P-7
Influence of bisulfite on the release of flavour volatiles from Schiff bases
and N-adducts. Mechanisms for flavour release and factors influencing flavour stability are
very important concerns in both the food and beverage industries. Understanding
the interactions of key flavour compounds such as carbonyls with nitrogen
containing groups, eg amino acids and proteins, of the food matrix are
fundamental to the governance of the flavour profile in many foods and
beverages. Beer is a typical example where such interactions have been proposed
with significant implications to the stability of saturated and unsaturated
carbonyl compounds and their subsequent release. Reaction of the carbonyl
compounds with the amino acids and proteins of the wort has been proposed with
Schiff bases, or imines, implicated in the mechanism of release of the carbonyl.
The lowering of temperature, the decrease in pH and the consumption of the free
amino acids by the yeast during fermentation have been mooted for unlocking the
carbonyl from the Schiff base. However, the Schiff base is simply too unstable
in an aqueous environment to provide a gradual liberation of the carbonyl
compound. Given the inherent instability of Schiff bases we have reviewed the
chemical literature describing amino acid/protein interactions with carbonyl
compounds (in particular alkenals) and evaluated the stability of the adducts
that are formed. In this work we report results from our work on model systems
(appropriate to beer) investigating the influence of bisulfite on amino-carbonyl
adducts, and the influence of amino acids on the stability of bisulfite adducts
of aldehydes. 1H-NMR evidence of model systems suggests that the N-adducts of
saturated aldehydes and alkenals, Michael adducts and Schiff bases, are
relatively unstable, particularly in the presence of bisulfite. Bisulfite,
whether produced by yeast during fermentation or added before bottling, has a
profound effect on the stability of beer. The bisulfite is a proven antioxidant
in beer and is also proposed to confer stability through the formation of
adducts, particularly with saturated and unsaturated carbonyl compounds. The
multivarious roles of bisulfite in beer appear to depend on the interplay
between several linked chemical equilibria, which directly reflect the
complexity of flavour stability.
Jean-Pierre Dufour. Studies: MSc., 1975-PhD., 1979 (Louvain). Research
fellow (Johns Hopkins University, School of Medicine, Baltimore, MD) 1979-1981.
Appointments: Catholic University of Louvain, Professor 1981-1993 (Head of the
Department of Brewery and Food Industries, 1987-1993); Universidade Catolica
Portuguesa, Escola superior de Biotechnologia (Porto, Portugal), visiting
Professor 1989-1994; University Senghor (Alexandria, Egypt), associated
Professor 1992-1995; Expert for EEC and UNIDO (Implementation of sorghum malt
for the production of lager beer in Africa) (1994-1996); University of Otago,
Dunedin, New Zealand, Professor (1995-present), Chairperson and Head of
Department of Food Science. Expertise: Flavour science, fermentation science and
technology, malting and brewing sciences, yeast biochemistry/enzymology. Active
member of EBC Brewing Science Group, ASBC, IOB, Institute of Food Technology,
American Chemical Society, New Zealand Institute of Food Science and Technology.
Vice-president and Fellow of the New Zealand Institute of Food Science and
Technology, New Zealand delegate to IUFoST.
JEAN-PIERRE DUFOUR and A. R. Hayman. Departments of Food Science and Chemistry,
University of Otago, POB 56 Dunedin, New Zealand.