Known acid-tolerant organisms

Listeria monocytogenes is another major concern to the food industry, due to its inherent resistance to several preservation practices and the high mortality rate of listeriosis in susceptible populations. Of particular concern is its ability to grow at refrigeration temperatures and on dry surfaces, and to tolerate acidic conditions (Barker and Park, 2001). 

A significant ATR has been found in S. typhimurium, following culture in acidic environments (pH 3.0). Modifications in the membrane fatty acid composition of S. typhimurium, however, have not been found dependent on the pH or the acid used for acidification (Alvarez-Ordonez et al., 2009). Shigella is another organism known to survive for extended periods in unfavorable conditions such as high acid or high temperature (Lin et al., 1995 Tetteh and Beuchat, 2003). 

Heavy losses are experienced in the feed and beverage industry because of certain yeasts being able to grow at low ambient pH in the presence of weak-acid preservatives (Macpherson et al., 2005). The yeast Z. bailii is able to grow at pH as low as 2.2, although it is known that these abilities can be affected by the acidification agent and the solute used to reduce the aw, as well as by other solutes present in food (Thomas and Davenport, 1985 Lenovich et al., 1988). 

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Synthesis of compatible solutes and production of stress-tolerant plants The cellular response of salt- and drought-tolerant organisms to both long- and short-term sah-nity stresses includes the synthesis and accumulation of a class of osmoprotective compounds known as compatible solutes. These relatively small, organic osmolytes include amino acids and derivatives, polyols and sugars, and methylamines. The osmolytes stabilize proteins and cellular stractures, and can also increase the osmotic pressure of the ceU [45]. This response... 

In 1991, Cottier et al. classified the acid catalysts, known to be active for the production of HMF, in five groups organic acids, inorganic acids, organic and inorganic salts, Lewis acids, and others [60]. As reported in Table 1, heterogeneous catalysts are rarely used for the production of HMF mainly due to the difficulty to design a water-tolerant solid catalyst. 

The demand for environmentally friendly chemistry and its widespread applicability have made water an increasingly popnlar solvent for organic transformations. Mixtures of water and other solvents snch as tetrahydrofnran are now commonly anployed for a number of organic transformations. For instance, the Lewis acid catalysed aldol reaction of silyl enol ethers, commonly known as the Mnkaiyama aldol reaction, which was firstly reported in the early seventies, can be carried ont in snch media. With titanium tetrachloride as the catalyst this reaction proceeds regioselectively in high yields, but the reaction has to be carried ont strictly nnder non-aqneons conditions in order to prevent decomposition of the catalyst and hydrolysis of the sUyl enol ethCTS. In the absence of the catalyst it was observed that water had a beneficial influence on this process (Table 4, entry D) . Nevertheless, the yields in the nncatalysed version WCTe still unsatisfactory. Improved results were obtained with water-tolerant Lewis acids. The first reported example for Lewis acid catalysis in aqueous media is the hydroxymethylation of silyl enol ethers with commercial formaldehyde solution using lanthanide trillates. In the meantime, the influence of several lanthanide triflates in cross-aldol reactions of various aldehydes was examined " " ". The reactions were most effectively carried out in 1 9 mixtures of water and tetrahydrofnran with 5-10% Yb(OTf)3, which can be reused after completion of the reaction (Table 19, entry A). Although the realization of this reaction is quite simple, the choice of the solvent is crucial (Table 20). 

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