Food microbiology, the most delicious of all microbiology fields, encompasses many subfields, one of which is fermentation processes. Lots of foodstuffs require microbial metabolic processing to change the chemical composition of food or drink – from fermenting ethanol in alcoholic beverages to lactic acid in cheese and yogurt, microbes contribute a good deal to the everyday foods we eat.
But it’s not as simple as the conversion of one molecule into another: as we’ve recently discussed on this blog, microbes contribute multiple metabolic byproducts that influence the flavor palate. Once a good batch is made, the microbes used for that batch – be it from beer, yogurt, bread, or kombucha – can be used to inoculate the next batch. This is known as the starter culture, and is one way to ensure similar flavor profiles from batch to batch.
New research published in Applied and Environmental Microbiology shows this starter culture can be manipulated to change the flavor profile of a common favorite food – chocolate. After cocoa beans are harvested, they are ground into a pulp, which is then fermented spontaneously, meaning that the final product is influenced by the luck of the local microbiota at the cocoa farm. The mix of cocoa pulp fermenters includes lactic acid bacteria, acetic acid bacteria, and yeasts, which vary in concentration based on fermentation stage.
Previous research has suggested the acetic acid bacteria have little influence on the final flavor, while the aromatic compounds produced by yeast do play a large role. Because of the potential economic impact that a systematic starter culture could have, food scientists have attempted to institute starter cultures to generate a consistent product. They’ve met limited success, however, as wild microbes introduced during the pulping process outgrow to dominate the culture.
Scientists working in Dr. Kevin Verstrepen’s lab hypothesized that novel yeast hybrids would be better suited for starter cultures if the hybrids combined flavor production with thermotolerance. The group had recently learned that withstanding higher temperatures allowed yeast to better compete with the natural microbial fermenters, and they thought changing the metabolic profile of these thermotolerant yeast would allow them to both add flavor and be aggressive and competitive.
To generate yeast with the desired characteristic profile, the researchers took strains of Saccharomyces cerevisiae that had previously been identified as thermotolerant and useful for cocoa pulp fermentation and bred them with S. cerevisiae strains that produce isoamyl acetate or long-chain acetate esters, the compound that confers fruity flavor to beer and wine. Many of the hybrid progeny inherited both characteristics, and three strains were selected for further testing.
Importantly, the hybrids tested were able to establish and outgrow other yeasts found in the fermenting pulp. Within 24 hours, the inoculated hybrid population dominated the fermentation culture, and remained dominant for the rest of the fermentative process – unlike the aromatic parents, which experienced population declines after 48 hours. But dominant growth doesn’t matter if the chocolate product varies little from the original thermotolerant parental strain.
To test fermentation differences, the pulp was further processed into cocoa liquor and analyzed for chemical composition. The isoamyl acetate (fruit flavor) concentration of the liquor correlated strongly with that of lab-scale fermentation experiments, which means food scientists can correctly predict the behavior of these hybrids in a complex, multispecies fermentation environment. Perhaps most importantly, however, an expert panel noted significant differences between the hybrids and their parental strains – describing the hybrid-produced chocolate as having more “fruitiness,” the goal of these manipulated strains.
Natural fermentation has clearly been successful for generations of chocolate makers. However, some of the volatile compounds produced by natural fermentation include isobutyric acid, diacetyl, and acetoin, which confer buttery or even rancid flavors – not what most chocolate-lovers want to taste. This research shows proof-of-principle that, similar to alcohol products, yeast metabolic pathways can be manipulated to produce desired flavors and consistent products. While home-brewing may be less common for cocoa pulp, this nevertheless may allow commercial and DIY candymakers to choose the source of chocolates to mix and match flavor profiles in a creative and consistent way.
-- Julie Wolf