Processing and key nutrients proteins

The impact of any continuous-flow process on nutritional components in a food will be similar in type to the appropriate in-container process. However, as explained earlier, the difference in z values between microbiological death and biochemical degradation means that the level of that impact will generally be substantially lower. Much of the detailed work on changes to nutrients during continuous-flow heat processing concerns milk and milk products, as research in this area has been on-going since the 1950s. As the technology is more recently being applied to other products, a large body of work on further foodstuffs is progressing and will continue to do so.

In general, a variety of changes in proteins can occur at elevated temperatures, especially denaturation and its associated changes in properties such as the waterholding capacity, viscosity or whippability, development of flavours (especially sulphurous), aggregation or formation of precipitates. Proteins are known to have a maximum thermostability at their isoelectric points although at pH values on either side, thermostability decreases at different rates depending on the type of protein and its environment. The effect of inorganic salts may also be important; at low levels of salts, stability is generally lower but, at high levels, can either reduce or increase stability. Lysine and cysteine are degraded by heat; losses of the latter rarely exceed 25% during in-container processing and are generally negligible in UHT processing.

In milk, the proteins are divided into two types, caseins (in micelle form) and whey proteins. The whey proteins (a-lactalbumin, b-lactoglobulin, bovine serum albumin and immunoglobulins) are in solution and start to denature at temperatures as low as 70-90°C. Typical denaturation levels in UHT are given for pasteurised milk as 5-15%, direct heating systems as 50-75%, indirect heating systems 70-90% and in-bottle sterilised as 80-100% (Renner, 1979). However, denaturation has little or no effect on their nutritive value, biological value or true digestibility. Unfolding and denaturation of b-lactoglobulin is responsible for release of volatile sulphurous compounds and the subsequent flavour of UHT milk. Oxidisation of these compounds during storage, either by residual oxygen in the product or leakage of oxygen into the container, will reduce this cooked flavour in the first few days after production. Again, direct heating processes were found to give a lower level of volatile sulphurous compounds, partly due to removal of the compounds during flash-cooling and despite the lower residual level of oxygen also due to this.

The casein micelles are relatively heat stable and only minor changes are found. However, this reaction is important, because denatured whey proteins aggregate onto the casein and interfere with coagulation by acid or rennet, giving a looser curd. The milk is not suitable for use in cheesemaking.

Soy milk is increasing in popularity as a product in Europe and the USA but raw soybeans have been long known to have a poor nutritive value due to the high levels of trypsin inhibitors in them. It is important nutritionally to reduce these inhibitors to less than 10% of the original concentration, at which point they will not interfere with biological value of the protein. The conditions required to achieve this are quite severe, for instance the D value at 143°C is between 56 and 100s, depending on the pH. Thermal denaturation of the inhibitor has been reviewed by Kwok and Niranjan (1995).

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