Evaluation of abilities of Pleurotus pulmonarius crude extract as a natural food additive in roasted chilli paste.

Abstract

The purpose of this study was to determine the effectiveness of the crude extracts in controlling Staphylococcus aureus and Bacillus cereus, as well as the shelf life of the roasted chilli paste. Crude extract was obtained from the volva of Indian oyster mushrooms (Pleurotus pulmonarius) and used to replace preservatives in roasted chilli paste. Thirty isolates of S. aureus and B. cereus were inhibited by crude extract through the disc diffusion method. At a concentration of 2,500 µg/ml, the crude extract inhibited 23 (76.67%) and 20 (66.67%) isolates of S. aureus and B. cereus, respectively, with an inhibition zone size of 7-10 mm. The broth microdilution method was employed in this study to determine the Minimum Inhibitory Concentration (MIC) of the crude extract, and it was observed that the extract at a concentration of 1,000 µg/ml was able to inhibit 18 (60.00%) and 16 (53.00%) isolates of S. aureus and B. cereus, respectively. For the purpose of Minimum Bactericidal Concentration (MBC), the crude extract at a concentration of 1,000 µg/ml was found to kill 16 (53.00%) and 14 (46.00%) isolates of S. aureus and B. cereus, respectively. At a concentration of 4,000 µg/ml, the crude extract had an inhibitory effect from hour 6, lowering S. aureus (Sa16 and Sa21) and B. cereus (Bc5 and Bc12) below the limit of detection (less than 2 log cfu/ml) at 24 h. The ability of the crude extract to inhibit both types of bacteria was observed to be unaffected by heat treatment to a temperature of 65ºC for 30 min (pasteurisation) as well as by no heat treatment at all. The phenolic compounds of the crude extract treated by pasteurisation and without heat treatment were higher than that of the crude extract treated by sterilisation at 121ºC for 15 min. The quality and safety of roasted chilli paste were analysed during storage at 25ºC for 8 months. The quality and safety of roasted chilli paste with crude extract were within the acceptability limit of water activity, total viable count, mould, S. aureus, B. cereus, C. perfringens, and Salmonella spp. as recommended by Thai community product standard. The ability of crude extracts to inhibit S. aureus (Sa16) and B. cereus (Bc5) in roasted chilli paste was then investigated, and it was observed that the crude extract at a concentration of 4,000 µg/ml was able to reduce the S. aureus (Sa16) and B. cereus (Bc5) population to 1.24 and 2.52 log cfu/g, respectively, at 8 months. Therefore, the use of crude extract from Indian oyster mushrooms in conjunction with heat pasteurisation can be employed to improve the shelf life of roasted chilli paste.