Effects of industrial processing methods on skimmed camel milk properties

Camel milk (CM) has an integral role in the diet of the population in the arid and semiarid regions of Africa and Asia where scarce agricultural areas, high temperatures and small amount of precipitation. Recent studies have shown that it has potential therapeutic effects, including anti-cancer, hypo-allergic and anti-diabetic properties. Nowadays, CM has become increasingly commercialised and consumed in urban areas; which has led to an increased interest in the processing of CM to improve its microbial quality and extend its shelf-life. However, there is still a scarcity of available information regarding the effects of different processing methods (e.g. thermal and high-pressure treatments) on CM properties. Therefore, the aims of the current research were to characterise and quantify CM proteins and to evaluate the effect of high-temperature short-time pasteurisation (HTST), ultra-high-temperature (UHT) and high-pressure processing (HPP) on the physical, chemical and the organoleptic properties of skimmed CM in comparison to bovine skimmed milk. Capillary electrophoresis (CE) was successful in identifying and quantifying the major whey and casein proteins in CM (chapter 3). Major variations were found between camel and bovine milk in terms of both concentration and composition of whey and casein proteins. Unlike bovine whey, camel whey had no β-lactoglobulin (β-lg) and instead a high concentration of α-lactalbumin (α-la) followed by lactoferrin (LF) and serum albumin (SA) was observed. β-casein (β-CN) was the main camel casein followed by α-casein (α-CN) while ҡ-casein (ҡ-CN) represented only minor amount. These variations were found to have an impact on the technological properties of CM, and quality of dairy products made from CM. In general, HTST (72oC for 15s), UHT (140oC for 5s) and HP (200 to 800 MPa at 20oC for 30 min) treatments significantly affected components of skimmed CM and their functional properties (chapter 4). UHT treatment resulted in the highest levels of denaturation of whey proteins and greatest colour change of CM compared to the HTST and HP treatments. Casein micelles size of CM was significantly decreased after both heat and HP treatments. While, bovine micelles size increased after UHT treatment. Similar to bovine milk, the rennet coagulation time (RCT) of CM was significantly delayed and coagulum strength (G') decreased after HTST pasteurisation. UHT treatment hindered the coagulation of milk from both species. In contrast, HP treatment at 200 and 400 MPa increased the RCT of CM and G' value was the highest after treatment at 200 MPa. Unlike bovine milk, HP treatment at pressures higher than 400 MPa impaired the rennet coagulation properties of CM. The volatile profile of skimmed CM subjected to HTST, UHT, and HP treatments was found to differ from the volatile profile of raw CM (chapter 5). HTST pasteurisation and UHT treatment resulted in an increase of aldehydes, furans, and terpenes content in CM. Moreover, the increase of heat severity during the UHT treatment led to the formation of sulphur compounds in CM. On the other hand, HP treatments tended to enhance the formation of alcohol and ketones in CM. Both thermal and non-thermal treatments had limited effect on amino acids and lactose content of skimmed CM. The volatile profiles and sensory properties of HTST pasturised and UHT skimmed CM were different to pasteurised and UHT bovine skimmed milk. Heated CM samples were described as having attributes such as cardboard, musty, sulphur odours, as well as sour, savoury, aged, and whey taste/flavours. While, bovine milk samples were described as having cooked milk, creamy, and dairy aroma. Overall, conventional heat treatments resulted in the formation of volatile compounds which were responsible for off-flavours in processed CM.