2009_Book_FoodChemistry

190 3 Lipids
Table 3.24. Free fatty acids in butter (sweet cream)
samples of different quality
Fatty acid
Butter
A B C D E
(mg/kg)
4:0 0 5 38 78 119
6:0 0 4 28 25 46
8:0 8 22 51 51 86
10:0 38 58 104 136 229
12:0 78 59 142 137 231
14:0 193 152 283 170 477
Aroma a 2.3 2.8 3.0 4.6 5.4
a Classification: 2 not rancid, 3 slightly rancid, 4 rancid,
5 very rancid.
3.7.1.2 Polar-Lipid Hydrolases
These enzymes are denoted as phospholipases,
lysophospholipases or glycolipid hydrolases, depending
on the substrate.
3.7.1.2.1 Phospholipases
Phospholipase A 1 . The enzyme is present together
with phospholipase A 2 in many mammals
and bacteria. It cleaves specifically the sn-1 ester
bonds of diacylphosphatides (Formula 3.53).
Phospholipase A 2 . Enzymes with sn-2 specificity
isolated form snake and bee venoms. They are
very stable, are activated by Ca 2+ -ions and are
amongst the smallest enzyme molecules (molecular
weight about 14,000).
both acyl groups in diacylphosphatides, is
controversial. Other than the phospholipases A 1 ,
A 2 , C and D, the B-type could not be isolated
in its pure form. A phospholipase B has been
enriched from germinating barley. However, the
B-specificity appears to be only a secondary
activity because the enzyme hydrolyzes the acyl
residue of lysolecithin considerably faster than
the acyl residues of lecithin.
Phospholipase C. It hydrolyzes lecithin to a 1,2-
diacylglyceride and phosphoryl choline. The enzyme
is found in snake venom and in bacteria.
Phospholipase D. This enzyme cleaves the
choline group in the presence of water or an
alcohol, such as methanol, ethanol or glycerol,
yielding free or esterified phosphatidic acid. For
example:
Phosphatidylcholine + ROH
−→ Phosphatidyl-OR + Choline
R:H,CH 3 ,CH 2 ,CH 2 (OH) − CH(OH) − CH 2
(3.54)
Phospholipase D cannot cleave phosphatidyl inositol.
The enzyme is present in cereals, such as
rye and wheat, and in legumes. It was isolated and
purified from peanuts.
Lysophospholipases. The enzymes, hydrolyzing
only lysophosphatides, are abundant in animal
tissue and bacteria. There are lysophospholipases
that split preferentially 1-acylphosphatides while
others prefer 2-acylphosphatides, and a third
group doesn’t differentiate at all between the two
lysophosphatide types.
3.7.1.2.2 Glycolipid Hydrolases
(3.53)
Phospholipase B. The existence of phospholipase
B, which hydrolyzes in a single-step reaction
Enzymes that cleave the acyl residues of monoand
digalactosyl-diacylglycerides are localized
in green plants. A substrate specificity study
for such a hydrolase from potato (Table 3.25)
shows that plants also contain enzymes that are
able to hydrolyze polar lipids in general. The
potato enzyme preferentially cleaves the acyl
residue from monoacylglycerols and lysolecithin,
whereas triacylglycerols, such as triolein, are not
affected.