cells changes the water activity which results in lowering the water content, causing osmotic stress. And due to this, microbes increase their cytoplasmic cells through various processes, for instance, humidity, air circulation, limitation of nutrition, and tempura which contribute to the stress [42].
2.5.3 Pressure
Elevated pressure when applied to the microbes results in altering the genes, metabolisms, and morphology of the cell. It is known that microorganisms adapt easily to the environment using various methods. They utilize diverse protection processes by activating the expression of the genes, staying in the dormant state, and producing the resistant mutant. There are different ways microorganisms adapt to the adverse condition, such as they create spores which hardly changes its morphology during application of pressure. As mentioned above, the resistance power to pressure in the stationary phase higher as compared to the exponential phase [44]. As the structure of the cell does not change in the stationary phase due to the protection of the membrane, the tolerance stress level gets elevated [45]. Listeria innocua endured in replicated milk with supplementary magnesium, calcium, citrate, and phosphate [27]. Magnesium has the stabilizing agent for ribosomes and calcium aids in maintaining the outer wall of the membrane of the cell. Sucrose guards bacteria by stratifying the functionality of membrane proteins [46].
2.6 Various Techniques for Thermal Inactivation
2.6.1 Infrared Heating
2.6.1.1 Principle and Mechanism
Infrared falls in the electromagnetic spectrum fluctuating between 0.78-1000 μm which is between ultraviolet and microwave radiation. Heat is generated due to the motion of the molecules which is both rational and vibrational in nature. Unlike conventional heating where the heating is done by convection at the first from the surface and from inside of the product, it is done by conduction, here infrared offers radiation from outside the surface and conduction from inside of the food product [47]. Numerous studies have been done to verify the anti-microbial effect of IR when applied to various food products such as honey, cheese, milk, fruits, and other liquid and semiliquid products [48, 49]. Infrared works well even for powder products mainly spices and with time varied of the word have also been performed to analyze the effectiveness of the IR treatment for effectiveness in reducing microbial activation. Several factors significantly affect the microbial log reduction but the utmost responsible factor is the temperature and wavelength of infrared radiation. Some of the other factors are as following power, water activity, moisture content, bandwidth, and depth of the food sample [50].
Infrared offers a similar effect of thermal inactivation mechanism as seen in the microwave heating and ultraviolet light which causes DNA damage along with heating through induction and of course. By thermal heating, the inactivation of microorganisms becomes easier; it can destroy or damage different parts of the cell structure which mention in the order of damage magnitude that is protein >RNA >cell wall >DNA. To confirm the effects and study the mechanism of heating for the inactivation of microbes, various methods like fluorescent and spectroscopic probes were utilized for paprika powder performed by [51]. The result inferred from the treatment that radiation of the infrared waves created all-around injury of the cell which includes inactivation of RNA polymerase of the microbial cells, which simultaneously prevents the transferase reaction by combining it with subunits of the ribosome. Much effective and advanced decontamination is visible in this treatment as the waves of the infrared have extended energy levels, and distribution of this energy is very efficient as compared to any other traditional method of heating which mostly uses the fluorescent probe for molecular-level analysis. Analysis using the digital method showed the best results as compared to the conventional one in terms of analyzing the relationship of the targeted organism and the environment they are present in real time.
2.6.1.2 Application for Inactivation in Food Sector
Different studies were conducted which explain the different effect of the parameters of infrared radiation. Bacillus cereus on paprika powder was studied by applying radiation at 11 kW/m2 and 5 kW/m2 at a temperature of 95 °C [51]. From the studies it showed that maximum injury of microbes was seen at aw 0.5 and at aw 0.8 the overall log reduction was seen was 0.7 and 1.6 log 10 CFU/g at 5 and 11 kW/m2. From this, it was inferred that Bacillus cereus is susceptible to heating through infrared, plus also preserving the effects of the main product. A similar effect was studied for oregano powder where disinfection of the microbes was done for investigation [52].
Table 2.1 Application of thermal techniques in food industry.
| Technique applied | Food product | Targeted microorganism | Treatment parameters | References |
|---|---|---|---|---|
| Infrared Heating | Rice Powder | Bacteria & moulds | Wavelengths: 3.2, 4.5 and 5.8Time interval: 10, 20 & 30s | [91] |
| Almonds | Enterococcus faecium | Temperature: 70 °CTime: 1 h | [92] | |
| Garlics (shredded) | Aspergillus niger | Wavelength: 3.3 μm | [93] | |
| Oregano | Bacillus cereus | Temperature: 90 °CTime: 10 min | [52] | |
| Microwave Heating | Bay leaves | Counts of bacteria | Power density: 32.14–142.85 W/gTime: 150s | [94] |
| Peanuts | Aspergillus flavus | Power levels: 360, 480 & 600 W | [95] | |
| Infant Formula Mix | Cronobacter sakazakii | Power levels: 800 & 900W | [65] | |
| Ashitaba leaf powder | Colonies count | Power: 300 WTime: 1 h | [96] | |
| Radiofrequency Heating | Broccoli | Bacteria | Heating: 6kW | [74] |
| Barley Grass | Aspergillus and E. coli | Heating: 6kWFrequency: 27.12 MHzGap (electrode): 120-240mm | [96] | |
| Walnuts (shredded) | Staphylococcus aureus | Time: 40 minFrequency: 27 MHzGap (electrode): 19 cm |