Please use this identifier to cite or link to this item: https://scholarbank.nus.edu.sg/handle/10635/153218
Title: MICROBIAL STUDY OF EXTENDED SHELF LIFE SYSTEM FOR FOODS
Authors: MATHEW LAU THYE NGAK
Issue Date: 2001
Citation: MATHEW LAU THYE NGAK (2001). MICROBIAL STUDY OF EXTENDED SHELF LIFE SYSTEM FOR FOODS. ScholarBank@NUS Repository.
Abstract: Ready to eat salads do not undergo heat treatment, hence sanitization is the key first step in the elimination or reduction of pathogens or spoilage bacteria. The salad studied in this project is a coleslaw salad with a mix of cabbage (Brassica oleracea), carrot (Daucus carota), onion (Allium cepa), sugar and mayonnaise. A total of 186 sets of salad samples were evaluated for the efficacy of sanitizer on salad preparation. The vegetables were trimmed and cored after an initial peel/removal of the outermost layers or leaves. The bacterial flora isolated from the raw cabbage were, Aeromonas cevae, Brevedimonas vesicularis, Burkholderia cepacia, Chryseonomas indologenes, C. luteola, Lactobacillus spp., Pseudomonas flourescens and P. putida. Pseudomonads comprised up to 80% of the bacterial population found. No bacterial pathogens were detected in the samples tested; yeast and molds were rarely found. The processed salad items were subsequently quartered and soaked for 3 minutes in a sanitizer wash tank at 4 °C, diced before mixing and packing. Sanitizers studied in this project were sodium hypochlorite, hydrogen peroxide and peroxyacetic acid (PAA) based sanitizers. It was found that there was a significant difference (p<0.01) in Log reduction in the presence of 100 ppm and 150 ppm free chlorine. There were reductions of 2.19 and 2.37 Log CFU/g when sanitized with 100 and 150 ppm free chlorine, respectively. There were increases in bacterial count ranging from 0.52- 0.91 and 0.33- 0.63 Log CFU/g during processing (dicing, mixing and packing) for 100 ppm and 150 ppm treated salad mix respectively. It was noted that Pseudomonas constitutes 98% of the remaining flora after sanitization. Amongst the 3 sanitizers used, PAA showed highest reduction of bacterial flora of 2.4 Log CFU/g. Salad isolate, B. cepacia was inoculated onto cabbage leaves before treating with hypochlorite, hydrogen peroxide or PAA. Under Scanning Electron Microscopy bacterial biofilm was observed on non-sanitized cabbage leaves. However, severe membrane damage was noted on treated B. cepacia. It was noted that bacteria in biofilm were less susceptible to membrane damage by the sanitizers. Physical evidence of bacterial entry into plant leaves via the stomata and their presence amongst the leaf cells was observed. PAA was shown to be a more effective sanitizer when the cross section study using Confocal Laser Scanning Microscope showed that bacterial cells within biofilm sited in the deeper parts of the biofilm was not affected by hypochlorite sanitizers but was inactivated by PAA.
URI: https://scholarbank.nus.edu.sg/handle/10635/153218
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