The right bacteria can strengthen the flavor of cheeses.
A GOUDA OR A CHEDDAR TAKES ON its full gustatory quality only after several months, and many cheeses, even ones that have been aged for a long time, do not have the powerful flavor that one might want. The aging of cheese has been a lively topic of debate in the gastronomic world for centuries. The milk that is curdled and seeded with lactic bacteria acidifies in the course of maturing: The transformation of the milk sugar lactose into lactic acid prevents contamination by pathogenic microorganisms, and the lactic bacteria release aromatic compounds that contribute to the taste of the cheese. Mireille Yvon and her colleagues at the Institut National de la Recherche Agronomique (INRA) station in Jouy-en-Josas have studied which strains of lactic bacteria produce the greatest quantity of these aromatic compounds.
The INRA biochemists knew that the flavor patiently acquired by cheeses results from the microorganisms responsible for maturing. Fats and sugars are progressively transformed, and proteins are dissociated into their constituent elements, amino acids, which are then transformed into aromatic molecules. For example, the amino acids leucine and valine produce compounds having a cheese note, whereas phenylalanine, tyrosine, and tryptophan are the precursors of floral and phenolic notes (as they are described by trained tasters).
Does the slow dissociation of proteins into amino acids limit the formation of aromatic compounds? No, for the direct addition of free amino acids does not improve the taste of cheeses any more than does the seeding of milk by lactic bacteria (whose capacity for dissociating proteins has been increased by genetic engineering). In the latter case, the amino acids are released in greater quantities, but the taste is not changed. In the early 1990s, Yvon and her team concluded instead that what limits the development of flavor is the transformation of amino acids into aromatic compounds.
Stimulating Additions
Two biochemists in the Netherlands, W. Engels and S. Visser at Wageningen University, noticed that the flavors typical of Gouda were obtained when methionine was added to lactic bacteria (without milk) and went on to identify two enzymes that seemed to be responsible for the phenomenon. Yvon and her colleagues, for their part, had observed in vitro that lactic bacteria degrade certain amino acids to form aromatic compounds such as aldehydes and carboxylic acids. The first step in this transformation, known as transamination, involves a reaction between an amino acid and a molecule named ketoglutarate, which produces a keto acid in addition to glutamate (a molecule that, as we have seen, is used as a flavor enhancer in Asian cuisine and in many commercial products because it communicates the taste that we know today as umami). During transamination, an amine group (–NH2)is converted from an amino acid to a keto acid, which is then chemically modified and transformed into aromatic compounds.
In 1997, the biochemists at Jouy succeeded in purifying and characterizing aminotransferase, the enzyme in the lactic bacterium Lactococcus lactis, which is responsible for the transamination of leucine and methionine. Nonetheless, under actual aging conditions, the presence of this enzyme does not significantly affect the aromatic quality of cheeses. Why? Was the diffusion of reactive agents in the lactic bacteria too slow? Did the lactic bacteria lack the molecules necessary for them to receive the amine groups?
The INRA researchers tested the second hypothesis by seeding warm (pasteurized) milk with lactic bacteria and then adding rennet (which curdles the milk and transforms it into cheese). In this way they obtained a curd that they then molded and pressed and finally immersed in a brine enriched with ketoglutarate. They followed the transformation of the amino acids during the aging process, and a panel of tasters analyzed the development of the cheese’s odor.
In the control cheeses, which had not been enriched by ketoglutarate, very few amino acids were dissociated, and the odor was weak. By contrast, the addition of ketoglutarate augmented the transformation of several amino acids. The transformation of ketoglutarate led to the formation of powerfully aromatic compounds, such as isovalerate in the case of leucine and benzaldehyde in the case of phenylalanine, demonstrating that the odor of the cheeses was increased by the addition of ketoglutarate. Similar results were obtained for Cheddars, tested for comparative purposes.
In Search of Efficient Microorganisms
While they were studying the effects of adding ketoglutarate, the INRA biochemists observed that the glutamate produced during transamination is transformed by an enzyme produced by other bacteria in ketoglutarate. Because glutamate is abundant in milk, even before aging, they had the idea of introducing the gene for dehydrogenase glutamate, the enzyme they had discovered in a lactic bacterium, which they suspected would produce ketoglutarate from glutamate.
The effects of introducing this gene were followed in vitro and in a control cheese. The modified lactic bacteria were found to trigger the transamination of the amino acids no less completely than lactic bacteria to which ketoglutarate had been added. What is more, lactic bacteria containing the dehydrogenase glutamate gene produced more highly aromatic carboxylic acids.
It is clear, then, that fortified bacteria can be used to make better cheeses. The problem is that genetically modified organisms are not universally accepted by consumers. Therefore biochemists are searching for strains of lactic bacteria that naturally produce dehydrogenase glutamate. In this case, at least, genetically modified organisms will have served as a research tool. Is this not one of their chief advantages?