Study on the structural characteristics and in-vitro digestion behaviours of NZ goat and sheep milk

Dr Siqi Li (Postdoctoral Fellow), Riddet Institute, Massey University

New Zealand’s goat and sheep milk industries have seen rapid growth over the past years, playing an increasingly important role in diversifying the NZ dairy export portfolio. Ruminant milks from different species have different compositions and consumer perceptions of their health benefits, providing opportunities for innovation and development of high-value products for global consumers.

Beyond the bulk composition of milk, the structural assemblies of milk components, such as the casein micelles and fat globules, play important roles in the technological and nutritional properties of milk and milk products. These milk structures vary naturally between the ruminant species, breeds and seasons. For example, goat milk has smaller fat globules and larger casein micelles than cow milk. Besides natural variations, processing technologies used in the dairy industry can alter these structures of milk. Recent research has shown that the digestion behaviour and nutritional outcomes of foods can be modulated by altering food structures. Our research aims to provide a better understanding of the natural differences in the compositional and structural characteristics of NZ ruminant milks (cow, sheep and goat), and how these characteristics change under common industrial processing treatments. We have studied the digestion behaviours of these processed milks from different species in a dynamic in vitro digestion system, the Human Gastric Simulator (HGS). The most common method for studying food digestion in vitro employs a “static” system, where the food and the simulated digestive fluids and enzymes are added into a closed vessel, adjusted to a fixed pH value and mixed by shaking or stirring. In such systems, the simulation of the physical movements of the stomach is oversimplified and the structural changes of ingested foods during digestion are not simulated well. The dynamic HGS utilised in our research however, was designed to simulate the peristaltic movement of the stomach based on the amplitude and frequency of contraction reported in humans. This is vital to gastric digestion and absent in “static” systems. This makes the HGS much closer to a ‘real life’ gastric system. In the HGS, a latex vessel serves as the stomach chamber. The continuous contraction of stomach walls is simulated by a series of moving rollers, controlled by motors on four sides of the stomach chamber (as shown in the photograph).

Schematic illustration of the Human Gastric Simulator (Guo et al. 2015)

References: Guo, Q., Ye, A., Lad, M., Ferrua, M., Dalgleish, D., & Singh, H. (2015). Disintegration kinetics of food gels during gastric digestion and its role on gastric emptying: An in vitro analysis. Food and Function. Li, S., Pan, Z., Ye, A., Cui, J., Dave, A., & Singh, H. (2022). Structural and rheological properties of the clots formed by ruminant milks during dynamic in vitro gastric digestion: Effects of processing and species. Food Hydrocolloids.

The HGS is also capable of simulating the continuous secretion of gastric juice, the emptying of digested food from the stomach and controlling the temperature of digestion at 37°C. Although the HGS is still a simplified representation of the human stomach, the system enables a more realistic simulation of the dynamic digestion process and the study of the changes in the structures and physicochemical properties of foods. From our studies, we found that all of the selected ruminant milks first coagulated in the stomach into structures similar to ‘cheese curds’ before further breakdown, hydrolysis and emptying from the HGS. This is due to the action of the digestive enzyme pepsin in the gastric juice, which acts on milk proteins similarly to chymosin in rennet used for cheese making. The structures of the milk curds in the stomach varied considerably, ranging from that of a ‘firm mozzarella cheese ball’ consistency to that of ‘loose and wet cottage cheese’, and was determined by the milk’s ruminant species and the processing treatment used. Goat milk formed the softest curds among the three species. Following homogenisation and heat treatments, milk from all the species formed looser curd structures. Differences were also found in the breakdown rate of the curds during digestion and the emptying of different milk components into the small intestine for further digestion and absorption. Milks that formed loose and soft curds were emptied from the HGS faster and had higher digestion rates than those that formed firm and intact curds. Findings from this study can inform the design and development of milk products from different ruminant species via different processing treatments to achieve different digestive outcomes. For instance, rapid delivery of proteins and amino acids is desired for products promoting muscle growth whereas a slowly digested milk may prolong satiety and assist in weight management. Premium products with tailor-made nutritional outcomes can be developed to meet the diverse and ever-changing needs of consumers. This could support a diversified portfolio of NZ dairy products that convey science-based wellbeing messages to global consumers.

This study is part of the MBIE-funded New Zealand Milks Mean More (NZ3M) Endeavour programme. The programme aims to achieve mechanistic insights into the structural characteristics and digestive dynamics of NZ ruminant milks and deliver superior digestive and nutritional outcomes. For more information on the NZ3M programme, please contact Prof Warren McNabb (+64 6 951 7742; W.McNabb@massey.ac.nz).

Coagulum structure of ruminant milk during gastric digestion (Li et al. 2022)