Comparison of Spoilage Degree of Seafoods Inoculated with Yeasts or Bacteria Singly

Abstract

Sterile seafood media were prepared by autoclaving fish muscle homogenate to which an equivalent amount of saline had been added. Common mackerel (Scomber japonicus), black tiger shrimp (Penaeus monodori) and neon flying squid (Ommastraphes bartrami) were used to prepare growth media. In order to compare the spoilage patterns of yeasts and bacteria, 5 yeast species and 4 bacterial species isolated from seafoods were inoculated at 105 cells per gram into the 3 kinds of seafood media, and incubated at temperatures of 5, 15 and 25° C for up to 2 weeks. Sensory signs of spoilage such as softening, odor or slime were observed, the pH values were taken after 2, 4 and 7 days of incubation, and total volatile basic nitrogen (TVB-N) values were determined according to the microdiffusion method of Conway after 2 weeks of incubation. Results by yeast strains. (1) Candida lipolytica: The muscle tissue of the mackerel medium incubated at 25°C began to soften after 2 days (pH6.13), and was damaged after 4 days (pH 6.91). Ammoniacal odor and slime were produced after a week (pH 7.93), and a strong putrid smell was generated after 2 weeks (pH 8.27). The TVB-N value in the mackerel medium at 25°C amounted to 634 mg-N/100g. The production of TVB-N was highest by C. lipolytica among the 5 yeast species tested. (2) Trichosporon pullulans: This strain showed an optimum temperature of 15°C, giving higher values of TVB-N at 15°C than at 25°C. The TVB-N value in the mackerel medium at 25°C was 121.3 mg-N/100 g. (3) Tr. cutaneum: This strain affected the 3 kinds of media to almost the same degree. A strong ammoniacal odor was detected in the squid medium at 25°C (pH 8.20, TVB-N of 116.7 mg-N/100 g). (4) Cryptococcus laurentii: This strain especially affected the squid media. (5) Candida sake: Although the TVB-N values were the lowest among the 5 yeast species, the values in the squid medium at 15 and 25°C exceeded the spoilage value at an early stage— 30 mg-N/100 g. Results by the bacterial strains. (1) Pseudomonas fluorescens: The spoilage patterns and the amounts of TVB-N produced were almost similar to those in the case of C. lipolytica. However, the values at 5°C were higher than those of C. lipolytica. (2) Bacillus sub-tills: The spoilage patterns were similar to those of Tr. cutaneum. (3) Alcaligenes sp. © 1987, Japanese Society for Food Hygiene and Safety. All rights reserved.