Assessment of the entrapment of free fatty acids in goat milk by β-cyclodextrin and reduction of goaty flavour
Goat milk and its products have a significant role in human nutrition in many developing countries and some developed countries. Goat milk feeds more people in developing countries than cow milk. It is also useful as a food for people with cow milk allergies and gastrointestinal disorders. In developed countries, goat milk is often perceived as ‘healthier’ than cow milk and possibly for this reason has a growing market. However, its characteristic goaty flavour is a barrier to increased consumption. Prior research has shown that β-cyclodextrin, which can trap small hydrophobic molecules, reduces goat milk flavour due to certain free fatty acids (FFAs) when added at low concentrations to goat milk and goat milk products. This thesis explores the chemistry of this phenomenon. The concentration of FFAs responsible for goat milk flavour was increased when a lipase from Pseudomonas fluorescens was added. After addition of lipase, β-cyclodextrin was added at various times up to four hours. In all cases, FFA concentrations were increased by β-cyclodextrin, which at first sight is contrary to the idea that β-cyclodextrin reduces goat milk flavour. A chemical model to explain this paradox was developed. It was proposed that β-cyclodextrin increased FFA concentration by a mass action effect, because as a trap for FFAs it was a chemical ‘sink’ so enhancing lipase activity. The method to measure FFA concentration in these experiments determines total FFAs, trapped or otherwise, so it was of major interest to show whether or not these FFAs remained trapped in β-cyclodextrin, in suspension or solution, and were therefore unavailable for odour/flavour sensing in the headspace above goat milk. This was tested by dynamic headspace analysis of four goat milk treatments: milk; milk plus β-cyclodextrin; milk plus lipase; and milk plus lipase plus β-cyclodextrin. β-Cyclodextrin alone in milk reduced the profile of FFAs in the headspace, particularly of octanoic acid. Lipase greatly increased the headspace profile of FFAs as expected, but when β-cyclodextrin was also present, the profiles were even lower than for milk plus β-cyclodextrin. The reason for this apparent synergism is unknown, but it confirms why β-cyclodextrin is so effective in reducing goat milk odour/flavour in spite of its mass action effect. The remarkable effectiveness of β-cyclodextrin in reducing goat milk odour might also be due to preferential binding of goat-milk characterising branched chain fatty acids, like the potent 4-methyloctanoic acid, but which are present in only low concentrations in goat milk fat. A spectrophotometeric competition experiment with phenolphthalein showed that although branched chain fatty acids were more strongly bound by cyclodextrins including β-cyclodextrin than their straight chain geometric isomers (nonanoic for 4-methyloctanoic acid), the difference was not marked. It was concluded that the remarkable effectiveness of β-cyclodextrin in reducing goat milk flavour is best explained by its ability to bind nearly all FFAs.