Chemical Markers for Processed and Stored Foods - American

Chapter 6

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0631.ch006

Principles and Applications of Chemical Markers of Sterility in High-Temperature— Short-Time Processing of Particulate Foods 1

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Hie-Joon Kim , Irwin A. Taub , Yang-Mun Choi , and Anuradha Prakash 3,4

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U.S. Army Natick Research, Development and Engineering Center, Kansas Street, Natick, MA 01760-5018 Institute of Biotechnology, Korea University, 1 Anam-dong, Sungbuk-ku, Seoul 136-701, Korea Department of Food Science and Technology, Ohio State University, Columbus, OH 43210 2

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Continuous sterilization and aseptic packaging technologies have a great deal of potential to produce shelf-stable foods in convenient packages. A direct measurement of time-temperature history within food particulates is not practical in continuous, high temperature/short time (HTST) processes. The yield of thermally produced compounds offers an alternative as a time temperature integrator and as a chemical marker of sterility. One such a compound, 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one (M-1), is formed at sterilizing temperaturesfromD-glucose or D-fructose and amines through 2,3-enolization under weakly acidic or neutral conditions. Another marker, 4-hydroxy5-methyl-3(2H)-furanone (M-2), is formed similarly from D-ribose or D-ribose-5-phosphate. Application of these compounds to mapping lethality distribution within food particulates in two volumetric heating processes, ohmic heating and microwave sterilization, is demonstrated. Conventional thermal processing, such as retorting, relies on heat transfer from the surrounding heat source, often through a liquid medium, to the center of particulate foods. Therefore, when producing shelf-stable foods, a certain amount of overprocessing takes place by the time commercial sterility is achieved at the cold spot of the food particulates. Such overprocessing could be avoided if the particulates are sterilized by heat generation throughout the volume. 4

Current address: Department of Food Science and Nutrition, Chapman University, Orange, CA 92666 0097-6156/96/0631-0054$15.00/0 © 1996 American Chemical Society Lee and Kim; Chemical Markers for Processed and Stored Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


6. KIMETAL.

Chemical Markers of Sterility in HT-ST Processing 55

Ohmic heating and microwave sterilization are two volumetric heating technologies available to food processors. In ohmic heating, the electrical conductivities of the fluid and the particulates are important parameters (7,2). In microwave sterilization, heat generation depends on the dielectric loss factor of the food materials (3,4). For industrial applications, both ohmic and microwave processes are carried out in a continuous mode. In ohmic heating, foods are continuously pumped through sets of electrodes under high voltage, holding tubes, and cooling tubes, and then aseptically packaged (5). In microwave processing, prepackaged foods are sterilized, under high pressure, with microwaves from magnetrons above and below the foods moving on a conveyer belt (6). 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 paper, we will discuss how thermally produced compounds can be used as chemical markers of sterility in ohmic heating and microwave sterilization. Selection of the Chemical Markers Destruction vs. Formation. When looking for chemical markers of sterility, one is tempted to look for compounds that are destroyed at sterilizing temperatures for the simple reason that the chemical identity and the assay method is already known to die investigator. Several examples were listed by Kim and Taub (7). This approach has a limitation, because a typical chemical 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 compound. For example, the 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 firstorder reaction, are related by eq. (1). k = 2.303/D

(1) 1

Thus, the rate constant for destruction of thiamin is 0.0094 min" at 122°C. For commercial sterility, 5-7 min heating at 121°C is usually required (9). After 5 min heating at 122°C, e"* equals 0.954 and only 5% loss of thiamin will take place, which is difficult to measure accurately. On the other hand, some reactions such as enzyme inactivation are too fast at sterilization temperatures and would be useful only as markers for pasteurization. However, if one were to turn attention to the products of such slow reactions, accurate determination becomes much easier, because one starts with a zero baseline. The product (marker) concentration approaching a limiting value exponentially can be expressed as follows:

Lee and Kim; Chemical Markers for Processed and Stored Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

CHEMICAL MARKERS FOR PROCESSED AND STORED FOODS


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Figure 1 Contour diagram for water-extractable compounds in beef heated for 1, 6, and 15 min at 121°C. The x-axis is chromatographic retention time, the y-axis is uv wavelength, and the z-axis is absorbance.


6. KIM ET AL.

Chemical Markers of Sterility in HT-ST Processing57 M(t)M„ = 1-6-*

(2)

For kt « 1 , e"* can be approximated as 1 - kt and eq. (2) becomes


M(t)/M„ = kt

(3)

which indicates that the marker concentration is directly proportional to the heating time at a given temperature. Particularly interesting possibilities exist in the case where two markers are formed with different rates and activation energies (70). 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 compounds that need to be ruled out include volatiles and unstable intermediates that rapidly undergo subsequent reactions. Preferably, the marker compound should be easily extracted with 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. Figure 1 shows contour diagrams of spectrochromatograms of water-soluble compounds from beef heated for 1, 6, and 15 min at 121°C using pressurized steam. The three-dimensional spectrochromatogram was obtained using anion exclusion chromatographic separation and photodiode array detection (7). It is clear that the compound with elution time of 5.8 min and absorption maximum of 285 nm (M-2)(7) is formed quite rapidly and approaches a limiting value after 15 min heating. The formation of another compound with elution time of 4.2 min and absorption maximum of 298 nm (M-l)(7) is slower and still ongoing after 15 min. These compounds are easily extracted with water and determined by liquid chromatography (7). Spectrophotometric detection at a fixed uv wavelength (285 or 298 nm) could be used for simultaneous determination of both M - l and M-2 without any interference. The heated sample or the extract could be frozen and stored for several days without affecting the analysis. Identification. Purification and identification of M - l and M-2 as 2,3-dihydro3,5-dihydroxy-6-methyl-4(H)-pyran-4-one and 4-hydroxy-5-methyl-3(2H)furanone, respectively, by mass spectrometry have been published (7,11). Analysis of different types of foods heated similarly at sterilizing temperatures revealed that M - l is formed in meats and vegetables and M-2 is formed in meats only. Another compound, 5-hydroxymethylfurfural, appears to be a useful marker in heating fruits andfruitjuices (M-3)(7).


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CHEMICAL MARKERS FOR PROCESSED AND STORED FOODS

Earlier Work and Precursor-Marker Relationship


Both 2,3-dihydro-3,5-dihycfroxy^ (referred to as M - l for convenience) and 4-hy

Chemical Markers for Processed and Stored Foods - American

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