1.2 Ultraviolet Light
Ultraviolet light (UV-C) treatment is a prominent non-thermal processing technique employed to kill microbes in fluid milk without subjecting the milk to heat (Bintsis et al., 2000; Matak et al., 2005; Rossitto et al., 2012; Cappozzo et al., 2015; Crook et al., 2015; Krishnamurthy et al., 2007; Christen et al., 2013; Bandla et al., 2012; Gunter-Ward et al., 2018; Alberini et al., 2015; Koutchma et al., 2019). This technique has a lot of advantages over conventional heat pasteurization, which can reduce flavor and cause nutrient loss, and this process seems to be highly energy efficient. The drawback of this technique is that treatment results in a lesser reduction in microbes because of its penetration into opaque liquids (Ansari et al., 2019). This problem can be overcome by using turbulent flow reactors, which allow the fluid milk to be exposed to UV light uniformly.
The electromagnetic spectrum of the UV radiation ranges from 100–400 nm, which can be classified into three ranges based on biological effects as well as photochemical properties, i.e. UV-A, B and C (315–400 nm, 280–315 nm, 200–280 nm) (Bintsis et al., 2000; Martysiak-Zurowska et al., 2017). The photochemical changes that occur in nucleic acids and proteins within the cell membrane when fluid milk is subjected to UV-C treatment lead to the inactivation of microbes present in the sample. The cells will die due to the interaction of photons with cystine and thymine nucleoside bases, which leads to the formation of cross-linked photo products like cyclobutyl pyrimidine dimers, which can stop DNA translation, transcription and replication processes, which results in the loss of microbes’ cell function and finally leads to microbial cell death (Martysiak-Zurowska et al., 2017; Gayán et al., 2013).
In food industries, generally UV-C light in the wavelength ranged from 250 to 260 nm is being used for the inactivation of microbes like bacteria, viruses, mold, yeast and bacterial spores (Crook et al., 2015).
The use of conventional heat pasteurization techniques for fluid milk has several limitations like higher energy cost, protein denaturation on account of high heat, deterioration of the milk’s technological properties, nutrient loss and flavor degradation. Therefore, owing to this effect, implementing non-thermal processing techniques is necessary to retain the biological and functional properties of fluid milk. On this occasion, UV-C light technology plays a predominant role in retaining the above properties without causing any serious technological effects in the samples.
Papademas et al. (2021) studied UV-C light’s technological effect on various food-borne microbes, which are artificially inoculated in donkey milk. The various food-borne microbes inoculated in the donkey milk in their studies were S. aureus, B. cereus, L. inoccua, E. coli, Salmonella enteritidis and Cronobacter sakazakii. The inoculated milk was treated with a UV-C dose up to the maximum extent of 1,300 J/L. Except for L. innocua, all other food-borne pathogens were destroyed by the treatment exposure to UV-C light at 200–600 J/L. They concluded that UV-C light is a promising non-thermal technology that can be commercially exploited to safeguard raw milk against various food-borne or food spoilage microbes.
The count of L. monocytogenes was not reduced and exhibited greater UV-resistant power, which may be ascribed to possession of a thick peptidoglycan cell wall that will prohibit UV photon penetration within bacterial cells; also, L. monocytogenes can naturally cope with DNA damage with better DNA repair mechanisms as compared to other food-borne microbes like E. coli (Baysal, 2018; Cheigh et al., 2012; Beauchamp and Lacroix, 2012). In bovine milk, a very high dose of UV at 2,000 J/L was required to reduce L. monocytogenes by a 5 log reduction. With respect to goat milk, a 5 log reduction of L. monocytogenes was achieved when the goat milk was subjected to a UV dose of 15.8 mJ/cm2 (Matak et al., 2005; Crook et al., 2015; Lu et al., 2011). The variation in UV dose level for the log reduction of microbes is attributed to the presence of various compositions of fat and total solids in different milk samples like goat, bovine and donkey.
1.3 Ultrasonication
Ultrasonication is simply sound waves that can exhibit high frequencies of more than 20 kHz. It has got its wide applications in food processing industries; the application can vary based on its usage, which may be either low- or high-energy ultrasounds. The frequency of the low ultrasound will be 2–3 MHz with intensities of less than 1 Wcm–2. It can be used for detecting foreign particles in both raw material and processed products, and it can also be used for characterizing various food samples (Rezek Jambrek et al., 2010). For the dairy industry, ultrasound with high-energy frequency is preferred, i.e. 18–100 kHz with a sound intensity of greater than 1 Wcm–2.
Several researchers reported that this kind of high-energy ultrasound can be potentially applied in dairy processing industries for the homogenization of milk, inactivation of bacterial enzymes and extraction of chymosin and β-galactosidase (Rezek Jambrak et al., 2009; Jelicic et al., 2010; Bosiljkov et al., 2011). The mechanism involved in ultrasound application lies in the formation of cavitation, i.e. the mechanical nature that leads to the formation and implosion of bubbles in liquid (Brncic et al., 2010). The bactericidal effect of ultrasound can be achieved as a result of implosion, which results in high temperatures, i.e. 5,500° C as well as higher pressures of 50 MPa (Villammiel and de Jong, 2000; Piyasena et al., 2003).
The use of ultrasound has a lethal effect on microbes and is widely used for preserving foods (Entezari et al., 2004; Piyasena et al., 2003; Zenker, 2004). The cavitations are of four kinds, which can be classified based on generation mode, viz. optic, particle, acoustic and hydrodynamic. In food applications, only hydrodynamic and acoustic cavitation were used (Gogate and Kabadi, 2009), as they bring physic-chemical changes to the ultrasound-exposed material.
Rezek Jambrak et al. (2011) conducted a study to notice the impact of ultrasound on model system physical properties prepared with whey protein concentrates (WPC) or whey protein isolates (WPI) after incorporating sucrose and milk powder and also without its addition. The samples after sucrose and milk powder incorporation were subjected to high-power ultrasound treatment of 30 kHz frequency for a period of 5–10 min. After treatment, the samples were analyzed for their solubility, foaming and emulsifying properties, thermophysical and rheological properties. Their study shows that the microstreaming and cavitation effects lead to the occurrence of protein denaturation and thereby exhibit greater influence on all the observed properties. It also further reduced the solubility of protein for whey protein concentrate and whey protein isolate samples when compared with untreated samples. The ultrasound-treated samples exhibited significant raise in foam volume after incorporating either sucrose or milk powder. The samples developed with whey protein concentrates and isolates reduced the emulsion stability indices as well as emulsion activity. The treatment of ultrasound for rheological parameters indicates that it doesn’t change the flow behavior indices resulting in noticeable changes in consistency coefficients (k). The treatment had further reduced the starting melting as well as freezing temperatures fo...
