Mixing in milk

The majority of the world's milk is produced by cattle, buffaloes, goats, sheep, and camels. The cow is the species that produces milk the most widely among the others. With a population of more than 1.5 billion people, the globe produced around 683 million tonnes of cow milk (CM) in 2018 [1]. The production of buffalo milk, which is common in many nations, is second in the globe after that of CM and accounts for over 13% of all milk produced globally [2]. More than 29 million tonnes of milk were generated by dairy goats and ewes in 2018 [1]. The majority of dairy ewes live in the Mediterranean and Black Sea areas, where the locals greatly appreciate and often utilise dairy products (such cheese) as standard elements in meals. On the Indian subcontinent, dairy goats are mainly found in low-income, food-deficit nations where their products are a vital source of nutrition. Nevertheless, high-income and technologically advanced nations (such as France, Spain, and Italy) also have a sizable animal population. Around 35 million camels are in existence worldwide, and they produce about three million tonnes of milk annually [2].
The provision of functional meals is seen to be a possible use for milk and dairy products [3]. This is due to the range of vital ingredients they contain, including proteins, polyunsaturated fatty acids (FAs), vitamins, and minerals, as well as the ease with which lactic acid generating bacteria (LAB) may be added during their manufacturing. Camel milk (CaM), goat milk (GM), ewe milk (EM), and buffalo milk (BM) have all gotten a lot of attention during the last several decades. This is primarily because they are acknowledged to have a larger potential for functional meals from a nutritional standpoint than CM.
In terms of fat, lactose, protein (particularly casein), vitamins, and minerals, BM is superior than CM [4,5]. More specifically, in terms of the lipid fraction, BM is often linked to greater levels of saturated fatty acids and lower levels of unsaturated fatty acids than CM, which includes more medium-chain fatty acids (C8:0 to C12:0). Compared to CM, BM had considerably greater concentrations of myristic acid (C14:0) and palmitic acid (C16:0) and lower concentrations of stearic acid (C18:0) in the long-chain fatty acids category. BM has more total trans fatty acids (C18:1 trans + C18:2 c9 tr11) and rumenic acid (C18:2 c9 tr11, the primary conjugated linoleic acid; CLA) than CM [6].
According to most presentations, GM has higher concentrations of vitamin A, thiamine, and niacin [7,8] as well as structural variations in the two main whey proteins found in CM, -lactalbumin and -lactoglobulin. Due to the smaller distribution of fat globules in CM, the difference in digestibility rates has also been linked to the variation in tolerance. This characteristic is influenced by the structural and molecular characteristics of -lactalbumin and -lactoglobulin, as well as by the small-diameter fat globules that enable greater digestion as compared to CM [9,10,11,12]. Additionally, GM can be viewed as a natural source of oligosaccharides made from lactose. Compared to CM, it has a better lipid composition with greater levels of conjugated linoleic acid, short FA, vitamins (A and B complex), and calcium [13,14,15]. As a result, it may have health benefits. However, GM also contains a lot of saturated fatty acids and very little polyunsaturated fatty acids, which are frequently associated with the emergence of cardiovascular diseases.

Due to its richness in minerals, higher protein, beneficial fat, and functional bioactive peptides [16], the demand for EM milk is increasing in the global market. It is characterized by the presence of small fat globules with an easily oxidizable membrane. Lipolysis in EM cheeses is faster than in CM cheeses, contributing to an important and typical flavor development [8] due to a higher content of short-chain FAs [17]. EM contains higher protein and fat levels than CM [18]. Compositional differences between EM and CM, mainly in proteins and fats, account for the different technological and sensorial characteristics of cheeses.

CaM presents a high nutritional value and plays a key role in providing milk of superior quality (e.g., more vitamin C, minerals (e.g., K⁺, Cu²⁺ and Mn²⁺), essential and polyunsaturated FAs than CM [19,20]). It is also thought to exhibit properties to manage chronic ailments [21]. The interest in dairy products obtained from CaM has increased in the past decade and the production of CaM on a large commercial scale from modern camel farms is growing [22]. Nowadays, in Mauritania and the United Arab Emirates, many milk and dairy products are produced and marketed (e.g., pasteurized milk, milk powder, fermented liquid milk, and cheese [21,23]). However, the use of CaM in processed food products is very limited and faces difficulties due to the coagulation properties of CaM [24].

In addition to the nutritional functionality that can be provided by these non-cow milks, their properties can contribute to the provision of specific technological functionalities to dairy products (e.g., texture, viscosity, melting, and color). This can be useful for the current consumption trend. Indeed, it is recognized that the modern lifestyle of consumers drives new models of dairy products, especially for cheese consumption (e.g., in fondue and pizza). The nutritional and technological functional properties of these milks and dairy products can participate in producing diversity and catering variations in consumer preferences that can be useful for milk and dairy industries in meeting this demand.

Despite the availability of scientific knowledge about the positive aspects of the consumption of non-cow milk and its derived dairy products [25], its production in some countries is scarce (e.g., Brazil, Morocco, Algeria), limiting its processing into dairy products (e.g., cheeses). This is detrimental because in some regions, mainly rural areas, the valorization in differentiated products may contribute to their economic sustainability. However, for some of the derived products, the flavor of the milk is different; it is stronger than CM, which constrains its acceptability to consumers. In this context, the production of dairy products using mixtures of milk species (e.g., GM with CM) could be an interesting and feasible opportunity for the expansion of the dairy industry in many regions and could equally strengthen the non-CM production chain. Moreover, mixing milk from different species can be a way of improving the quality of fermented dairy products and developing new ones with specific nutritional (biochemical), physicochemical, sensory, and rheological properties. Regarding this opportunity, it is of the utmost importance to characterize the quality features of products derived from mixing different milk species in order to obtain products with proper characteristics and satisfactory acceptance by consumers.

Cheese represents one of the most popular food products in the world. This is probably thanks to its richness in nutritional components like proteins, short-chain FAs, vitamins (e.g., riboflavin, thiamin, vitamin B12), and minerals (e.g., calcium, phosphorus). The number of scientific studies conducted on the characterization of cheeses produced from mixtures of different species has increased from year to year. The production of 150 papers was observed from 1990 to 2008, while 350 papers were published from 2014 to 2019 (Scopus database on 1 July 2020; keywords: cheese, mixture and milk). Therefore, the present narrative review will gather and focus on the different studies that have provided a clear comparison between cheeses produced from one species of milk (i.e., CM, BM, GM, EM, and CaM) and cheeses produced from mixing different milk species (e.g., CM/GM). Moreover, this review will discuss in detail the effect of milk species and milk ratios on the biochemical (nutritional), physicochemical, structural, sensory, and rheological properties of cheese . Finally, this paper will present some future research trends in order to improve the knowledge of these types of cheese properties.