Fermentation Microorganisms and Flavor Changes in Fermented Foods

Food fermentation processes often result in profound changes in flavor relative to the starting ingredients. However, fermenting foods are typically very complex ecosystems with active enzyme systems from the ingredient materials interacting with the metabolic activities of the fermentation organisms. Factors such as added salt, particle sizes, temperature, and oxygen levels will also have important effects on the chemistry that occurs during fermentation. This is a brief review of recent research on flavor changes in food fermentations. The emphasis will be on the role of lactic acid bacteria in changing the compounds that help determine the character of fermented foods from plant-based substrates. Introduction Lactic acid bacteria influence the flavor of fermented foods in a variety of ways. In many cases, the most obvious change in a lactic acid fermentation is the production of acid and lowering pH that results in an increase in sourness. Since most of the acid produced in fermentations will be produced by the metabolism of sugars, sweetness will likely decrease as sourness increases. The production of volatile flavor components tends to be the first mechanism considered for the development of flavor specific to a particular fermented food. In addition to this direct mechanism, however, there are less direct ways in which fermentation microorganisms affect flavor. Lowering the pH in lactic acid fermentations may reduce the activity or completely inactivate enzymes in the plant that generate either flavor components or flavor precursor compounds. Finally, the fermentation microorganisms may directly metabolize precursor flavor compounds or flavor components themselves. Some examples of these different flavor modification mechanisms will be given. Prevention of flavor formation Purge-and-trap analysis of the volatile components found in cucumber slurries before and after cucumbers were fermented in a 2% reduced-salt brine. Comparison of volatile components before and after fermentation led to the conclusion that the major effect of the fermentation on flavor volatiles was to prevent enzymatic formation of E,Z-2,6-nonadienal and 2-nonenal by enzymes present in cucumbers (Zhou and McFeeters 1998). These aldehydes are the major compounds responsible for fresh cucumber flavor (Schieberle and others 1990). However, a few days into cucumber fermentation, the pH drops low enough to inactivate the enzymes which form these compounds when cucumber tissue is disrupted. Among the volatile components identified in the fermented cucumbers, only benzaldehyde, ethyl benzene, and o-xylene were not observed in fresh cucumber slurries (Table 1). The lack of the flavor impact of volatile aldehydes is certainly the major effect of the fermentation on flavor. More recently, Marsili and Miller (2000) found a low volatility flavor impact compound in fermented pickled cucumber brines. Addition of saturating salt to brine samples heated to 50 °C, sampling with an SPME (solid-phase microextraction) fiber and followed by GC-olfactometry led to recognition of a compound with an odor close to that of the fermentation brine. The compound with a fermentation brine odor was identified as trans-4-hexenoic acid. They also tentatively identified the presence of cis-4-hexenoic acid. In a reconstitution experiment, a solution that contained 25 ppm trans4-hexenoic acid, 10 ppm phenyl ethyl alcohol, 0.65% lactic acid, 0.05% acetic acid, and 8% NaCl had an odor very similar to that of brine from fermented cucumbers. The concentrations of the lactic acid, acetic acid, and NaCl are reasonable for commercial brines after the completion of fermentation. The addition of phenyl ethyl alcohol gave only a small improvement in the odor match. Thus, the trans-4-hexenoic acid was the key component in the simulated brine solution. Unfortunately, the origin of trans-4-hexenoic acid in fermentation brines is not known. Volatile odor changes caused by lactic acid fermentation In contrast to the cucumber fermentation, where most volatile components did not change substantially, Czerny and Schieberle (2002) observed a number of changes in odorants when whole meal wheat flour was fermented by a commercial sourdough starter culture. Using aroma extract dilution analysis, they identified 14 compounds that had substantial odor intensity in fermented sourdough. All of these odorants were present in dough before and after fermentation, so the fermentation did not result in formation of compounds not present in the flour, nor did fermentation completely remove odor components. However, most of the compounds changed during fermentation, with changes ranging from 7-fold decreases to 9-fold increases in concentration (Table 2). Unsaturated aldehydes present in the flour decreased in the fermentation. As would be expected for heterofermentative lactic acid bacteria, there was a major increase in acetic acid. In addition, components such as 2and 3-methylbutanal and 2and 3-methylbutanoic acid, which form as a result of amino acid degradation, also increased. Flavor precursor formation caused by lactic acid fermentation When sourdough bread is baked, one of the characteristic odor compounds generated in the baking process is 2-acetyl pyrroline, which has a roasted, popcorn-like odor, with an odor threshold of only 20 ppt (Schieberle 1995). The precursor of 2-acetyl pyrroline during baking is ornithine. Ornithine is synthesized from free arginine by heterofermentative lactic acid bacteria. De Angelis and others (2002) have shown that Lactobacillus sanfranciscensis has the enzymes arginine deiminase and ornithine transcarbamoylase, which can convert arginine to ornithine. However, L. sanfranciscenAuthor McFeeters is with the U.S. Dept. of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Dept. of Food Science, NC State Univ., Raleigh, NC 27695-7624 (E-mail: rfm@unity. ncsu.edu).