Chemical markers of sterility

Kim, H.-J. Taub, I.A. Choi, Y.-M. Prakash, A. Principles and applications of chemical markers of sterility in HTST processing of particulate foods. ACS Symposium Series 631, American Chemical Society, Washington, DC, 1996b. 

Principles and Applications of Chemical Markers of Sterility in H h-Temperature— Short-Time Processing of Particulate Foods... 

In either case, the time-temperature measurement within the moving food particulates is difficult, and consequently assuring commercial sterility without overprocessing is not a straightforward matter. In this p ier, we will discuss how thermally produced compoimds can be used as chemical markers of sterility in ohmic heating and microwave sterilization. 

Destruction vs. Formation. When looking for chemical markers of sterility, one is tempted to look for compounds that are destroyed at sterilizing temperatures for tire simple reason that the chenucal identity and the assay method is already known to the investigator. Several examples were listed by Kim and Taub (7). This approach has a limitation, because a typical chenucal reaction in foods is much slower than bacterial destruction at high temperatures and one has to be able to measure a small loss of the compoimd. For example, tire D-value (time required to reduce the concentration by 90%) for destruction of thiamin is 244 min at 122°C (8). The D-value for destruction of B. stearothermophilus is about 1 min at the same temperature. The D-value and k, the rate constant for a first-order reaction, are related by eq. (1). 

Ease of Detection and Stability. Numerous compounds are thermally produced in foods, but not all are suitable as chemical markers of sterility. Some of the compoimds that need to be ruled out include volatiles and unstable intermediates that rapidly undergo subsequent reactions. Preferably, the marker compound should be easily extracted wi an aqueous solvent and easily determined without many additional operations. The marker should also be stable during analysis. In situ analysis would be ideal however, accurate quantitation by simple in situ methods, such as surface fluorescence or near infrared measurements, is questionable. 

The reported chemical markers are useful markers of sterility, which is an important quality index in shelf-stable foods. The use of the markers to map lethality distribution in particulate foods has been demonstrated. The markers can be used for validating and optimizing new thermal processing technologies such as ohmic heating and microwave sterilization. 

The chemometric principle was used to derive a guideline for obtaining a simple yes or no answer about the sterility of food particulates heated at aseptic processing temperatures. A quadratic temperature pulse model was used to estimate bacterial destruction from the fractional yield of thermally produced chemical marker compounds (2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one, M-1, and 4-hydroxy-5-methyl-3(2H)-furanone, M-2) and the rate constants and the activation energies of the chemical and bacterial systems. The model yielded a conservative estimate of lethality at the center of meat-balls heated under different time-temperature conditions. A scheme for determining the minimum marker yield for a designated F -value is provided. 

Several chemical marker compounds formed at sterilizing temperatures from precursor compounds inherently present in the foods have been reported (Kim and Taub, 1993 Kim et al, 1994). In order to apply these chemical markers to real processing conditions, where lethality accumulates under nonisothermal conditions, accurate informations about the reaction rate constants and their temperature dependence (activation energy or Z-value) are needed. The variability of the precursor concentration within the particulates and among different particulates also needs to be considered carefully. 

---