**3.3 Skim milk**

 A multistage processing technique (like skim milk development) involves wide use of heat treatment for milk preservation [62]. The formation of Maillard intermediates and glycation products during manufacture of dairy products has been studied [63]; the focus of these studies was the reduction in nutritional values, e.g., lysin [64].

### *3.3.1 Biofilm resistance of the bacteria in a milk powder processing factory*

 In the dairy industry, it is known to use a closed production system without removing or opening equipment using the CIP process. In terms of economic benefits, short cleaning procedures and long-term use of equipment in processing lines are common [65]. As a result, bacteria remain on the surface of the device and can accumulate in hard-to-reach places, such as dead ends, cracks, seals, and valves, where the complexity of cleaning and disinfection is difficult [66]. Undesirable biofilms on the surfaces of food processing have certain properties, such as increased tolerance to antimicrobial agents, increased secondary metabolites, etc. These are the potential cause of bacterial contamination [67]. In dairy industry, the presence of such biofilms leads to contamination after processing, shortens the shelf life, and promotes the transmission of diseases [68].

### *3.3.2 Presence of spore-forming bacteria in skim milk*

The existence of spore-forming bacteria in milk is a critically important issue in the dairy industry. Bacterial endospores can survive in harsh environmental conditions such as high heat, low pH, desiccation, or cleaning and sanitizing chemicals [69]. Compared with vegetative cells, spores have also been found to attach more readily to stainless steel, leading to the formation of biofilms that can promote bacterial contamination within dairy processing plants. Spores present in final products can germinate and produce enzymes that decrease the quality and shelf life of dairy product; it causes the significant economic losses [70]. Additionally, some spore formers such as *Bacillus cereus* and *Bacillus subtilis* can produce toxins that are responsible for food poisoning [71]. *Thermophilic geobacillus* spp. and *Anoxybacillus*  spp. are other spore formers of importance to the dairy industry, as they are commonly present in dairy powders and evaporated milk [70].

### **4. Factors affecting milk production and composition**

The milk quality is influenced by many factors acting together and influences each other [72]. One of the most important factors is disease that adversely affects livestock systems that leads to decrease in yield, income, and survival of livelihood. The impact of livestock diseases is complicated and often exceeds the impact on the respective producers [73]. Selection of dairy animals, nutritional

 management, advances in milking technology, and mammary gland of the dairy animals are the other factors that are also associated with milk yield as well as its composition [74].
