Protein complex, fruit juice

The flow nephelometric technique is a very fast and reliable method for studying the interaction between tannins and proteins (Figure 16.7) [84]. The possibility of changing the flow and changing the gradient of the different substrates involved is very convenient and versatile and the technique can be applied to complex samples such as different phenolic extracts and beverages (red and white wines, fruit juices, beers). 

Emit juice clarification is the oldest and largest use of pectinase. The traditional way to prepare juices is by cmshing and pressing fruit pulp. The raw juice is a viscous liquid with a persistent cloud of cell wall fragments and protein complexes. Addition of pectinase lowers the viscosity of the juice and modifies the haze particles. These sediment and can be removed by centrifugation or (ultra)filtration. Pectin lyase or the combination of polygalacturonase with pectin esterase are found optimal for this application. 

In some cases, however, the formation of salts and complexes is an unwanted phenomenon, as protein salts and complexes with polyphenols form hazes and sediments in preserved fruit, fruit juices and beer. Protein hazes can also form in white wines after bottling, where the main factor is inorganic sulfate ions. Protein complexes with transition metals are often coloured, and their formation in processed foods is generally undesirable. An example is the protein conalbumin occurring in egg white, which readily forms coloured complexes with metal ions (pink with Fe +, yellow with Cu + and Mn +) in media of pH > 6 through tyrosine and histidine residues of the polypeptide chain. Complexes with iron ions often cause discoloration of egg products, but in media with pH < 4, these complexes dissociate to the original colourless compounds. 

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