Biomimetic Investigation of Coffee Flavor: A Wake Up Call for Reductionism

Food science tends to take a reductionist approach to evaluating complex foodstuffs, analyzing individual components and extrapolating the results to obtain a comprehensive, if somewhat artificial, understanding of the entire product. However, as is true for other scientific enterprises as well, this can result in an obscured impression of reality. For this reason, many scientists strive to develop experiments that allow them to probe as realistic a simulation as possible. In regards to biologically oriented fields, the term “biomimetics” has been coined to encapsulate such technologies. In their article investigating the aromatic profile of roasted coffee beans, Poisson et al employ biomimetic “in-bean” experiments to gain a more accurate understanding of the formation of odorant molecules during roasting (article abstract). Although previous studies have investigated flavor formation in coffee, the vast majority of these have been conducted using vastly simplified model systems that may not accurately reflect the normal processes occurring inside coffee beans. Studies comparing the flavor profiles of beans roasted whole to those ground before roasting have demonstrated the indispensability of the whole bean environment to normal flavor formation.

Poisson et al utilize a relatively new approach to circumvent the insufficiencies faced by experiments based on model systems. They extract the flavor-precursor molecules from unroasted, green coffee beans by soaking them in hot water. The beans can then be reconstituted either with the natural bean extract or with synthetic “biomimetic” solutions containing the most likely principle precursors to coffee bean flavor molecules (as suggested by experiments in model systems). The reconstitution step allows the scientists to intervene and alter the composition of the replenishing extract, so that they can trace the result of various precursors after the “spiked” beans have been roasted. By controlling the types of precursors present in the unroasted beans, the scientists can determine the effect of the presence or absence of particular precursors on the formation of various flavor molecules upon roasting.

The procedure also enables the to spike the beans with isotope-labeled precursors, which contain carbon-13 (heavier than the more abundant carbon-12 – see http://en.wikipedia.org/wiki/Isotope for more information on isotopes). Upon roasting, these carbon-13 labeled precursors are incorporated into odorant molecules, and can be identified using mass spectrometry (a technique that identifies molecules based on their masses – therefore if the mass is greater than expected for the compound containing only carbon-12, the difference between the expected and experimental value tells you how many carbon-13 atoms were incorporated). Therefore, if exhausted green coffee beans are spiked with only a particular labeled precursor, the scientists can trace which odorant molecules contain the carbon-13 isotopes (and can also identify the number of isotopes, and therefore the number of precursors incorporated). Such a determination of the fate of various precursors provides significant insight into the mechanism of formation of these odorant molecules during roasting.

Through use of this in-bean approach, Poisson et al were able to confirm many of the formation pathways proposed based on model systems. However this method also revealed a variety of alternative pathways that had not been suggested due to simplification in these models. Unlike previous experiments, the in-bean experiments allowed researchers to maintain the complexity of the coffee bean, and thus did not eliminate molecules that could be potentially important contributors to mechanisms of flavor formation. Though the researchers feel that improvements to this method are still necessary (optimization of the reconstituting mixture and reconstitution efficiency, for example), they stress the importance of biomimetic techniques in determining the nature of reactions that occur during food processing of any kind. This type of approach is analogous to in vivo studies in biomedical research, which are seen as an imperative step toward gaining a full understanding of biological processes. Food science must adopt such a standard, as in order to obtain the most accurate and informative results, studies of food processing and cooking must be performed under conditions as authentic as possible.

Poisson, L.;Schmalzried, F.; Davidek, T.; Blank, I.; Kerler, J. Study on the Role of Precursors in Coffee Flavor Formation Using In-Bean Experiments. J. Agric. Food Chem., [Online] 2009, 57 (21), 9923–9931. http://pubs.acs.org/doi/full/10.1021/jf901683v (accessed November 15, 2009).

No More Green Eggs and Ham: Flavor Development in Dry-Cured Spanish Jamón

In Spain, ham is a way of life. Not only does jamón feature prominently on most traditional Spanish tables, but its unique flavor profile also occupies the time and resources of many Spanish scientists. And all for good reason – Spanish dry-cured hams are some of the most delectable in the world, and who wouldn’t like to know the reason for that? In the current issue of the American Chemical Society’s Journal of Agricultural and Food Chemistry (JAFC), a group of scientists from Britain and Spain report their latest findings on the biochemical changes that occur during the curing process and contribute to the characteristic flavor and texture of jamón (the abstract can be found here). Others have previously determined that the lengthy curing process (which can last anywhere from nine months to more than two years and involves at least five steps – refrigeration, salting, resting, drying, and ripening) facilitates the degradation of muscle proteins into their constituent parts, known as amino acids. These amino acids can be thought of as the alphabet from which all proteins “words” are formed, contributing to the enormous diversity of proteins in existence. Free amino acids are known to impart a variety of flavors in both animal- and plant-based foods, and one, glutamic acid, is responsible for the distinctive “fifth taste” of umami.

In the JAFC article, Mora et al. investigate the degradation of creatine kinase (CK), a particular protein found in muscle cells. Physiologically, CK is involved in cell metabolism (breakdown of chemical substances into the cell’s primary unit of energy – adenosine triphosphate, or ATP), but it also plays a role in converting muscle to meat, and its degradation seems to be related to meat quality. Using samples from traditionally cured hams, the scientists were able to distinguish 58 different peptide sequences (a peptide is a chain of amino acids too short and simple to be considered a protein), which are the fragments of CK resulting from degradation by enzymes (proteins with the ability to catalyze biochemical reactions - in this case, cleavage between two particular amino acids). The scientists used a technique known as MALDI-TOF mass spectrometry to distinguish the peptides, separating them by mass, thus allowing the researchers to determine the sequence of each peptide. By comparing the peptide sequences both with each other and with the complete protein sequence, Mora et al. were able to determine which types of enzymes are likely to be responsible for the protein degradation that occurs during dry-curing, as the enzymes are known to cleave at specific locations in peptide sequences. They found that two major classes of enzymes are involved, exopeptidases (exo- meaning outer, -ase being the suffix denoting an enzyme – thus an enzyme which cleaves an particular amino acid from the end of a peptide chain) and endopeptidases (endo- meaning inner – so an enzyme responsible for cleaving a peptide chain anywhere other than the ends). Many such enzymes have been found in hams after 12-15 months of curing, and seem to be the major facilitators of free amino acid production in dry-cured ham.

The JAFC paper provides further evidence of protein degradation in dry-cured Spanish hams, with the characterization of nearly sixty peptide fragments of the CK protein. Moreover, the existence of these fragments suggests the action of exo- and endopeptidases in the liberation of free amino acids from muscle proteins during curing. Understanding of these processes provides insight into the generation of the unparalleled flavor and texture of Spanish jamón. A more robust understanding of this flavor profile could allow us to modify the curing procedure to further enhance desirable flavors, or may even allow us to appropriate combinations of flavor molecules unique to jamón and creatively apply them to other food products.

Mora, L.; Sentandreu, M.A.; Fraser, P.D.; Toldra, F.; Bramley, P.M. Oligopeptides Arising from the Degradation of Creatine Kinase in Spanish Dry-Cured Ham. J. Agric. Food Chem. [Online] 2009, 57, 19, 8982-8988. http://pubs.acs.org/doi/abs/10.1021/jf901573t (accessed October 15, 2009).