Thermal Food Engineering Operations. NITIN KUMAR

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(also for spores) and oxidant that is capable of generating chemicals that can oxidize hydroxyl radicals (OH˙) [43]. Peroxidases convert to alcohol and water by reducing hydrogen or organic peroxides.

      2.5.2 Osmotic Stress

      The requirement of water in the food system varies and is calculated through the water activity (aw). The addition of solutes in the microbial 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].

      2.6.1 Infrared Heating

       2.6.1.1 Principle and Mechanism

      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].

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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]