Research Note

Stability of feed enzymes in physiological conditions assayed by
in vitro methods

Johan Inborr and Anne Grönlund

Inborr, J. & Grönlund, A. 1993. Stability of feed enzymes in physiological condi-
tions assayed by in vitro methods. Agric. Sci. Finl. 2: 125-132. (Finnfeeds Interna-
tional Ltd., Market House, High Street, Marlborough, Wiltshire SNB IAA, United
Kingdom and Cultor Ltd., Technology Centre, FIN-02640 Kantvik, Finland.)

A series of in vitro incubations were carried out to investigate the stability of two
enzyme preparations in conditions similar to those in the upper gastrointestinal tract of
monogastric animals. The two enzyme products, one crude xylanase from Trichoderma
longibrachiatum (Multifekt K) and the other a specifically manufactured feed enzyme
(Avizyme SX®), were subjected to incubations at low and neutral pH with and without
proteolytic enzymes (pepsin and pancreatin). Wheat gluten was employed together with
the crude xylanase to investigate its potential as a stabilising agent. Due to the buffering
effect of Avizyme SX®, incubations were carried out with (pH 2.5) and without (pH 3.2)
addition of either citric orhydrochloric acid. Incubation of the crude xylanase at low pH
followed by incubation at neutral pH resulted in negligible loss of xylanase activity
whereas (Lxylosidase recovery fell to57 per cent of the initial value (P<0.05). Addition
of wheat gluten resulted in full recovery of (3-xylosidase. The recoveries of both
(3-glucanase and xylanase were significantly (P<0.05) lower than the initial values after
incubation of Avizyme SX® in pH 2.5. However, with no pH adjustment (pH 3.2) the
recoveries were significantly higher (P<0.05 for (3-glucanase and P<o.lo for xylanase).
The results from the pepsin and pancreatin incubations showed similar trends as the
ones of the pH stability experiments. Consequently, gluten addition and no pH adjust-
mentgave the highest enzyme activity recoveries.

The results suggest that partial enzyme inactivation may occur due to the low pH and
proteolytic activities and hence in the Gl-tract ofmonogastrics. Feeds and feedstuffs can
due to their buffering capacity and possibly by providing substrates for the enzymes
markedly reduce the rate of inactivation. Results from a number of pig and poultry
experiments appear to support this assumption. In vivo recovery measurements using
animal models are needed to substantiate this.

Key words: xylanase, (5-glucanase, (5-xylosidase, wheat gluten, enzyme survivability,
pH, pepsin, pancreatin, Multifect K, Avizyme SX®.

Introduction

Since the early 1980’s the use ofenzymes in animal
feeds has received much attention both in the aca-
demicand commercial worlds. There is now a con-

response in animal performance to enzyme supple-
mentation of feeds, which has been extensively
reviewed by Chesson (1987) and Dierick (1990).
With increased commercial use of feed enzymes
stability has become a frequently debated issue. In

siderable amount of published work showing the particular, enzyme stability during feed manufac-

125

Agric. Sei. Fint. 2 (1993)

https://www.c-info.fi/en/info/?token=4JsLZy_JePuxY5DQ.1uCBue1ti2gKUdfti73VJg.kx2iArkmLUiuZCOUKRJ51DSSEa7pg8-O1e4dOJkcIW2N2pDhr1xgPK9vL5qAn-EfZaFjHLvLsmN3Bfp3TW6Yr2Yu_1BI1IVEpqao2E0cp7BY3_oVVyfWkjJnIc7Kqwzr9EYvA7nMgqserC3pvZGPeE1vyhsXFIo-h-k8nQhiAUlFcSUiEvLs6gioaemMFIrBaVEqrtHWrlwnZE7XijULNJK_pyg9EWgo7QT0oWccig6ijpPrQY6RpiRqeVa4NGofpNWWnm7x6WZL8ctZgigl9u0YNyCEI5AR-Kc


Research Note

turing, due to the different heat treatments (pellet-
ing, expansion, extrusion etc.) now being employed
in the feed industry, has caused some concern. Fur-
thermore, once the enzymes enter into the gastroin-
testinal tract, they are subjected to a range ofcondi-
tions that can cause inactivation. Firstly, gastric pFI
may be as low as 1, but usually around 2 in an
empty stomach, with rapid increase to between 4
and 5 as ingested food arrives (Kidder and Man-
ners 1978). Keeping in mind that enzymes are
proteins and, therefore, subject to proteolysis, low
pFI in combination with pepsin would appear to be
a potential hazard for the enzyme molecules. Sec-
ondly, the pancreatic proteases e.g. trypsin and chy-
motrypsin represent a major threat to the enzymes
due to their abundance and high activity in duode-
num.

Cellulases and hemicellulases are the most com-
monly used enzymes in feed applications. These are
mainly microbial enzymes produced by different
fungi and bacteria. In general, the inherent heat
stability of mesophilic microbial enzymes is poor,
although there are considerable differences be-
tween enzymes from different sources in this re-
spect (Godfrey and Reichelt 1983). Through
modem biotechnology a number of stabilisation
methods have been developed. For example, it is a
known fact that substrates and inhibitors that bind
specifically to the native conformation of an en-
zyme will increase the stability (PACE 1990). This
methodof stabilisation is called immobilisation and
has been shown by Simons and Georgatsos

(1990) to decrease heat inactivation of barley (3-
glucosidase. Other promising stabilisation methods
involve adding crosslinks (disulfide bonds) to, sub-
stituting amino acids (PACE 1990) and crystal-
lisation of the enzyme proteins (St. Clair and
Navia 1992).Furthermore, De Cordt et al. (1992)
found that polyols and carbohydrates are powerful
stabilisers for both dissolved and immobilised B.
licheniformis ot-amylase.

A series of in vitro experiments were designed to
investigate the survivability ofa few main activities
of two enzymes subjected to conditions similar to
those prevalent in the digestive tract of pigs and
poultry. Two enzyme preparations, one crude and
one specifically manufactured feed enzyme prod-

uct, were employed. Xylanase and B-xylosidase ac-
tivities of the crude enzyme preparation and B-glu-
canase and xylanase of the feed enzyme product
were investigated.

Material and methods

A liquid crude xylanase preparation (Multifect K,
Genencor International Ltd., Helsinki, Finland)
from a Trichoderma longibrachiatum fermentation
and a stabilised(liquid enzymes adsorbed and dried
onto a wheat-based carrier; Patent FI 77359) com-

(Sömercial feed enzyme product (Avizyme SX ,

Finnfeeds International Ltd., Marlborough, Wilt-
shire, United Kingdom) containing (3-glucanase
and xylanase from Trichoderma longibrachiatum
were subjected to treatments in conditions similar
to those in the stomach (proventriculus) and small
intestine of pigs and poultry. A schematic descrip-
tions of the incubation procedure employed is pre-
sented in Figure 1.

Incubation in low and neutral pH

The conditions employed in these incubations were
identical with those described below with the ex-
ception that no pepsin and pancreatin were used.

Incubation with pepsin and pancreatin

Crude xylanase preparation

20 ml of pepsin solution (Merck 7189), containing
0.25 protease units (haemoglobin, pH 1.2, 25°C)
per ml, in 0.1 M Mcllvaine’s buffer (pH 2.5) was
equilibrated at 40°C. 10ml of xylanase solution in
Mcllvaine’s buffer (1 ml xylanase/50 ml buffer)
was added and thesolution diluted with buffer to 50
ml followed by incubation at 40°C, pH 2.5 for 30
minutes. The reaction was stopped in an ice/water
bath immediately after incubation. pH was adjusted

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Agric. Sei. Fint. 2 (1993)



Research Note

with 0.5 M NaiHPOd to 5.0 and the final volume
was 100 ml. Xylanase and P-xylosidase activities
were measured. The pepsin treated sample (10 ml)
was then adjusted to pH 7.0 with 0.5 M NaaHPOa.
30 ml of pancreatin solution(Merck 7230), contain-
ing 30 protease units per ml, in Mcllvaine’s buffer
(pH 7.0), was added, and the solution diluted to 50
ml and adjusted to pH 7.0 with Mcllvaine’s buffer.

The solution was incubated at 40°C for 30 minutes,
after which time the reaction was stopped in an
ice/water bath. The pH was adjusted to 5.0 by 0.5
M citric acid and xylanase and (5-xylosidase activ-
ities measured.

A parallel incubation was carried out with wheat
gluten (2% w/v) solubilised in a NaCl solution
(0.9% w/v) as a stabiliser. When wheat gluten was

Fig. 1.Flow chart diagramme of the incubations.

127

Agric. Sei. Finl. 2 (1993)



Research Note

used, samples were filtered through fibre glass filter
paper before enzyme activity analysis.

Commercialfeed enzyme

Two proteolysis procedures were carried out. In the
first one, the pH was kept constant at 2.5 by addi-
tion of 6 N HCI, whereas in the second one no acid
was added during the pepsin treatment, and the pH
rose to 3.2 (buffering effect of the feed enzyme
product).

20 g feed enzyme premix was suspended in 0.1
M Mcllvaine’s buffer (pH 2.5) and the pH was
either maintained at 2.5 by acid (6 N HCI) addition
or allowed to rise due to the buffering capacity of
the feed enzyme product (parallel incubation). 10
ml pepsin solution was added to give a final volume
of 70 ml. The pH was adjusted after incubation to
5.0 using 2.5 M NaOH and samples were taken for
enzyme activity analysis (P-glucanase and xyla-
nase) after filtration through fibre glass filter paper.

The procedure continued with 70 ml unfiltered
pepsin-treated sample, employing the same condi-
tions as with the crude xylanase preparation but
without addition of wheat gluten. 15 ml pancreatin
solution in 0.1 ml Mcllvaine’s buffer (pH 7.0) was
added giving a volume of 85 ml during the incuba-
tion. After the pH adjustment (15 ml 2 M citric
acid) the final volume was 100 ml. P-glucanase and
xylanase activities were measured after filtration.

of reducing sugars (expressed as glucose equival-
ents) in one minute under the conditions described.

Xylanase (EC 3.2.1.8; endo-(3-(l,4)-xylanase)
activity (modified from Poutanen et al. 1986)was
determined using 1% oat spelt xylan (Sigma X-
-0376) in Mcllvaine’s buffer at pH 5.0 as substrate.
0.2 ml of suitable enzyme dilution was incubated
with 2.0 ml of substrate solution at 40°C for 30
minutes. The reducing sugars were assayed as de-
scribed above. One xylanase unit is the amount of
enzyme that liberates 1 pmol of reducing sugars
(expressed as xylose equivalents) in one minute
under the conditions described.

P-xylosidase (EC 3.2.1.37; 1,4-p-D-xylan xylo-
hydrolase) activity was determined (modified from
Deleyn et al. 1978) using 2 mmol/1 p-nitrophenyl-
P-D-xylopyranoside in Mcllvaine’s buffer at pH
5.0 as substrate. 0.2 ml of suitable enzyme dilution
in deionized water was incubated with 2.0 ml sub-
strate solution in 40°C at pH 5.0 for 30 minutes.
After adding 1.0 ml of Na2CC>3, absorbance was
measured at 400 nm. One P-xylosidase unit is the
amount of enzyme that liberates one pmol of p-ni-
trophenol in one minute under the conditions de-
scribed.

Each enzyme activity measurement was carried
out in duplicates or triplicates unless otherwise
stated. Values are expressed as means with standard
deviations of each sample. Means were separated
where appropriate by paired t-test.

Enzyme activity analysis

P-glucanase (EC 3.2.1.6; endo-p-(l,3)(l,4)-gluca-
nase) activity was determined spectrophotometri-
cally using 1.0% barley P-glucan (Biocon Bio-
chemicalsLtd., Ireland) in Mcllvaine’sbuffer at pH
5.0 as substrate. 0.2 ml of suitable enzyme dilution
in deionized water was incubated with 2.0 ml of
substrate solution at40°C for 30 minutes. Reducing
sugars were assayed by addition of 3.0 ml 3,5-dini-
trosalisylic acid (DNS) reagent (Sumner and
Somers 1949), boiling for 5 minutes, cooling and
measuring absorbance at 540 nm. One P-glucanase
unit is the amount of enzyme that liberates 1 pmol

Results and discussion

pH stability

Incubation in pH 2.5 did not reduce the xylanase
activity of Multifect K, whereas the P-xylosidase
activity was reduced to 57% (P<0.05) of the initial
value (Table 1). Subsequent incubation at pH 7
only marginally reduced xylanase activity and had
no effect on P-xylosidase. Addition ofwheat gluten
markedly increased the recovery of P-xylosidase
after incubation in pH 2.5 (P<0.05) and pH 7
(P<0.10), resulting in full recovery of this activity.

Maintaining pH at 2.5 by addition of citric acid

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Research Note

Table I . Initial and residual xylanaseand (i-xylosidasc activities of Multifect K, measured at pH 5, after incubation at acidic
and neutral pH with and without pepsin and pancreatin, and with (+ ) and without (-) gluten (means ± sd).

Xylanase p-xylosidase

Gluten - + - +

U/g % U/g % U/g % U/g %

Initial pH 5 4510 ±BO 100 4970 + 27 100 44 ± 3 100 30 ± 2 100
Residual pH 2.5 4440* 98 4780* 96 25» ±1 57 30» ± 1 100

pH 7 4070 ± 235 90 4720* 95 25» ±1 57 30" ± 2 100
Pepsin pH 2.5 4270* 95 4520* 91 30> ± 2 68 18» ±0 60
Pancreatin pH 7 2590» ±4 57 3400* 68 25 + 5 57 29 + 2 97

' result of one measurement
x differ from initial value (P<0.05)
v differ from initial value (P <0.10)
» differ from corresponding control ( + ) (P<0.05)
b differ from corresponding control ( +) (P<0.10)

Table 2. Initial and residual P-glucanase and xylanase activities of Avizyme SX, measured at pH 5, after incubation at
acidic and neutral pH with and without pepsin and pancreatin, and with (+) and without (-) pH adjustment (means ± sd).

3-glucanase Xylanase

pH adjustment l + +

U/g % U/g % U/g % U/g %

Initial pH 5 870 ± 5 100 850 + 15 100 460 ± 16 100 467 + 38 100
Residual pH 2.5/3.2 370» ± 6 43 830* ±2B 96 340» ± 6 75 510b ±2o 109

pH 7 390" ±8 45 790» ± 28 92 260" ±2 57 422" ±6 90
Pepsin pH 2.5/3.2 370» + 17 42 850» + 13 100 360» ± 3 78 510" ±lB 110
Pancreatin pH 7 310» + 9 36 760» ±3O 89 260»'± 12 56 470" ±2l 100

1 ( + ) means pH kept constant at 2.5 by addition of either 2 M citric acid (pH stability) or 6 N HCI (pepsin stability), (-)

means no acid addition during the incubation with pH rising to 3.2.
< differ from initial value (P<0.05)
v differ from initial value (P<0.10)
» differ from pH 2.5 (P<0.05)
“ differ from corresponding control ( + ) (P<0.05)
b differ from corresponding control (+ ) (P<0.10)

resulted in 43% (P<0.05) recovery of (3-glucanase
and 75% recovery of xylanase (P<0.10) in
Avizyme SX (Table 2). Subsequent incubation at
pH 7 had no effect on the (3-glucanase activity,
whereas xylanase was reduced to 57% (P<0.05) of
the initial activity. When pH was not adjusted, lead-
ing to a 0.7 unit increase in pH, (3-glucanase recov-
ery after the first incubation (pepsin in pH 3.2) was
96% (P>0.05) and after the second incubation (pan-

creatin in pH 7) 92%.The corresponding recoveries
for xylanase were 109 and 90%. This difference
was not significant (P>0.10).

From these results it appears that low pH per se
did not cause any dramatic reductions in enzyme
activity and that addition of wheat gluten may re-
duce the degree of inactivation of more sensitive
enzymes i.e. in this case (3-xylosidase. Whether this
effect is due to an increased dry matter content of

129

Agric. Sei. Finl. 2 (1993)



Research Note

the system or e.g. binding between the gluten and
the enzyme is not known. Interestingly, with wheat
being the carrier material of Avizyme SX thus
providing the system with gluten, xylanaserecover-
ies were lower with and higher without pH adjust-
ment than when gluten was added to Multifect K.

Pepsin and pancreatin stability

Xylanase and p-xylosidase recoveries after incuba-
tion of Multifect K with pepsin was 95 and 689?)

without and 91 and 60% withadded gluten, respect -

ively (Table 1). These effects were not significant
(P>0.10), with the exception being p-xylosidasc
when incubated at pH 2.5 with added gluten
(P<0.10). After the subsequent incubation in pan -

creatin the corresponding recoveries were 57 and
57% and 68 and 97%, respectively (P>0.10). Thus
P-xylosidase seemed to be more sensitive to pepsin
than xylanase. Incubation with pancreatin tended to
further reduce enzyme activity, except the activity
of P-xylosidase when gluten was added.

p-glucanase activity of Avizyme SX® decreased
to 42% (P<0.05) of the initial valueafter incubation
in pepsin with pH kept at 2.5 by addition of hydro-
chloric acid and was further reduced to 36% after
incubation in pancreatin (Table 2). With no pH
adjustment, the P-glucanase recoveries after pepsin
treatment was 100% and after incubation with pan-
creatin 89% (P>0.10). These recoveries were sig-
nificantly higher than when pH was kept at 2.5
(P<0.05). Xylanase recoveries after pepsin and
pancreatin incubations withpH adjustment were 78
and 56% of the initial activity (PcO.10), respect-
ively. Without pH adjustment xylanase was fully
recovered. These recoveries were significantly

higher than with pH adjustment (P<0.10).
Addition of wheat gluten to the enzyme/substrate

systems tended to increase the final enzyme recov-
eries but this effect was not observed at pH 2.5,
except for p-xylosidase after incubation at pH 2.5
without pepsin. De Cordt et al. (1992) found that
both polyols and carbohydrates increased the tem-
perature stability of bacterial a-amylase. The
mechanisms involved were not specifically eluci-
dated but "preferential protein hydration", changes
in the chemical potential of the proteins, changes in
the solvent dielectric constant, changes in the water
activity of the solvent system, degree of water or-
ganisation were suggested. It is possible that wheat
gluten used in this present experiment exerted one
of these effects leading to increased pH and prote-
olytic stability. Interestingly, the buffering capacity
of the feed enzyme product, probably due to the
cereal carrier material, had a clear stabilising effect
on the enzyme activities measured. Feed arriving in
the stomach of pigs exert similar pH buffering ef-
fects (Kidder and Manners 1978) and it can
therefore be assumed that the feed acts as a potent
stabilizer in the animal.

Results from a number of experiments with pigs
and poultry would suggest that sufficientactivity of
supplementary enzymes survive the potential haz-
ards of the GI-tract to improve animal performance
(Chesson 1987,Dierick 1990). Enzyme recovery
measurements in different segments of the GI-tract
would be needed to exactly establish the rate and
extent of feed enzyme inactivation in vivo.

Acknowledgements. The authors want to thank Ms Päivi
Rantalaiho and Mrs Jaana Oksanen at the Cultor Ltd. Tech-
nology Centre for technical assistance during these investi-
gations. This work was funded by Finnfeeds International
Ltd., Marlborough, Wiltshire, U.K.

References

Chesson, A. 1987. Supplementary enzymes to improve the
utilisation of pig and poultry diets. In: Haresign W. &

Cole D.J.A. (eds.). Recent Advances in Animal Nutrition
1987, Butterworths, London, p. 71-89.

Dierick, N.A. 1990. Biotechnology aids to improve feed and
feed digestion: enzymes and fermentation. Arch. Anim.

Nutr. Berlin 39: 241-261.
De Cordt, S., Saravia, J., Hendrickx, M., Maesmans, G. &

Tobback, P. 1992. Changing the thermostability ofBacil-
lus licheniformis a-amylase. Proc. Int. Symp. on ’Stabil-
ity and stabilisation of enzymes’. Maastricht, The Neth-
erlands. Poster.

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Research Note

Deleyn, F., Claeyssens M., van Beeumen, J. & De Bruyne,
C.K. 1978. Purification and properties of (1-xylosidase
from Penicillium wortmanni. Can. J. Biochem. 56:4350.

Godfrey, T. & Reichelt, J. 1983. Industrial enzymology.
582 p. Stockton Press, New York, NY.

Kidder, D.E. & Manners, M.J. 1978. Digestion in the pig.
204 p. Scientechnica, Bristol, U.K.

Pace, C.N. 1990. Measuring and increasing protein stability.
Tibtech 8: 93-98.

Poutanen, K., Puls, J. & Linko, M. 1986. The hydrolysis of
steamed birchwood hemicellulose by enzymes produced
by Trichoderma reesei and Aspergillus awamori. Appi.
Microbiol. Biotechnol. 28: 419-424.

Simons, G. & Georgatsos, J.G. 1990. Immobilisation of
barley (i-glucosidase on solid supports - yields and prop-
erties. Appi. Microbiol. Biotechnol. 33: 51-53.

St. Clair, N.L, & Navia, M.A. 1992. Crosslinked enzyme
crystals as robust biocatalysts. J. Amer. Chem. Soc. 114:
7314-7316.

Sumner, J.B. & Somers, G.F. 1949. Dinitrosalicylic acid for

glucose. In: Laboratory experiments in biological chem-
istry, 2nd ed., Academic Press, New York. p. 3839.

Manuscript received February 1993

Johan Inborr
Finnfeeds International Ltd,
Market House, High Street
Marlborough, Wiltshire SNB IAA
United Kingdom

Present address:
Finnsugar Gmbh
Raboisen 58
D-20095 Hamburg 1,Germany

Anne Grönlund
Cultor Ltd.,Technology Centre
FIN-02640 Kantvik, Finland

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Agric. Sei. Finl. 2 (1993)



Research Note

SELOSTUS

Rehuentsyymien stabiilisuus fysiologisissa olosuhteissa in vitro -menetelmin mitattuna

JohanInborr jaAnne Grönlund

Finnfeeds International Ltd. jaCultor Ltd.

Kahden entsyymipreparaatin, Trichoderrm longibrachiatu-
min tuottaman ksylanaasin (Multifekt K) jarehuentsyymi-
tuotteen(Avizyme SX®), stabiilisuutta tutkittiin in vitro -me-
netelmin sellaisissa olosuhteissa, jotka vallitsevat yksimahais-
ten ruoansulatuskanavan yläosissa. Ksylanaasipreparaatista
tarkasteltiin ksylanaasi- ja P-ksylosidaasiaktiivisuuksien-,
sekä Avizyme SX®:stä P-glukanaasi- ja ksylanaasiaktiivi-
suuksien stabiilisuutta. Aluksi tutkittiin entsyymiaktiivisuuk-
sien stabiilisuutta eri pH:ssa (2,5 ja 7) ja sen jälkeen pepsiinin
ja pankreatiinin vaikutusta entsyymiaktiivisuuksiin. Tutki-
muksessa selvitettiin myös vehnägluteenin entsyymiaktiivi-
suuksia stabiloivaa vaikutusta.

Inkubointi pH 2,s:ssä ja sen jälkeen pH 7:ssä ei vaikuttanut
merkitsevästi Multifekt K:n ksylanaasiaktiivisuuteen. Sen si-
jaan P-ksylosidaasiaktiivisuus aleni 57 %:iin alkutasosta
(P<0.05) ilman vehnägluteenilisäystä. Vehnägluteenilisäyk-
sellä P-ksylosidaasiaktiivisuus pysyi alkutasolla. Avizyme
SX*:n P-glukanaasi- jaksylanaasiaktiivisuus aleni merkitse-
västi kun sitruunahappoa lisättiin inkuboinneissa. Ilman hap-
polisäystä enstyymiaktiivsuudet pysyivät aikatasoilla.

Inkubointi pepsiinillä ja pankreatiinilla alensi merkitsevästi
(P<0.05) Multifekt K:n ksylanaasiaktiivisuutta. P-ksylosi-
daasi aleni 68 %:iin pepsiini-inkuboinnin jälkeen (P<0.10) ja
57 %:iin pankreatiini-inkuboinnin jälkeen (P>0.10). Pepsii-
niinkuboinnissa vehnägluteenilisäys ei parantanut stabiili-
suutta, mutta pankreatiini-inkuboinnissa vehnägluteenin lisä-
ys nosti P-ksylosidaasiaktiivisuuden 97 %;iin alkutasosta.
Avizyme SX®:n pepsiini- japankreatiini-inkuboinnit ja hap-
polisäys alensivat merkitsevästi (P<0.05) P-glukanaasi- ja
ksylanaasiaktiivisuutta. Ilman happolisäystä aktiivisuudet py-
syivat alkuperäisillä tasoilla.

Tulokset osoittavat, että entsyymien osittainen inaktivoitu-
minen voi tapahtua matalassa pH:ssa ja proteolyyttisissä olo-
suhteissa. Avizyme SX®:llä ja vehnägluteenilisäyksellä saatu-
jen tulosten perusteella on kuitenkin odotettavaa, että rehun
mahalaukun sisältöä puskuroiva vaikutus suojaa rehuun lisät-
tyjä entsyymejä inaktivoitumiselta suhteellisen tehokkaasti.
Rehuun lisättyjen entsyymien todellisen inaktivoitumisasteen
määrittämiseksi joudutaan kuitenkin suorittamaan vastaavat
mittaukset kohde-eläimiä käyttämällä.

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Agric. Sei. Fint. 2 (1993)