Chemistry Reference
In-Depth Information
presence of pyridoxal-5'-phosphate (PLP) under cheese-like conditions (pH
5.2 + 4% salt at 78C) resulted in the production of methanethiol, dimethyl
disulphide and dimethyl trisulphide. A 1000-fold increase in the production
of volatile sulphur compounds was observed when the reaction temperature
was raised from 7 to 378C. This reaction in conjunction with an adjunct
culture with higher peptidolytic activity may offer an improved method to
control and enhance the production of volatile sulphur compounds in cheeses
or enzyme-modified cheeses.
Amino acid degradation plays a vital role in flavour development in
cheese. A number of researchers have attempted to enhance the free amino
acid content of Cheddar cheese by direct addition of amino acids (Wallace
and Fox, 1997) or genetic modification of lactococci with increased amino-
peptidase N activity (McGarry et al., 1994; Christensen et al., 1995). How-
ever, an increased amino acid content in Cheddar did not affect flavour
development, which led Yvon et al. (1998) to hypothesize that the rate-limit-
ing factor in flavour biogenesis was not the release of amino acids but their
subsequent conversion to aroma compounds. Yvon et al. (1998) identified the
transaminase acceptor,
-ketoglutarate, as the first limiting factor in the
degradation of amino acids. Addition of
-ketoglutarate to Cheddar curd
resulted in increased volatile components originating from branched-chain
and aromatic amino acids (Banks et al., 2001). Results of a recent study
showed that Cheddar cheese made using an adjunct starter Lactobacillus
casei (genetically modified to enhance the expression of hydroxyl acid dehy-
drogenase, HADH) retarded flavour development (Broadbent et al., 2004).
HADH catalyses the conversion of
-KAs to
-hydroxy acids, which have
little or no importance from a flavour point of view. It may still be possible to
selectively suppress aromatic amino acid (Phe, Tyr, Trp)-derived off-flavour
compounds by overexpression of an alternative HADH with more narrow
specificity for aromatic amino acid-derived
-KAs.
The effect of membrane permeablisation, by treatment with the
bacteriocin lacticin 3147, on the branched-chain amino acid transamina-
tion by L. lactis IFPL359 was investigated by Martinez-Cuesta et al.
(2002). Membrane permeabilisation of the cells made them non-viable,
but they remained metabolically active and facilitated free amino acids
diffusion into the cell. These changes made intracellular enzymes more
accessible to their substrates and hence increased branched-chain amino
acid transamination.
The detailed understanding of the mechanistic pathways involved in the
degradation of amino acids to cheese flavour compounds and the capacity of
starter/non-starter bacteria to generate volatiles will not only result in
enhanced control/acceleration of cheese flavour development but also in
minimizing the occurrence of off-flavours.
