Microbial inactivation

Two other broad areas of food preservation have been studied with the objective of developing predictive models. En2yme inactivation by heat has been subjected to mathematical modeling in a manner similar to microbial inactivation. Chemical deterioration mechanisms have been studied to allow the prediction of shelf life, particularly the shelf life of foods susceptible to nonen2ymatic browning and Hpid oxidation. 

Orth has applied the concepts of the D-value as used in sterilization technology (Chapter 20) to the interpretation of challenge testing. E q)iessing of the rate of microbial inactivation in a preserved system in terms of a D-value enables estimation of the nominal time to aehieve a prescribed proportionate level of kill. Problems arise when trying to prediet the behaviour of very low levels of survivors, and the method has its detractors as well as its advocates. 

Nowadays, supercritical C02 is used in numerous technological processes for the production or treatment of new materials. Among others, the food industry is seeking new alternative techniques to protect and preserve foods. The use of C02, a non-inflammable, atoxic, relatively inexpensive gas, under supercritical conditions, could become relevant in this field. In fact, it has been demonstrated that the effect of microbial inactivation, assuring healthy food preservation is already consistent at more moderate pressures (lower than 200 bar) than those employed by traditional hydrostatic-pressure HPT methods. 

Bacterial inactivation is achieved by CO2 absorption in the liquid phase, even though the reason why it happens is still not clear. In this respect, batch- and semi-continuous operating modes are substantially different. In the batch system the residence time, i.e., the time of contact between gas- and liquid phase, must be sufficient to allow the diffusion of CO2 in the liquid, and is therefore a fundamental parameter to assure a desired efficiency. In the semi-continuous system the contact between the phases is localized in the surface of the moving micro-bubbles. In this second case, the efficiency of the process is influenced by temperature, pressure, gas flux, bubble diameter, and other parameters that modify the value of the mass-transfer coefficient. Therefore, it is not correct to use the residence time as a key parameter in the semi-continuous process. In fact, a remarkable microbial inactivation is reached even with an exposure time of 0 min (i.e., pressurizing and immediately depressurizing the system) these two steps are sufficient to allow CO2 to diffuse through the liquid phase. 

S.I. Hong et al. [42] confirmed that the inactivation rate increased with pressure, exposuretime and with decreasing pH of media. They stated that microbial inactivation was governed essentially by penetration of CO2 into cells, and its effectiveness could be improved by enhancing the transfer rate. Microbial reduction of more than six powers of ten occurred within 30 min, under a CO2 pressure of 2000 psi at 30°C. The authors hypothesized that cell death resulted from the lowered intracellular pH and damage to the cell membrane owing to penetration of CO2. 

A sourdough yeast strain (S. cerevisiae) and two bacterial species (S. marcescens, B. subtilis) were treated in a physiological buffer (PBS).The first experiments (with the yeast) were performed in a batch apparatus (Fig. 10) as well as in a semi-continuous apparatus (Fig. 12). As the latter device showed a higher efficiency in the microbial inactivation than the... 

HTP has already become an alternative method to UHT and it is used nowadays for commercial production. It allows a total microbial inactivation, by operating with a relatively low temperature, and retaining unchanged the delightful tastes and smells of foodstuffs. Nevertheless, the cost of such a process is still high because of the extreme range of operating-pressure values required, and therefore it is not yet economically competitive with the traditional processes. 

Procedures aimed at reducing or detoxifying aflatoxins and/or their effects have been reviewed by Phillips et al. (35), and include technological procedures for food and feeds and chemical degradation, as well as biocontrol and microbial inactivation, dietary modification and chemoprotection, and reduction in toxin bioavailability via selective chemisorption with clay. 

Haynes, R.H. (1966). The interpretation of microbial inactivation and recovery phenomena. Radiation Res. Suppl. 6,1-29. 

The quantity of EPS produced after fermentation was determined through dry weight measurements. The fermentation broth was heated at 80° 1°C for 10 min, to ensure microbial inactivation. A filtration was then conducted to remove the cells. To precipitate the EPS, ethanol P.A. (3 1) was added to the fermented broth. After total precipitation of the EPS present in the medium, the mixture (broth plus ethanol) was filtered through a 0.2-pm Millipore membrane using a Gouche crucible previously weighed. The obtained product was dried at 80 1°C until constant weight. All determinations were done in triplicate. 

Thiebaud, M., Dumay, E., Picart, L., Guiraud, J.P., Cheftel, J.C. 2003. High-pressure homogenization of raw bovine milk. Effects on fat globule size distribution and microbial inactivation. Int. Dairy J. 13, 427 439. 

For instance, if ampules of aqueous products were to be sterilized in a hot air oven, the mechanisms of microbial inactivation would still be by coagulation of intracellular proteins. However, heat transfer from hot air is much slower than heat transfer from steam, which is why this is not seen as a practical process. Microwave irradiation could be an alternative means of sterilizing aqueous pharmaceutical products utilizing the same antimicrobial mechanisms as steam certainly there is evidence that microwave killing patterns are mainly due to heat transfer with very little direct energy being absorbed from the microwaves. 

Even though the definition of sterility is an absolute condition, the effectiveness of the sterilization process can be determined by measuring the reduction of microbial population. Such measurements reveal the kinetics of microbial inactivation, and it is from the exponential nature of inactivation that the concept of sterility assurance level (SAL) is derived. This value... 

Chlorine contact time must be long enough to achieve the desired degree of microbial inactivation (i.e., attain the CT value that applies to the concentration of chlorine and the pH and water temperature). 

Singh, J. P. and Ghaly, A. E. (2006). Reduced fouling and enhanced microbial inactivation during online sterilisation of cheese whey using a UV coil reactor. Bioprocess Biosyst. Eng. 24, 269-281. 

Pressure affects a reaction system in two ways (1) reduction of the available molecular space, which has to do with conformation, and (2) increase of intrachain reactions (Hoover et al., 1989), which affect the d)mamics and reactions of biomolecules. The effect of HP on microbial inactivation, chemical or enzymatic reactions, and structural and /or functional properties of foods is based on the principle of Le Chatelier-Braun and on the State Transition Theory. 

An alfemative way to model microbial inactivation by means of HP nof using the first order kinetics model is the model of Fermi. This model demonsfrates fhat population decreases because of exposure fo lefhal agents such as heat, electric pulses, and radiation or ozone doses. The equation of Fermi relates the fraction of survivors S(P), fhe pressure applied P (MPa), the critical pressure Pc at which the survivors fraction is 0.5, and the constant k that indicates the rate at which inactivation is occurring (Palou et al., 1998). 

Cheftel, J.C. Review high-pressure, microbial inactivation and food preservation, Int. Food Sci. and Technol, 1, 75,1995. 

Microbial Inactivation in Aqueous Systems for Food and Pharmaceutical Processing... 

Spilimbergo S, Elvassore N, Bertucco A. Microbial inactivation by high-pressure. J Supercrit Fluid 2002 22 55-63. 

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