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    The signal-based microbial spoilage on New Zealand export lamb : thesis submitted to Massey University for the degree of Doctor of Philosophy
    (Massey University, 2019) Zhang, Yufeng
    Lamb is one of New Zealand’s primary exports; however, the spoilage of lamb causes considerable financial loss. Psychrotolerant, anaerobically growing spoilage bacteria are responsible for the spoilage of fresh chilled vacuum-packaged lamb exported from New Zealand. These spoilage bacteria interact with through a system called “quorum sensing”. Different types of quorum sensing signals are produced by spoilage bacteria in response to the bacterial cell population. Through this cell-to-cell interaction, the expression of certain genes is regulated, followed by changes in bacterial activity. The main objective of this research was to determine the influence of quorum sensing signals produced by those psychrotolerant, anaerobically growing spoilage bacteria on the spoilage of New Zealand lamb. Quorum sensing of psychrotolerant, anaerobically growing spoilage bacteria was studied using in vitro and ex vivo methods. Two types of in vitro quorum sensing signals (Type-I and Type-II) were identified for New Zealand reference type strains Hafnia alvei and Serratia liquefaciens, and Type-II quorum sensing signal was found from a lamb isolate Carnobacterium divergens. These two types of signals were also discovered in spoiled chilled vacuum-packed lamb. After random EZ-Tn5 transposon mutagenesis to luxI/R-type genes and luxS genes, which are responsible for Type-I and Type-II signals respectively in H. alvei, S. liquefaciens, and C. divergens, wild type and mutant strains were compared. Type-I quorum sensing signaling molecules influenced the expression of lipB that regulates the secretion of hydrolytic enzymes produced by H. alvei and S. liquefaciens. These enzymes are believed to contribute to lamb spoilage. Cinnamaldehyde added to fresh vacuum-packed New Zealand lamb as an inhibitor of quorum sensing enabled an extension of the shelf-life of lamb by 2-8 days, through deactivating the Type-I quorum sensing system. Inhibition of quorum sensing has the potential in lamb preservation.
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    Microbial spoilage of potato top pies : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science, Massey University, Palmerston North
    (Massey University, 1986) Masters, Fiona
    The microbial spoilage of Potato Top pies was investigated to try to provide a view of the events taking place during the spoilage process which renders these pies unsuitable for public consumption. Pies used in this study were obtained from a commercial pie manufacturer and were stored at 4°c, 25c and 37°c, and the effects of storage at these temperatures studied. During the examination biochemical techniques were used in conjunction with microbiological methods. Aspects gained from this study can be summarised as below: . Major flora of all pie component parts (meat filling, pastry surround, potato topping) of freshly cooked pies and of pies stored at the three temperatures consisted primarily of gram positive rods and cocci. The origins of these organisms could be traced directly back to various stages of manufacture. . Within 24-36 hours of storage at 25deg c and 37°c numbers of gram positive organisms could reach above 107/gram of pie component. . No obvious organoleptic spoilage took place. Lack of off odours (NH 3; H2S); the presence of amylase producing Bacillaceae; pH decreases in spoiled pies - suggested that the utilisation of low molecular weight compounds (such as glucose) resulted in saccharolytic spoilage of the pies. . Bacterial numbers in the pies stored at 4°c did not rise above 105/gram within 28 days of storage. . Isolated dominant bacterial species were identified as Bacillus, ,, Streptococcus, Staphylococcus and Micrococcus,with the Bacillus., species and Streptococcus species inhibiting the growth of the other organisms.
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    The development of hotcake products with reduced staling and reduction of microbiological growth : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand.
    (Massey University, 2012) Zhang, Enchong
    Staling and microbiological spoilage are major issues in the market development of hotcake products. This project is aiming at reduce the staling rate of hotcake product during storage and review the methods that could be effective in reducing microbiological spoilage of hotcakes at ambient temperature. The staling rate was reduced by incorporation of anti-staling ingredients into the formulation. A combination of anti-staling ingredients including Dimodan PH 320/B-M, a distilled monoglyceride; DATEM Palsgaard 3502, a Diacetyl Tartaric Acid Ester of Mono- and Diglycerides; and also Novamyl 10000 BG, a bake stable alpha amylase was effective to reduce the staling rate of hotcake when incorporate them into the hotcake formulation. The staling rate of hotcake products was reduced from 0.14N/day to 0.085N/day in commercial trial. In addition, the sensory results indicated the customers can not perceive a stale hotcake for the new formulation developed in this research and they also can not perceive the changes between original formulation and the new formulation. Two applicable antimicrobial spoilage approaches were used; these were to increase the level of calcium propionate preservative and to reduce the oxygen content level to below 1% using O2 absorber or 100% CO2 in the packaging. The commercial trial showed decreasing the oxygen content level to less than 1% in the packaging and increasing the level of preservatives increased the shelf life by 1 or 2 days under the ambient storage condition used.