Composition, Structure, and Digestive Dynamics of Milk From Different Species-A Review

Abstract

Background: The traditional dairy-cattle-based industry is becoming increasingly diversified with milk and milk products from non-cattle dairy species. The interest in non-cattle milks has increased because there have been several anecdotal reports about the nutritional benefits of these milks and reports both of individuals tolerating and digesting some non-cattle milks better than cattle milk and of certain characteristics that non-cattle milks are thought to share in common with human milk. Thus, non-cattle milks are considered to have potential applications in infant, children, and elderly nutrition for the development of specialized products with better nutritional profiles. However, there is very little scientific information and understanding about the digestion behavior of non-cattle milks. Scope and Approach: The general properties of some non-cattle milks, in comparison with human and cattle milks, particularly focusing on their protein profile, fat composition, hypoallergenic potential, and digestibility, are reviewed. The coagulation behaviors of different milks in the stomach and their impact on the rates of protein and fat digestion are reviewed in detail. Key findings and Conclusions: Milk from different species vary in composition, structure, and physicochemical properties. This may be a key factor in their different digestion behaviors. The curds formed in the stomach during the gastric digestion of some non-cattle milks are considered to be relatively softer than those formed from cattle milk, which is thought to contribute to the degree to which non-cattle milks can be easily digested or tolerated. The rates of protein and fat delivery to the small intestine are likely to be a function of the macro- and micro-structure of the curd formed in the stomach, which in turn is affected by factors such as casein composition, fat globule and casein micelle size distribution, and protein-to-fat ratio. However, as no information on the coagulation behavior of non-cattle milks in the human stomach is available, in-depth scientific studies are needed in order to understand the impact of compositional and structural differences on the digestive dynamics of milk from different species.

Keywords: composition; curd; digestion; fat; milk; protein; stomach; structure.

Figure 1

Proportion of dairy cattle and non-cattle milks produced globally in the year (A) 1961 and (B) 2018. (Source: FAOstat, March 2020).

Figure 2

Images of clots formed during the gastric digestion of 200 g of unheated (top row) and heated (bottom row) cattle skim milk at different digestion times. Source: Adapted from Ye et al. (107).

Figure 3

Schematic diagram of the possible mechanism of events during the formation of protein curds from (A) raw milk (unheated) and (B) heated milk during gastric digestion. Source: Adapted from Ye et al. (145).

Figure 4

Changes in the fat content (g/100 g milk) in clots obtained from (•) unheated (raw) and (°) heated cattle whole milk during gastric digestion. Source: Adapted from Ye et al. (151).

Figure 5

Images of clots formed during the gastric digestion of raw (unheated), homogenized, and heated homogenized cattle whole milk during 20 and 160 min of gastric digestion. Source: Adapted from Ye et al. (152).