfites to control browning in affected commodities. 

Because Actinidia fruits contain high levels of vitamin C, they are attractive materials for nutritional juice production. However, high vitamin C can cause nonenzymic browning in juice products during storage (Wong and Stanton, 1989). 

Joslyn, M. A., and Ponting, J. D. 1951. Enzyme-catalyzed oxidative browning of fruit products. Advances in Food Research 3, 1-44. 

Citric acid (pKi = 3.09 pK2 = 4.74 pKs = 5.41) is utilized in candy production, fruit juice, ice cream, marmalade and jelly manufacturing, in vegetable canning and in dairy products such as processed cheese and buttermilk (aroma improver). It is also used to suppress browning in fruits and vegetables and as a synergetic... 

Anthocyanins usually give a purple red colour. Anthocyanins are water soluble and amphoteric. There are four major pH dependent forms, the most important being the red flavylium cation and the blue quinodial base. At pHs up to 3.8 commercial anthocyanin colours are ruby red as the pH becomes less acid the colour shifts to blue. The colour also becomes less intense and the anthocyanin becomes less stable. The usual recommendation is that anthocyanins should only be used where the pH of the product is below 4.2. As these colours would be considered for use in fruit flavoured confectionery this is not too much of a problem. Anthocyanins are sufficiently heat resistant that they do not have a problem in confectionery. Colour loss and browning would only be a problem if the product was held at elevated temperatures for a long while. Sulfur dioxide can bleach anthocyanins - the monomeric anthocyanins the most susceptible. Anthocyanins that are polymeric or condensed with other flavonoids are more resistant. The reaction with sulfur dioxide is reversible. 

An excellent discussion of the many enzymes occurring in fruits is provided by Dilley (247). Of the numerous enzymes present in fruits, and as in the case of grapes, oxidizing enzymes have received considerable attention from researchers because their activity is usually accompanied by undesirable changes in raw product quality. A complete account of enzyme-catalyzed oxidative browning of fruits and their products is presented by Joslyn and Ponting (248). 

The simplest, but least accurate, method of assaying DPO activity is to record the final color yield when the enzyme is incubated with a suitable chromogenic substrate such as catechol, DOPA, or 4-methylcatechol. DOPA is the most frequently used substrate in colorimetric assays because it yields a dark brown/black end-product. In this reaction, catecholase catalyzes the conversion of DOPA to dopaquinone and then to the red dopachrome, which subsequently polymerizes to yield dark brown melanin-type pigments. Unfortunately, this simple procedure has serious limitations, as it measures the end-product of a sequence of reactions rather than the true initial reaction rate. Furthermore, because different substrates yield different final colors, valid kinetic comparisons between substrates are not possible. Nevertheless, this simple assay technique has proved adequate for useful comparative studies of the levels of enzymic browning in different fruit varieties and similar problems (Vamos-Vigyazo, 1981 Machiex et al., 1990). 

Phenolic compounds are of interest due to their potential contribution to the taste (astrin-gency, bitterness, and sourness) and formation of off-flavor in foods, including tea, coffee, and various fruit juices, during storage. Their influence on the appearance of food products, such as haze formation and discoloration associated with browning in apple and grape products, is also significant. Furthermore, analysis of these phenolic compounds can permit taxonomic classification of the source of foods. The importance of each phenolic compound and its association with the quality of various foods is described further in Sec. IV, on food applications. 

The phenolic composition of apple consists of cinnamic acids, flavonols, dihydrochalcones, and flavan-3-ols (50,56). In the apple fruit processing industry, hydroxycinnamic acid derivatives and flavan-3-ols are important due to their contribution to the astringency, haze, and browning in apple juice and cider. Chlorogenic acid represents the major hydroxycinnamic acid derivative. The flavan-3-ols (catechins) are present in the monomeric form as well as in oligomeric and polymeric forms (procyanidins) in apple and apple products (56). 

The colour of a soft drink or fruit juice may be assessed in a number of ways. If the product is clear it can simply be carried out by measuring the absorbance of the product at one or more wavelengths. The actual values chosen will depend on the particular colour of the product. For a yellow product, such as apple juice, wavelengths of 465, 430 or 420 ntn are often chosen to assess the colour. These values can then be expressed in European brewing convention (EBC) units by multiplication by a factor of 25. The actual Brix value chosen to assess the colour depends on the country however, levels between 11 and 12 are often taken as the norm. If dealing with a red-coloured product, then the assessment is generally carried out at 520 nm. Absorbance values are sometimes also taken at 420 nm in red or black juices to assess the brownness of the product. The two absorbance values are often used to express a colour ratio, which gives an indication of colour versus brownness ... 

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