Bacteriology of Spoilage of Fish Muscle

LERKE, PETER (University of California Medical Center, San Francisco), RALPH ADAMS, AND LIONEL FARBER. Bacteriology of spoilage of fish muscle. I. Sterile press juice as a suitable experimental medium. Appl. Microbiol. 11:458-462. 1963.-A sterile raw fish muscle press juice, diluted 1:4 with saline, has been prepared. This dilution greatly facilitated Seitz filtration and affected the spoilage properties of the medium only negligibly. At 5.5 C, the spoilage pattern of naturally contaminated diluted juice was almost identical to that of naturally contaminated fillets. This was shown by comparing the quantitative and qualitative aspects of the bacterial flora on the two substrates and by measuring the production of volatile reducing substances (VRS) and of trimethylamine (TMA). With the sterile raw muscle press juice, some preliminary data showed that individual members of the genera Achromobacter and Pseudomonas differ markedly in their spoilage capabilities: some grew but did not produce spoilage detectable either organoleptically or chemically; others gave rise to strong off odors and to high levels of VRS and TMA. The bacteriology of fish spoilage, the subject of a voluminous literature, has been reviewed by Griffiths (1937) and Shewan (1961). Most of the work reported has largely been concerned with the enumeration and partial identification of the bacteria found on fresh and spoiling fish. Results of standard biochemical tests on the cultures obtained served as a basis for inferences regarding the role of particular genera in spoilage. For example, organisms from spoiling fish, which could grow at low temperatures and exhibited proteolytic activity, were assumed to be responsible for spoilage (Shewan, Hobbs, and Hodgkiss, 1960). Since members of the Achromobacter and Pseudomonas groups predominated during advanced spoilage, it was concluded that these groups cause spoilage. However, it was not known whether all members of these groups are equally active in spoilage or whether other groups found on fish also participate in it. Those who attempted to study spoilage under "natural" conditions encountered difficulties. Most of the work was on whole fish, which consists of several different kinds of tissues. The contaminating flora of freshly caught fish fluctuated widely in kinds and numbers depending on size, location of fishing grounds, and season, thus producing varying patterns of spoilage, even within one species (Shewan, 1961). Under these conditions, the assessment of the role of any one bacterial group in spoilage was understandably difficult. Furthermore, the initial contaminating flora of fish prevented an accurate interpretation of the results obtained after the controlled introduction of specific bacteria. The first attempts at a different approach consisted in observing the effects of pure bacterial cultures Qp. heat-sterilized fish media (Hunter, 1922; Kimata, 1935). Next, odor production by Pseudomonas, Flavobacterium, and Achromobacter was studied on sterile raw fish muscle (Castell and Mapplebeck, 1952; Castell and Greenough, 1957; Castell, Greenough, and Jenkin, 1957; Castell, Greenough, and Dale, 1959; Castell and Greenough, 1959). Subjectivity of the odor descriptions limited the usefulness of the data. Many important questions remain. Are those bacteria that can spoil fish equally active, or do they differ both in the intensity and in the kind of spoilage they cause? Can weak spoilers act concurrently and additively? Can the consecutive activities of nonspoilers result in overt spoilage? What is the distribution of spoilers among the total bacterial population as spoilage progresses? In a series of reports, of which this is the first, the bacteriology of fish muscle spoilage will be discussed with the aim of elucidating the parts played by the bacteria found on raw fish during the course of spoilage, their activities, and the effects of various inhibitors of these activities. The first requirement was a suitable experimental substrate which could duplicate all the changes, subjectively and objectively observable, commonly associated with spoilage of the natural substrate (fish muscle). These include quantitative and qualitative aspects of the bacterial flora on the two substrates, and organoleptic and chemical changes. This substrate also should lend itself to quantitative inoculations to determine the relative activities of the bacteria studied. Finally, the substrate should be readily available and easily sterilizable without heat to preserve the native state of its constituents. Commercially available fillets were eliminated, mainly because of their contaminating bacterial population. 458 on O cber 3, 2017 by gest ht://aem .sm .rg/ D ow nladed fom SPOILAGE IN FISH MUSCLE PRESS JUICE Sterile fish muscle was eliminated because it is difficult to obtain and to inoculate quantitatively. Some sterile pieces were prepared, however, to serve as controls for the other methods. A liquid medium was sought in which bacterial cells could be uniformly distributed and their concentration could be determined with minimal experimental error. It was realized that spoilage normally occurs primarily on the surface of fillets (or whole fish), that the concentration of nutrients in a press juice may differ from that on the surface of a fillet, and that such factors as degree of aerobiosis may also play a role. Nevertheless, a raw fish muscle press juice was chosen as the medium most likely to satisfy the requirements. The feasibility of such a preparation had been shown (Anderson and Fellers, 1949). Fillets of Pacific Coast flat fish, locally called sole, were chosen as a starting material. Their ready availability throughout the year and their low fat content (largely precluding fat spoilage) were determinants in this choice. This paper reports the comparison of the spoilage p!atterns of raw fish muscle and of a press juice prepared from it, and shows that the latter can be used as a suitable experimental substrate for the study of fish muscle