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LOW LACTOSE YOGHURT

Death by yoghurt?

In the 1980s, yoghurt-making appeared in most biology and many science syllabuses as the example of biotechnology. The photo below is the cover of an ASE publication that had a substantial section on yoghurt-making. It was death by a thousand worksheets - if your photocopying account wasn't used up first.

The practical exercise described on the left is one example of how we tried to bring a different dimension to this type of activity.

A common practice in yoghurt manufacture is the addition of skimmed milk powder to improve the yoghurt's nutritional status and to thicken it. This also increases the lactose content of the yoghurt, and values ranging from 3.3-5.8% have been reported. Typically, only 15% of this sugar is utilised during fermentation by lactobacilli. Consequently such yoghurt may be unsuitable for those who are lactose-intolerant.

Food technologists have therefore investigated the possibility of making yoghurt from milk with a reduced lactose content. This procedure involves pretreating the milk with the enzyme ß-galactosidase (lactase) to hydrolyse its lactose to glucose and galactose. Two other advantages arise from this. Firstly, yoghurt made in this way sets more rapidly due to faster initial rates of acid production (although the final pH is similar to that of normal yoghurt). This difference has been attributed to increased fermentation of glucose by starter culture organisms, although the overall numbers and proportions of the bacteria in the yoghurt are not greatly altered. Secondly, the product is sweeter than that made with normal milk, reducing the need for additional sugar.

The change in pH can be measured as the yoghurt ferments, ideally using a datalogger. A comparison of the rate of fermentation using lactose-reduced and normal milk is worthwhile.

Copyright © National Centre for Biotechnology Education, 2006 | www.ncbe.reading.ac.uk