Applied processing

Common treatments or processes that can be applied to kill microorganisms or retard their growth include chilling, freezing, heating, dehydration or a combination of these processes, such as occurs during extrusion. The effects of chilling and freezing on micro-organisms are to retard the biochemical processes that occur within the cell and that are necessary for metabolism or growth. The application of low temperature has been discussed earlier in this chapter. Equally, the application of dehydration processes changes the water activity of the system in which the micro-organisms are found and the effects of water activity on micro-organisms have also been explained earlier in this chapter.

The effects of heating on micro-organisms result in damage to the cell. Typically, damage occurs at a number of sites on the cell membrane or the cell wall, or within the cell; in order for a cell to die a large number of sites must be damaged irreparably. If the cell is not killed and only a fraction of these sites have been damaged then the cell is able to repair this damage (resuscitation). This must be considered in all methods of enumeration of heat-treated micro-organisms since many micro-organisms cannot form colonies on agar plates that contain some selective components. Accordingly enumeration of micro-organisms on these agar plates might give deceptively low numbers of organisms. Equally, once the micro-organisms have resucitated they are free to grow within the system. Therefore, failure to enumerate sub-lethally injured micro-organisms that have not sufficiently resuscitated to grow on conventional microbiological culture medium can provide a false sense of security as these organisms can resuscitate and subsequently grow and produce spoilage of the system or result in infection.

The effect of heat on bacteria is modelled through the well known system of D and Z values. The D value is derived from the gradient of the line of the number of micro-organisms declining with time of exposure to a given heat process (Fig. 6.3). The D value is defined as the time required for a ten-fold reduction in the number of viable micro-organisms. The D values can be extracted from such a primary model and be plotted against their corresponding temperature. This creates a secondary model where the gradient of the line relates to the influence of temperature on the D value. The gradient of this line can be used to derive the Z value, which is defined as the temperature change required for a ten-fold reduction in the D value (Fig. 6.4).

Primary model

Primary model

Fig. 6.3 A representation of a primary model of the effect of time on the change in numbers of micro-organisms which can be used to derive the D value when modelling the inactivation of micro-organisms at a range of temperatures.

Fig. 6.4 A representation of a secondary model of the effect of temperature on D value which can be used to derive the Z value when modelling the inactivation of micro-organisms.

Fig. 6.4 A representation of a secondary model of the effect of temperature on D value which can be used to derive the Z value when modelling the inactivation of micro-organisms.

An additional preservation method that can sometimes be applied to co-products is the application of pressure. This may involve the use of high-pressure hyperbaric systems or lower pressure extrusion systems. However, the extent of pressure required to cause inactivation of micro-organisms is really very high, largely because bacterial spores are extremely resistant. For example spores of Clostridium sporogenes may require pressures of 680 MPa for one hour at ambient temperature to achieve a 5-log reduction (Crawford et al., 1996). However, other workers have found that for a similar reduction in numbers, in excess of 1000 MPa were required and even at 1500 MPa a reduction of only 1.5 log cycles of spores was achieved Rovere (1996). The most serious form of food poisoning is that associated with toxin production by Clostridium botulinum. Therefore, if there is a danger of this organism being present within co-products in the form of spores then these spores must be eliminated if there is a subsequent chance that they would germinate, and form cells that would then produce toxins. However, spores of this organism are among the most pressure tolerant. Non-proteolytic C. botulinum type E required about 830 MPa at 40 °C for 10 minutes to effect a 5-log reduction in number and about 230 MPa at 50 °C (Reddy et al., 1999). Spores of proteolytic C. botulinum type A required over 800 MPa at up to 88 °C for 9 minutes to effect a decrease of 3 log cycles (Reddy et al., 2003). However, in co-product streams we must consider not just harmful effects of organisms but spoilage effects, and the spores of some spoilage micro-organisms have even greater resistance to pressure and heat than C. botulinum. It is considered that the most resistant organism is Bacillus amyloliquefaciens. This forms highly pressure resistant spores and it has been suggested that this be adopted as the target organism for the development of ultra-high-pressure processes (Margosch et al., 2004).

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