Browning in (Darkening of) fruit juices is undesirable, detracting from the fresh appearance of the product. The colour (formation) can occur by the two quite different routes of Enzymic and Non-Enzymic Browning.
As the name implies, it is the action of enzymes naturally present in the fruit which cause the reactions leading to brown colouration. Polyphenoloxidase enzymes are located in fruit tissue separate to the juice, so pose no threat for intact fruit. But on processing when the fruit is disrupted, the enzymes are released and begin to catalyse reactions of phenolic compounds producing quinones, which then polymerise giving rise to melanins, the brown pigments. The effect is commonly noticeable on cutting an apple, when the flesh begins to brown quite quickly at the surface of the cut. The browning can be minimised by covering the cut in a highly acidic juice such as lemon or lime.
In fruit processing, there are three main methods to control the enzyme activity; elimination of oxygen, heat treatment and reduction of pH to 3 or less. In practice pasteurisation of juice is nearly always carried out, which effectively denatures the enzymes and avoids the need for pH reduction. For some particularly sensitive, low colour juices such as apple, fruit crushing and extraction may be carried out under a blanket of nitrogen to exclude contact with air.
Juices are complex (diverse) mixtures of materials including carbohydrates, proteins, amino acids, minerals, oils and acids. Perhaps not surprisingly, there are many possible chemical reactions between the various groups of materials. One of the more important classes from the point of view of degradation is the Maillard reaction. In the Maillard reaction, sugars initially react with amino acids to form glycosylamines, (Amadori compounds). Depending on conditions, the glycosylamines can then further react to produce the final degradation materials of Furfural and Hydroxymethylfurfural, or undergo Amadori rearrangement to give extremely complex mixtures which include long chain polymeric brown coloured pigments, melanoidins. Even today, there is not a complete understanding of the Maillard reaction. In practical terms the controlling factors are pH (basically a function of acidity), concentration, temperature and addition of antioxidant in the form of sulphur dioxide.
In fact there are three other possible non-enzymic routes to browning, that of caramelisation, ascorbic acid decomposition and oxidation. Caramelisation occurs when sugars are heated to high temperature in low water environments, so is not a factor for fruit juices. Ascorbic acid eventually decomposes to dehydroascorbic acid and then to 2.3 diketogulonic acid, so while initially it is a good antioxidant and inhibits both enzymic and non-enzymic causes of browning, it can ultimately lead to increased browning through involvement in Maillard reactions. Oxidation occurs when air gains access to the juice, such as through aeration during processing, permeation of oxygen through packaging, or when juices are stored in containers having a significant headspace.
To minimise non-enzymic browning, fruit juices are processed carefully to avoid aeration, then stored in full containers at low temperatures either frozen at -18°C when unpreserved, or chilled at 3 - 5°C if aseptically packed or when chemically preserved.
A highly effective method to control non-enzymic browning is by addition of sulphur dioxide, which amongst the permitted preservatives (additives) has a unique function in reversibly binding to active carbonyl sites, so interrupting the series of reactions that lead to browning.
In other classes of foods, the consequences of Maillard reactions have distinctly positive effects in generating flavours and colours, for example in coffee and meat products.