2009_Book_FoodChemistry

10.3 Aroma of Milk and Dairy Products 543
characteristic sour/pungent taste, which is intensified
by lactic acid. The two furanones HD3F
and EHM3F probably contribute to the nutty
note. Model experiments show that lactic acid
bacteria (Lactobacillus helveticus, Lactobacillus
delbrueckii) are involved in the formation of
HD3F.
At the Emmentaler pH of 5.6, magnesium
(threshold value: 3.5 mmol/kg) and calcium
propionate (7.1 mmol/kg) taste sweet. Consequently,
it is assumed that these propionates
contribute to the sweet note. On the other
hand, glutamic acid is an important taste substance,
which has the additional function of
neutralizing the bitter taste of amino acids
and peptides. Only if the concentrations of
these constituents climb too high on longer
ripening of Emmentaler, the effect of glutamic
acid is no longer sufficient and the bitter taste
appears. An off-flavor can also be formed if
there is a greater increase in the fatty acids
4:0–12:0.
The caseins are increasingly degraded during
longer ripening. Water-soluble peptides and
amino acids are formed which bind a part of the
ions. Thus, when chewing a cheese ripened for
a long time, the water-soluble portion of the ions
increases, possibly causing an intensification of
the salty taste.
It is probable that not only peptides, but also
other amides are responsible for the bitter taste
of cheese. For example, the presence of bitter
N-isobutyl acetamide has been detected in
Camembert cheese.
10.3.6 Aroma Defects
As already indicated, aroma defects can arise in
milk and milk products either by absorption of
aroma substances from the surroundings or by
formation of aroma substances via thermal and
enzymatic reactions.
Exogenous aroma substances from the feed or
cowshed air enter the milk primarily via the respiratory
or digestive tract of the cow. Direct absorption
apparently plays only a minor role. Metabolic
disorders of the cow can cause aroma defects,
e. g., the acetone content of milk is increased in
ketosis.
The oxidation of lipids is involved in the endogenous
formation of aroma defects. While very low
concentrations of certain carbonyl compounds,
e. g., (Z)-4-heptenal (1 µg/kg), 1-octen-3-one,
and hexanal, appear to contribute to the full
creamy taste, increased concentrations of these
and other compounds produce cardboard-like,
metallic, and green aroma notes. In butter, for instance,
the phospholipids of the fat globule membrane
are especially susceptible to oxidation. The
subsequent products get distributed in the entire
fat fraction and cause taste defects which range
from metallic to fatty and from fishy to tallowy.
Light can cause the degradation of methionine to
3-methylthiopropanal via riboflavin as sensitizer.
Together with other sulfides and methanethiol,
this sulfur compound produces the aroma defect
of milk and milk products called “light taste”.
A series of aroma defects are caused by enzymatic
reactions. These include:
• An unclean taste due to an increased concentration
of dimethylsulfide produced by psychotropic
microorganisms.
• A fruity taste due to the formation of ethyl
esters produced by psychotropic microorganisms,
e. g., Pseudomonas fragii.
• A malty taste due to increased formation
of 3-methylbutanal, 2-methylbutanal, and
methylpropanal by Strept. lactis var. maltigenes.
• A metallic taste in buttermilk due to (E,Z)-2,6-
nonadienol in concentrations >1.3µg/l. The
precursor is the triglycerol-bound α-linolenic
acid which is oxidized to 9-hydro-peroxy-
10,12,15-octadecatrienoic acid by oxygenases
from the starter culture. Proton catalysis liberates
(E,Z)-2,6-nonadienal which is reduced
to the corresponding alcohol by lactic acid
bacteria.
• A phenolic taste due to spores of Bacillus circulans.
• A rancid taste due to the release of lower fatty
acids (C 4 –C 12 ) by milk lipases or bacterial lipases.
• A bitter taste can occur due to proteolytic activity,
e. g., on storage of UHT milk. The milk
proteinase plasmin is inactivated on intensive
heating (142 ◦ C, >16 s). However, some bacterial
proteinases can still be active even after
much longer exposure to heat (142 ◦ C, 6 min).