Chemistry Reference
In-Depth Information
baking (trout, chicken breast) to 30-45%, cooking, chilling/reheating of
various vegetables, 25%; soaking and cooking of pulses, 30-70%; ionized
radiation of vegetables, 10-60%; storage of spinach, liver or strawberries in
the freezer over 6-8 months led to a loss of 17-40%; chilled storage of yoghurt
or fermented milk for 2 weeks had no effect. Storage of UHT milk at room
temperature for 8 weeks led to losses between 0 and 100%, dependent on the
degree of oxidation and enzyme activity (cf. Witth oft and J¨ gerstad, 2002).
13.8.1.
Functions of Folate
Folate serves as an enzyme cofactor; its principal function is to carry
one-carbon units, e.g. methyl-, formyl-, formimino-, methylene- or methe-
nyl- (see Figure 13.12). The origin of these carbon units is the catabolism
of amino acids like glycine, serine and histidine. Folates also serve as co-
enzymes providing the above-mentioned carbon-containing radicals for
the synthesis of purines and pyrimidine bases for DNA and RNA synth-
esis, i.e. folates are very important for cell replication and also repair
mechanisms involving DNA and RNA. Another role is the regeneration
of methionine from homocysteine by the enzyme, methionine synthase.
Vitamins B6 and B12 are also involved in this mechanism. Recently,
homocysteine has been recognized as an independent risk factor for the
development of atherosclerosis, and a sufficient folate level can help
reduce this risk by avoiding elevated homocysteine plasma levels (see
Herrmann et al., 2007).
13.8.2.
Sources of Folate
Tables 13.11 and 13.12 give an overview on the content of folate in
several nutrients, including milk, dairy products and cheese. Table 13.9 shows
the recommended daily uptake of folate published by the German Society for
Nutrition. It must be taken into account that the bioavailability of folates
from different sources can be very different, e.g.
<
5% from vegetables, 55%
from liver, 70-75% from egg yolk or up to 100% from pharmaceutical pre-
parations taken as supplements. The overall situation in humans is still
unclear, as only few studies, with contradicting results, have been performed.
Interestingly, some nations (e.g. Canada and the USA) add folate to basic
nutrients (in the USA to wheat flours, rice, corn meals, bread, pasta at a
concentration of 1.4 mg kg
-1
product) which has led to a reduction of neural
tube defects (see below). To improve the bioavailability of folate - and also
other secondary plant components - the uptake of dairy products was shown
to be helpful (Forss´n et al., 2000; Pfeuffer and Schrezenmeir, 2007). Another
important factor concerning bioavailability seems to be the folate-binding
