Teorías del Aprendizaje

Predictable response of cells to stressful situations is the cornerstone of the multiple-hurdle concept, whereby organisms are subjected to multiple deleterious compounds or processes designed to reduce or eliminate microbial loads. These stresses occur simultaneously, such as the addition of several antimicrobial agents (acid, bacteriocin, preservative) to a food, or sequentially, such as intervention strategies that occur during processing of muscle foods (acid sprays, hot water rinses). In sequential treatments, there may be more of an opportunity for strains to develop resistances to the antimicro-bial compound or treatment provided that time of exposure allows for such development.

Cells strive to maintain a balance between competing stresses that occur through exposure to sublethal conditions, namely, acidity, temperature, available water, starvation, and desiccation, among others. Generally, organisms survive and function over a wide range of acidity and alkalinity as a result of their ability to maintain homeostasis; however, exposure to adverse environments frequently induces a stress response in susceptible microorganisms. Response to such stresses often takes the form of specific stress proteins induced after sublethal exposure. It is hypothesized that these proteins protect against subsequent exposure to the same stress or possibly cross protect against other stresses (Davidson and Harrison, 2002). Adaptation to sublethal stresses allows organisms to adapt to higher levels of the same stress without being killed. For example, exposure to low concentrations of organic acids would cause organisms to become more resistant to higher concentrations of acids, thus potentially allowing them to survive exposure to commonly used antimicrobial intervention strategies. Depending on the length of time and exposure to mild acid conditions, cells can adapt by means of an acid tolerance response. Exposure to greater quantities of acid (lower pH levels) causes acid shock, which can lead to cell death. During the shock period, stress-induced proteins are formed, which can be specific to the type of shock conditions.

Adaptive responses to sublethal exposure to acids can often offer cross protection to cells when exposed to other stresses. Cross-protective effects have been demonstrated in E. coli between initial exposure to organic acids and subsequent exposure to salt (Garren et al., 1998; Jordan and Davies, 2001), radiation (Buchanan et al., 1999), and heating (Mazzotta, 2001; Ryu and Beuchat, 1998).

The adaptive effect has also translated to survival of microorganisms in acidic foods, such as E. coli O157:H7 in salami and apple cider (Leyer et al., 1995), S. Typhimurium in cheeses (Leyer and

Organic Acids 127 Johnson, 1992), and L. monocytogenes in fermented dairy products (Gahan et al., 1996). In addition, cell survival is affected by its previous growth environment, including oxygen conditions, growth phase, and temperature (Cheng and Kaspar, 1998). Conversely, pretreatment with acids increased sensitivity of S. Typhimurium to chlorine and iodine (Leyer and Johnson, 1997), leading to poten-tially more effective interventions.

Measurement of survival or injury of organisms subjected to stresses is dependent on the methods designed to recover organisms. It is assumed that inhibitory compounds that affect optimal recovery of cells would be avoided. It is interesting that higher survival levels were achieved in acid-injured E. coli when dilutions were made in osmotically stable diluents containing sucrose, sorbitol, glucose, or sodium chloride, but not glycerol (Jordan et al., 1999).

Acid adaptation is also seen in Gram-positive organisms, such as L. monocytogenes (Kroll and Patchett, 1992). Preexposure to mild acid conditions led to increased resistance to higher acid environments such as those encountered in the stomach or macrophage phagosome (O’Driscoll et al., 1996).

Inducement of stress conditions can result in the shortening of processing systems and can optimize the use of multiple hurdles as a preservation process (Brul and Coote, 1999; Brul et al., 2002). Using acid-adapted strains of salmonellae inoculated into lean beef tissue, Dickson and Kunduru (1995) demonstrated that exposure to organic acid rinses did not lead to more resistant organisms. It was further shown (Uyttendaele et al., 2001) that acid-resistant, acid-sensitive, and acid-inducible strains of E. coli O157:H7 presented no difference in their survival when inoculated into beef tissue treated with 1% and 2% buffered lactic acid.

Shadbolt et al. (2001) postulated that exposure of E. coli to stresses such as low water activity (0.90) or low pH (3.5) could produce a biphasic death phase. This was readily apparent if the first stress was low water activity. If the stresses were in reverse order, pH rapidly inactivated cells such that a biphasic death phase was not observed. The authors hypothesized that disruption of cell homeostasis created a large energy drain, thus sensitizing cells to future environmental stresses.

ASSAY

The various acids can be assayed by relatively simple procedures. Methods for determining acid content in foods are found in the Official Methods of Analysis of the Association of Analytical Chemists (Horowitz, 2000) or Handbook of Food Analysis (Nollet, 1996).

Volatile acids can be separated from foods using steam distillation. If only one acid is present, the distillate can be titrated for the particular acid. For a mixture of acids, the distillate is separated on a silicic acid column and the component acids can be identified using enzymatic, chromato-graphic (gas, paper, thin-layer, high-performance liquid chromatography), or electrochemical meth-ods. Qualitative or quantitative methods can be used depending on the degree of sensitivity required.

Often derivatives of the acids are required, which adds to sample preparation time (Gomis and Alonso, 1996).

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