A study of lactose in goat milk and the subsequent conversion to galacto-oligosaccharides

Abstract

Goat milk is more alkaline, and has better digestibility buffering capacities and therapeutic values than most other types of milk. So dairy goats and its milk productions have a significant place in the economy of many countries. Lactose is one of the main compounds found in goat milk and makes up approximately 4.4% of the total solids content. The lactose content of goat milk is a variable, it depends on the season, genotype, feeding, and lactation stage of the goat itself. In addition, lactose hydrolysis is often applied to food industrial fields (dairy food and non-dairy food) where the lactose molecules are turned into sweeter monosaccharides to form sweetening syrups and lactose-free dairy products which can alleviate the symptoms of lactose intolerance. The enzymatic hydrolysis of lactose is a transgalactosylic process which involves a multitude of sequential reactions with both disaccharides (other than lactose), and higher saccharides, which form intermediate products collectively known as galacto-oligosaccharides (GOS). GOS have an important role in promoting the proliferation of intestinal Bifidobacteria sp. and Lactobacilli sp. in the colon which provide protection from infections by inhibiting the growth of pathogens and modulating the immune system. The aims of this thesis was to quantify lactose contents in the milk of individual goats to determine the differences of lactose content using High-performance anionexchange chromatography (HPAEC) from two New Zealand goat farms (Lemuel Dairy Goats farm and Ian and Jan Webber’s farm) and with the same goats at different lactation stages within a given year; and to optimize lactose hydrolysis conditions by two different enzymes to increase the yields of GOS obtained from goats milk. Experiments conducted in this thesis revealed that the majority of the goat sampled from Lemuel Dairy Goats farm contained between 40 and 50 g/L of lactose and there were significant increases in the lactose concentration in milk with the earlier lactation stage. The goats sampled from Ian and Jan Webber’s farm were divided into two different groups depending on breed. Both breeds showed that the level of lactose concentration in milk fluctuated between goat and milkings in late lactation. Compared to the mean lactose concentration in different breeds, the Toggenburg goat content was lower than Saanen goats. There was no correlation found between the measured lactose content of goat milk and the particle size measured using Malvern Zetasizer of the milk obtained from both farms. Two commercial enzymes: Enzyme L6500 from Kluyveromyces lactis and Lactase 10 from Aspergillus oryzae, were employed to hydrolyze lactose buffered solutions to form a lactose hydrolysis model. The optimum conditions for hydrolysis of the lactose solution (for both lactose concentrations of 45 g/L and 90 g/L) were found to be as follows: pH value was 6.7, temperature was 45°C and lactose depletion time was 20 min using enzyme L6500. For enzyme Lactase, the optimum conditions of lactose solution (45 g/L) hydrolysis: pH value was 6.7, temperature was 60°C and lactose depletion time was 240 min. Comparing the capability of the two enzymes at lower lactose concentrations (45 g/L lactose solution and 10% goat milk), enzyme L6500 from K. lactis obtained higher yields for two GOS disaccharides measured compared to Lactase 10. In addition, a trisaccharide 6-galactosyllactose was produced by enzyme L6500 at higher concentrations of lactose substrates, but other trisaccharides of β(1-3) bonds and β(1-4) bonds were not obtained.