tailieunhanh - Advanced Topics in Mass Transfer Part 9

Tham khảo tài liệu 'advanced topics in mass transfer part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Mass Transfer Mechanisms during Dehydration of Vegetable Food Traditional and Innovative Approach 309 Fig. 1. Stability map of food as a function of water activity from Schmidt 2004 instance the out-growth of Clostridium botulinum spores one of the most dangerous pathogen microorganisms is inhibited at aw values Clavero et al. 2000 . The generation times of Listeria monocytogenes at aw values of and were reported respectively as h and h in systems at pH . Colatoni Magri 1997 reported that the minimum value for the Clostridium perfringens spore germination is . Beuchat 1985 reported that Staphylococcus aureus which is very tolerant to aw values shows a minimum for toxin production of and a minimum of for growth. In table 1 are reported the minimum aw values for some of the most important microorganisms for food degradation. It should be taken into account that the threshold reported in table 1 are often measured in model system with specific environmental conditions. In vegetable foods the minimum aw values for growth or toxin production of each microorganism must be considered as range of ranges of aw values inside which the true limiting value may change when other inhibitory factors are applied. In fact it should be considered that the resistance of microorganisms on aw values is affected by several parameters such as pH temperature oxygen concentration preservatives solutes etc. For instance the type and the amount of solutes in food their chemical composition greatly affect the tolerance of microorganisms on water activity. This is because microorganisms have different levels of ability to adapt at an hypertonic environment regaining turgor pressure and excluding certain incompatible solutes Kang et al. 1969 Gould Measures 1977 Christian 1981 Buchanan Bagi 1997 Lenovich 1985 Gould 1985 . For instance it was reported that the limiting aw values for several microorganisms is lower when sodium chloride rather than .