Food Chemistry

16.2 Individual Constituents 757

eraTriboliumandTenebrioand by the increase
in inhibitor concentration in tomato and potato
leaves after infection with the potato bug or its
larvae. Proteinase inhibitors from the potato also
inhibit the proteinases of microorganisms found
in rotting potatoes, e. g.,Fusarium solani.

16.2.3.4 Action on Human Enzymes

Inhibitor activity is normally determined with
commercial animal enzymes, e. g., bovine trypsin
or bovine chymotrypsin. The evaluation of a po-
tential effect of the inhibitors on human health
assumes that the inhibition of human enzymes is
known. Present data show that inhibitors from
legumes generally inhibit human trypsin to the
same extent or a little less than bovine trypsin. On
the other hand, human chymotrypsin is inhibited
to a much greater extent by most legumes.
Ovomucoid and ovoinhibitor from egg white as
well as theKazalinhibitor from bovine pancreas
do not inhibit the human enzymes. TheKunitz
inhibitor from bovine pancreas inhibits human
trypsin but not chymotrypsin. The data obtained
greatly depend not only on the substrate used, but
also on the enzyme preparation and the reaction
conditions, e. g., on the ratio enzyme/inhibitor.
The stability of an inhibitor as it passes through
the stomach must also be taken into account
in the evaluation of a potential effect (cf. Ta-
ble 16.15). TheKunitzinhibitor of soybeans, for

Table 16.15.Resistance of inhibitorsato pepsin at pH2

Source/Inhibitor Remaining
activityb(%)

Soybean,Kunitzinhibitor 0
Bowman–Birkinhibitor (BBI) 100
extract 30–40
Lima bean, BBI-type inhibitor 70–93
Kidney bean, BBI-type inhibitor 100
Kintoki bean, BBI-type inhibitor 100
Lentil, BBI-type inhibitor 83–100
Chick pea, inhibitors 100
Broad bean, trypsin-chymotrypsin
inhibitor 100
Moth bean, trypsin inhibitor 91
Broad bean, trypsin inhibitor 100
aDifferent incubation times.
bAgainst bovine and human trypsin and chymotrypsin.

example, is completely inactivated by human

gastric juice, but the Bowman–Birk inhibitor

from the same source is not. The available data

show that the average amount of trypsin and

chymotrypsin produced by humans per day can

be completely inhibited by extracts from 100 g of

raw soybeans or 200 g of lentils or other legumes.

16.2.3.5 Inactivation

The inactivation of proteinase inhibitors in the

course of food processing has been the subject of

many studies. In general, the inhibitors are ther-

molabile and can be more or less extensively in-

activated by suitable heating processes. In these

processes, both the starting material as well as the

process parameters are of great importance (time,

temperature, pressure, and water content of the

sample) (Table 16.16). Steaming of soybeans for

9 minutes at 100◦C causes an 87% destruction of

inhibitors (Table 16.17).

A decrease in the inhibitor activity can also be

achieved by soaking. A thermal step can then fol-

low under gentler conditions. Although the pro-

cessing of soybeans into protein isolates, textured

protein, or meat surrogates causes a decrease in

the inhibitor activity against trypsin, noticeable

activity can still be present (Table 16.18).

Soybeans promote the growth of rats to the same

extent as casein when about 90% of the inhibitor

activity is eliminated (Table 16.19).

16.2.3.6 Amylase Inhibitors

Relatively thermostable proteins, which have an

inhibitory effect on pancreatic amylase, are found

in aqueous extracts of navy beans, wheat, and rye.

As a result of the high thermostabilty, inhibitor

activity is also detectable in breakfast cereals and

bread.

The amylase inhibitor of navy beans is insta-

ble in the stomach and becomes active only af-

ter preincubation with the enzyme in the absence

of starch. As a result, it has no measurable influ-

ence on the digestion of starch by human beings.

Moreover, the average amounts of inhibitor in-

gested with the food are small compared to the

amylase activity present.