377
5.12 FOLACIN
5.11.6 Reactions
5.11.7.4 Fruits and vegetables
Biotin is stable when heated, in the light, in neutral and strongly
acidic solutions. It is unstable in alkaline media. It is easily oxidised
by hydrogen peroxide and other oxidants to a mixture of isomeric (+)- and (–)-sulfoxides (5-88 and 5-89), eventually to
biotin sulfone (5-90). Sulfoxides also form as metabolic products of microorganisms. They are fully available, whereas sulfone is
unavailable as a vitamin. Nitrogen of a ureido ring may be nitrosated
to nitrosobiotin (5-91) in the presence of nitrites or nitrogen oxides.
During preservation of fruit and vegetables about 30% of biotin is
lost. This large proportion is caused by leaching and can therefore
be regarded as a loss only when the brew is not consumed.
5.12 Folacin
5.12.1 Structure and terminology
Folacin is the name for the biologically active derivatives of folic
(pteroylglutamic) acid (5-92), formerly also known as vitamin
B9 , vitamin Bc or vitamin M. The basis of its structure is pteroic
acid, systematic name 4-[(pteridine-6-ylmethyl)amino]benzoic
acid, which is hypothetically derived from 6-hydroxymethylpterin
and 4-aminobenzoic acid. The carboxyl group of pteroic acid is
conjugated, via an amide bond, with one or more l-glutamic acid
units, typically with three to eight molecules (n = 3–8). These
compounds are called glutamyl peptides or folate polyglutamates.
The active form is (6S)-5,6,7,8-tetrahydrofolic or (6S)-5,6,7,8tetrahydropteroylglutamic acid (5-93), for short H4 PteGlu or
FH4 , with a reduced pteridine (more precisely pyrazine) ring.
Heptaglutamate was previously known under the names of chick
growth factor or vitamin B11 .
O
O
N
N
NH
H
H
COOH
S
5-91, nitrosobiotin
5.11.7 Changes in foods
Biotin is very stable during food processing. Losses in hydrothermal
processes are mainly caused by leaching into the water.
5.11.7.1 Meat and meat products
Biotin retention in cooked meat is relatively high, at about 80%.
5.12.2 Biochemistry
5.11.7.2 Milk and dairy products
Many microorganisms and plants are capable of de novo synthesis of
7,8-dihydrofolic (7,8-dihydropteroylglutamic) acid (7,8-H2 PteGlu,
5-94). Animals can only reduce this form of vitamin to tetrahydrofolic acid. They are also able to reduce the folic acid to
dihydrofolic acid and can hydrolyse or add additional glutamic
acid residues. The starting compound for the biosynthesis of 7,8dihydrofolic acid is guanosine 5 -triphosphate (GTP). Reactions
with 4-aminobenzoic acid (which results from chorismic acid via
4-amino-4-deoxychorismic acid) and with glutamic acid proceed
in the later stages of biosynthesis.
The activity of folacin is similar to the activity of cobalamins. It
is linked with the transfer of one of the carbon functional groups,
such as methyl (CH3 –), methylene (–CH2 –), formyl (–CH=O)
and other groups whose donor is mainly choline, glyoxylic acid,
serine and other compounds. These functional groups are bound
Copyright © 2014. John Wiley & Sons, Incorporated. All rights reserved.
Pasteurisation of milk results in 10–15% losses of biotin. Milk
drying losses are higher, but subsequent storage does not influence
the vitamin level substantially. Biotin concentration in cheeses is
lower by about 20–35% compared with milk. The biotin content of
yoghurts is influenced by the microflora (Lactobacillus spp.) used,
and is lower than in milk by about 45–60%. Some bacteria (e.g.
bacteria of the genus Micrococcus) produce biotin, and its content
in yoghurt may then increase by 5–25%.
5.11.7.3 Cereals and cereal products
Biotin losses during the baking of bread and other cereal products
are small. Cooking legumes results in loss of vitamin, which reaches
5-15%, depending on the time of soaking.
O
4a
HN3
4
5
2
8
1
H 2N
N
8a
10
N
9
6
7
N
H
5´
6´ 1´
O
COOH
7´
α
3´ 2´
N
H
β
H
N
γ
O
4´
n
COOH
COOH
N
4-aminobenzoic
acid
pteroic acid
5-92, folic acid
Velisek, J. (2014). The chemistry of food. Retrieved from http://ebookcentral.proquest.com
Created from espol on 2019-10-22 08:53:00.
glutamic
acid
glutamic
acid
0
