Symposium

Starch Hydrolysis by the Ruminal Microflora12 @

SUSAN

F. KOTARSKJ,3 RALPH D. WANISKA*

KERRY K. THURN

Upjohn Laboratories, The Upjohn Company, Kalamazoo, MI 49001 and *Dep@jy@eflt of Soil and Crop &iences, Texas A&M University, College Station,

ABSTRACT

5, 6). These factors can all have an effect on the efficiency of starch utilization and food conversion by

The effects of grain type and processing

on nuninalstarch digestionare well documentedbut

the ruminant. If starch fermentation rates are too slow, total tract digestion of starch may be incom

poorly understood at the biochemical and molecular levels. Waxy grains have starches high In amylopectin and are more readily digested than nonwaxy grains.

plete (2, 3). If rates are too rapid and food intake is high, the buffering and absorptive capacity of the

However,the compositionof the endospenncell matrix and the extent to which the starch grarailes are em bedded within ft also affect starch digestion rates. Con tinued work is needed to detennine the infbience of specific cell matrix proteins, protein-starch interactions and cell wall carbohydrates on starch availability. The microbial populations that metabolize starch are diverse, differing in their capacities to hydrolyze starch granules and soluble forms of starch. Surveys show that the amylases are under regulatoty control in most of these or@sms,

but few studies

have addressed

gnitmil

compensate

acids

for the

produced

large

by

amounts

the

of

nmilnal

ments stemming from acute rwninal acidosis (7—9). The impact of plant, microbial and host factors on niminpi

starch

biochemical

the types of

digestion

is poorly

understood

at the

and cellular levels. Such information

Is

needed to increase the potential to manipulate ru minpl

search in these areas, coupled with the deVelOpment and

starch

digestion

rates

and

improve

the

effi

ciency of starch utilization by the niminant. The objective of this review is to provide an overview of

use of isogeneic or near-Isogeneic grain cultivars with characteristics,

not

microflora. This may cause reduced food intake asso ciated with subacute acidosis, or overt clinical all

Impact of other plant polymerson their synthesis. Re defined endosperm

may

fermentation

amylolytic enzymes produced, their regulation and the

biochemically

7X 77843

will

enhance our ability to Identify mechanisms to maul

how starch chemistry and structure within the grain

pulate rominalstarch digestion.J. Nutr. 122: 178-190,

matrix

1992.

influence

starch

utilization

by the

rwnln@1

microflora and to highlight the information available on starch

INDEXING KEY WORDS:

minal

hydrolysis

and utilization

by individual

ru

species.

. starch •ruminal bacteria • ruminants • sorghum digestion

CHEMISTRY AND STRUCTURE OF STARCH IN GRAIN ENDOSPERM Cereal grains are a major component of diets used for intensive production of ruminant livestock in the United States. The primary nutrient in grain fed to ruminants

is

starch,

which

constitutes

Starch is a heterogeneous polysaccharide composed of two polymers, amylose and amylopectin. Amylose is a linear molecule

roughly

of 9()O—3000D-glucopyranose

60—80%of the grain's weight (1). The first site of

residues linked by a-l,4 bonds. Amylopectin is a

starch digestion is the rumen, in which starch is fermented by the resident microflora. Rwninal starch

larger, highly branched

polymer,

averaging

1O&@1O@

fermentation rates vary and are influenced by the grain type, the method of cereal processing, diet and ruminpnt rutyiinal

species starch

(2—6).In turn, the rate and extent of digestion may influence the com

position of the microbial fermentation niniinpl niminpi

1Presentcdat the 31st Annual Ruminant Nutrition Conference, entitled “Starch Digestion: Understanding and Potential for Im

acid produced,

provement,― at the AnnualMeetingof the Federationof Ametican Societies for Ex@mental Biology, April 1, 1990, Wuhington, DC. 2Guest editor for this symposium was C. B. Thonrer, Department of Animal Sciences, University of Arizona, Tucson,

pH, the amount of starch available for post digestion and the biochemical and physical

form in which starch is presented (within the re imaining

plant

0022-3166/92

matrix)

for postruminal

$3.00 C 1992 American

Institute

digestion

AZ 83721.

(2,3,

of Nutrition.

3To whom correspondence should be addressed.

Received 26 June 1991. Accepted 16 July 1991.

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SYMPOSIUM: STARCH DIGESTION

179

glucose residues and consisting of a-l,4--linked r glucan ch@'ins joined at branch points by a-l,6 bonds (1, 10—12).These polymers are deposited within en dosperm cells in semicrystalline granules whose shape (lenticular, polyhedric Qr spherical), size (1-38 pm) and percent content of amylose and amylopectin

semicrystalline arrangement (1, 10, 11). If water is present, the polymers become hydrated, increasing their susceptibility to enzymatic hydrolysis (1, 10-12). During processing, the peripheral and comeous regions of the endosperm retain their

vary

more starch diimiige and gelatinization

with

plant

species

and

cultivar

(1,

1 1, 12).

Starches purified from nonwaxy and heterowaxy cul tivars of various grains contain 14-34% amylose.

structure longer than the floury endosperm. Thus, occur in the

floury endosperm than in the corneous or peripheral endosperm during cereal processing (17).

Starches from waxy cultivars have