For mono- and oligosaccharides, little direct degradation occurs at temperatures typical of UHT processing but there are several reactions that occur that may affect nutritional quality. Firstly, Maillard reactions may occur, depending on the composition of the food, i.e. the presence of reducing sugars and amino acids. This is covered in Chapter 11 and will not be further discussed here. It is interesting to note that one of the intermediate compounds formed during the reactions, 5'-hydroxymethylfurfural (HMF), has been used to indicate the level of heat treatment received by milk.
Secondly, another reaction of note in milk is the formation of lactulose, an epimer of lactose formed during heating, which has also been used to distinguish between levels of heat treatment. This compound has been used in European Union legislation to distinguish between pasteurised, UHT-sterilised and incontainer sterilised milks, and can also be used to distinguish between direct-and indirect-heating processes. Although lactulose has a laxative effect at around 2.5mg/kg, milks fortunately do not reach this level, being around 2mg/kg for in-bottle sterilised and much lower for other milks.
Native starches will gelatinise at relatively low temperatures (60 to 85°C) and have been found to break down under the high temperatures and high shear rates present in an UHT process, giving a product with a vastly reduced viscosity after heat treatment. To overcome this, it is necessary to use chemically-modified starches which have been specifically devised to be stable under these conditions. Rapaille (1995) found a highly crossbonded and hydroxypropylated waxy maize starch performed best in both plate and direct-heating UHT processes. Low cross-bonded starches, however, were found to foul the process plant rapidly and gave a high viscosity during the preheating stages of the process.
UHT processing has not been found to cause any physical or chemical changes to fats in milk and milk products. Milk is usually homogenised during treatment, and some instability of the milk fat globule may occur due to denaturation of the proteins in the milk fat globule membrane. Because of this, homogenisation after the heat treatment section is usually preferred, especially in direct-heating systems, but has the potential to cause recontamination of the product due to leakage through the homogeniser seals and general difficulty with cleaning this area. It is important to have a well designed homogeniser with aseptic design and steam seals on the piston seals.
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