Physico-chemical, biological and nutritional properties of Australian camel milk and Its powder

Abstract

Camel milk is an increasingly popular alternative to bovine milk around the world due to its reported nutritional properties and its potentially therapeutic benefits. Although Australia is home to one of the largest dromedary camel herds in the world, studies of Australian camel milk are lacking. This project aims to provide a comprehensive understanding of the physicochemical properties of Australian camel milk, with a primary emphasis on proteins.The major nutritional components, physical properties and whey protein profile of Australian camel milk samples collected over four seasons were initially analysed. The composition and physical properties of Australian camel milk varied with season, milking frequency and yield. Higher contents of fat, total solids and lactose were observed in spring samples compared with those in other seasons, while no significant (P < 0.05) differences were found for protein and ash content between the four seasons. Milk viscosity was found to be stable over the year but a smaller average particle size was observed in summer and autumn samples. Lactose content was positively correlated with milk yield in the four seasons. The milk viscosity, particle size and other components such as fat, protein and ash content were, however, normally negatively correlated with milk yield. A proteomic study showed that, in general, summer camel milk contained higher amounts of functional whey proteins, such as lactotransferrin, peptidoglycan recognition protein 1, osteopontin and lactoperoxidase.A highly sensitive, high-throughput, fluorescent method was developed for the determination of activities of enzymes with antimicrobial properties in milk, including xanthine oxidase (XO), polyamine oxidase (PAO) and lactoperoxidase (LPO). The hydrogen peroxide generation by milk XO/PAO was coupled to the horseradish peroxidase conversion of Amplex® Red (1-(3,7-dihydroxyphenoxazin-10-yl)ethanone) to its fluorescent product, resorufin. LPO activity was measured by the direct oxidation of Amplex Red by LPO, following the addition of hydrogen peroxide. These enzyme activities varied significantly between camel, bovine, goat and human milk, suggesting that different innate immune systems may be present in milk from different species. Analysis of the enzyme activities in seasonal camel milk samples revealed that summer and winter samples had the highest LPO and PAO activities, respectively, over the year. The secretion of LPO into milk varied between individual camels, while the PAO activity was negatively correlated with milk yield in all the seasons. An antimicrobial role of PAO may be important for the innate immune system in camel milk.The abundant amount of β-casein and lactotransferrin, coupled with the absence of β-lactoglobulin in camel milk, makes it a promising alternative base material for making human infant formulae. The protein digestibility of camel milk was therefore compared with that of bovine and human milk using an in vitro digestion system simulating the infant gastrointestinal tract. Camel milk formed a single clot during gastric (stomach) digestion, while no visible clot was formed in bovine and human milk. During the gastric phase of digestion, casein was the most obviously digested protein in bovine milk, while the whey proteins were most prominently digested in camel milk. The soluble milk proteins remaining in the gastric digesta were rapidly and extensively digested in the intestinal phase. A gradual hydrolysis of the proteins incorporated in the camel milk clot was observed and very small amount of the clot remained at the completion of intestinal digestion period. Different peptide profiles were observed from camel, bovine and human milk following digestion. The largest number of potentially bioactive peptides were identified in camel milk after intestinal digestion, using PeptideRanker predictions. This work provides some initial insights on the protein digestibility of camel milk under infant digestion conditions, and presages further studies on the infant consumption suitability of camel milk proteins.Finally, to increase the potential availability of camel milk, camel milk powders were produced by freeze drying or spray drying with/without pre-concentration by spray dewatering or reverse osmosis. An improved retention of lactoperoxidase activity was observed in freeze-dried camel milk powders (18-50%) when compared to spray-dried camel milk powders (9-14%). A total of 108 proteins were quantified, on a relative basis, in the camel milk powders versus unprocessed camel milk, using Sequential Window Acquisition of All Theoretical Mass Spectra (SWATH-MS). Smaller magnitude changes in protein profile were observed in directly freeze-dried or spray-dried camel milk powder, with the freeze-dried sample showing the closest protein profile to that of unprocessed milk, while reverse osmosis had the greatest impact in changing the milk protein profile. Among the bioactive proteins identified, lactotransferrin and oxidase/peroxidase had the most significant decreases following processing, while glycosylation-dependent cell adhesion molecule 1, peptidoglycan recognition protein 1, and osteopontin generally increased. The reverse osmosis-concentrated and freeze-dried camel milk powder exhibited the highest anti-Listeria activity among all the powders produced. The results suggest that the dehydration method should be carefully selected depending on the use and desired characteristics of the camel milk powder.