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Department of Animal Sciences, University of Illinois, Urbana, Illinois 61801
3To whom correspondence should be addressed at Department of Animal Sciences, University of Illinois, 132 Animal Sciences Laboratory, 1207 W. Gregory Drive, Urbana, IL 61801. E-mail: g-fahey{at}uiuc.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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KEY WORDS: • starch • total dietary fiber • digestion • fermentation • in vitro • dog
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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Although most fermentation occurs in the large bowel, a few studies
(Ruseler-van Embden et al. 1992
, Zentek 1995
) suggest that fermentative activity can occur in the small
intestine. Åman et al. (1995
) indicated that
substantial degradation of mixed-linked ß-glucans may occur in
ileostomy subjects, presumable due to bacterial fermentation in the
small intestine. The ileum of humans has been reported to contain
bacteria in concentrations of
105–106 colonies/g of
contents (Drasar and Hill 1974
). Small intestinal
bacteria ostensibly could affect digestive processes occurring at this
site. Relatively little data are available on the effects of starch and
fiber fractions in selected food and feed ingredients on small
intestinal and large bowel digestibility characteristics.
The objectives of this research were to first compile a starch and
fiber fraction database for common food and feed ingredients. The
general categories studied were legumes, cereal grains, cereal and
potato flours, grain-based food products and reference substrates.
Second, in vitro ileal digestible starch
(IDS)4
and total tract digestible starch (TDS) values were determined using a
monogastric starch digestion model. Finally, the ileal disappearance
and fermentative characteristics of selected food and feed ingredients
were determined using ileal microbes from dogs in an in vitro model.
Information gained in this experiment will aid in the understanding of
effects of microbes in the distal small intestine on the starch and
fiber fraction of food and feed ingredients. The dog was used in the in
vitro experiments as an animal model for humans. Both dogs and humans
are omnivorous monogastrics. The lower gastrointestinal tract of the
dog, like that of humans, contains numerous endogenous species of
bacteria (Balish et al. 1977
; Davis et al. 1977
) that contribute significantly to colonic fermentation
(Banta et al. 1979
). The contribution of the large bowel
to total digestive tract volume is also similar in the dog (14%) and
the human (17%), in contrast to that in the pig (48%) and the rat
(61%) (van Soest 1995
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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Chemical analyses.
All substrates (legumes, cereal grains, cereal and potato flours,
grain-based food products and reference substrates) were analyzed
for dry matter (DM), organic matter (OM), Kjeldahl nitrogen (N)
(Association of Official Analytical Chemists 1985
) and
total dietary fiber (TDF) (Prosky et al. 1984
).
Insoluble fiber (I) was determined according to the method of
Prosky et al. (1992
). Soluble fiber (S) was calculated
by subtracting the I from the TDF. Total fat content was determined by
acid hydrolysis followed by ether extraction according to the
American Association of Cereal Chemists (1983
) and
Budde (1952
).
Starch fractions [free glucose (FG), rapidly digestible starch (RDS),
slowly digestible starch (SDS) and resistant starch (RS)] of samples
were determined according to the methods of Muir and O’Dea (1992
and 1993
). Total starch (TS) values were determined
according to the method of Thivend et al. (1972
). Both
starch fractionation and TS assays used dimethyl sulfoxide to
disassociate the retrograded amylose (Englyst and Cummings 1984
).
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Experiment 1: Quantification of starch and fiber fractions and in vitro IDS and TDS values for common food and feed ingredients |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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Substrates used in the in vitro experiment consisted of seven legumes (black beans, red kidney beans, lentils, navy beans, black-eyed peas, split peas and northern beans) and nine cereal grains (barley, corn, white rice, brewer’s rice, brown rice, wheat, millet, oats and sorghum), all purchased from local vendors. The substrates were ground through a 2-mm screen in a Wiley mill. Seven flours (corn, wheat, rice, potato, soy, barley and sorghum) were obtained from a pet food manufacturer. Flours had been prepared according to the normal methods of grinding, fine milling, sieving and steam processing. Other substrates included six prepared grain-based food products (macaroni, spaghetti, corn meal, rice bran, rolled oats and hominy grits) purchased from local vendors. The final set of samples included three reference substrates: corn starch (73% amylopectin, 27% amylose; Sigma Chemical Co., St. Louis, MO), potato starch (approximately 80% amylopectin, 20% amylose; Sigma Chemical Co.) and amylomaize (Crystalean; almost 100% amylose; Opta Food Ingredients, Bedford, MA). These standards were included as part of each fractionation method to validate the efficacy of the experimental conditions imposed (i.e., a database containing information on key response criteria measured in this experiment was available for these standards, and any deviations in results obtained with these standards resulted in invalidation of the entire set of substrates being studied).
Donors.
Two mixed-breed purpose-bred mature female ileally cannulated dogs
(Walker et al. 1994
) with hound bloodlines had ad
libitum access twice daily to a commercial diet (Diamond Petfoods,
Meta, MO) containing 
21% crude protein (CP) and 12% fat for
14 d before the collection of feces. Major ingredients in the diet
included ground corn, poultry by-product meal, chicken fat and beet
pulp. Dogs were housed in a temperature-controlled room in 1.2
x 3.1-m solid-floor pens. Free access to water was provided
at all times.
Monogastric in vitro digestion model.
This model represents a combination of three assays used to determine
the amount of digestible starch at the ileum and in the total
gastrointestinal tract. The method of Muir and O’Dea (1993
) was used to determine the amount of starch digestion in
the stomach and small intestine by measuring glucose in the supernatant
resulting from acid-enzyme digestion of the substrate. Each
substrate in triplicate was exposed to pepsin/hydrochloric acid,
amyloglucosidase and 
-amylase. Tubes containing reagents but no
substrate were run as blanks. Glucose concentrations then were
determined on the supernatant. Glucose was measured according to a
glucose oxidase method (Glucose Test Kit 510-A; Sigma Chemical Co.).
Glucose concentration was determined by reading the absorbance of
individual samples at 450 nm on a DU 640 spectrophotometer (Beckman
Instruments, Schaumburg, IL) and comparing those values against a
glucose standard curve. IDS was determined by subtracting (FG x0.9)
from (total glucose/original sample weight) present in the supernatant
after 15 h of digestion. The 0.9 used in the calculation of IDS is
a correction factor for the difference in weight between an FG unit and
a glucose residue in starch. Because the measurement of glucose is used
to determine starch content, the correction factor is needed. The
substrate remaining after simulated stomach and small intestinal
digestion then was used in a model that simulated large bowel
fermentation (Bourquin et al. 1993
). Freshly voided
feces from two dogs was diluted (1:10) in anaerobic diluting solution.
This inoculum was used to inoculate all substrates individually for
each dog. Substrates were incubated in an in vitro medium (Table 1
) at 39°C for 12 h. TS was determined in the pellet that remained
after simulated large bowel fermentation according to the method of
Thivend et al. (1972
) with dimethyl sulfoxide
solubilization of amylose. TDS was determined by subtracting [(total
glucose/original sample weight) x 0.9] in the remaining sample
from the percentage TS.
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Experiment 2: Determination of the ileal disappearance and fermentative characteristics of selected food and feed ingredients |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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All substrates were the same as those described for expt. 1.
Donors and collection methods.
Six mixed breed purpose-bred female ileally cannulated dogs
(Walker et al. 1994
) had ad libitum access twice daily
to the diet used in expt. 1 for a 14-d period before the collection of
ileal effluent. Dogs were housed in a temperature-controlled room
in 1.2 x 3.1-m clean-floor pens. Free access to water was
provided at all times. Fresh ileal fluid was collected from each dog
for 15-min intervals in a Whirlpak bag (Pioneer Container Corp.,
Cedarburg, WI) until sufficient amounts needed to inoculate all tubes
were obtained. At the end of each 15-min period, the bags were removed
and replaced with new ones. Bags containing samples were sealed
immediately after expressing excess air, placed inside a prewarmed
thermos (37°C) and transported to a laboratory within the same
building for processing.
Medium composition and substrate fermentation.
The composition of the medium used to culture the ileal microflora is
presented in Table 1
. All medium components except the
vitamin mixes were added before autoclaving. The vitamin mixes were
aseptically added after they were filter-sterilized.
On arrival in the laboratory, fresh ileal samples were immediately
pooled under anaerobic conditions and diluted 1:10 (v/v) in a 39°C
anaerobic dilution solution (Bryant and Burkey 1953
) by
blending for 10 s in a Waring blender. Blended, diluted ileal
effluent was filtered through four layers of cheesecloth, and the
filtrate was sealed in 125-mL serum bottles under
CO2. Appropriate sample and blank tubes
containing 26 mL of medium and 300 mg of substrate were aseptically
inoculated with 4 mL of diluted ileal effluent. Tubes were flushed with
CO2 and capped with stoppers equipped with
one-way gas release valves. Blank tubes contained 4 mL of inoculum
and 26 mL of medium but did not contain any substrate.
Triplicate tubes were placed in a forced air incubator at 39°C with periodic mixing for each fermentation time period (2.5, 5 and 7.5 h). At the appropriate time, tubes were removed from the incubator and processed immediately. A 2-mL aliquot was removed from each tube for short-chain fatty acid (SCFA) and lactate analyses. The remaining 28 mL was combined with 112 mL of 95% ethanol and allowed to set for 1 h to precipitate the soluble polysaccharide fractions. To recover unfermented residues, samples were filtered through tared Whatman 541 filter paper and washed sequentially with 78% ethanol, 95% ethanol and acetone. Samples then were dried at 105°C, weighed, ashed in aluminum weigh boats (500°C) and weighed again to determine OM disappearance (OMD). In vitro OMD (percentage) was calculated as {1 - [(OM residue–OM blank)/original OM]} x 100, where OM residue is the OM recovered after 2.5, 5 or 7.5 h of fermentation; OM blank is the OM recovered in the corresponding blank after the same fermentation times; and original OM is the OM of the substrate placed in the tube. Corrected OMD was calculated as the 2.5-, 5- and 7.5-h OMD minus the 0-h OMD.
The 2-mL aliquot of fluid removed from the sample tubes for SCFA and
lactate analyses was immediately added to 0.5 mL of metaphosphoric acid
(250 g/L), precipitated for 30 min and centrifuged at 20,000 x g for 20 min. The supernatant was decanted and frozen at
-20°C in microfuge tubes. After freezing, the supernatant was thawed
and centrifuged in microfuge tubes at 10,000 x g for
10 min. Concentrations of acetate, propionate and butyrate were
determined in the supernatant using a Hewlett-Packard 5890A Series
II gas-liquid chromatograph and a glass column (180 cm x 4 mm
i.d.) packed with 10% SP-1200/1%
H3PO4 on 80:100 mesh
Chromosorb WAW (Supelco, Bellefonte, PA). SCFA concentrations were
corrected for by the blank tube production of SCFA. The supernatants
also were analyzed for lactate concentration according to the
spectrophotometric method described by Barker and Summerson (1941
).
Statistical analysis.
The General Linear Models procedures of SAS (1994)
were used to analyze
data from these experiments. In expt. 1, the experimental design was a
randomized complete block design with the two fecal donors serving as
blocks. Donor x substrate was used in the statistical model. In
expt. 2, the experimental design was a factorial arrangement of
substrates within groups (legumes, cereal grains, cereal and potato
flours, grain-based food products and reference substrates) and
fermentation times (0, 2.5, 5 and 7.5 h). Arithmetic means are
reported along with the SEM for each group of substrates.
When significant (P < 0.05) differences were
detected, individual means were compared using the least significant
difference (LSD) method of SAS (1994)
.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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The chemical composition of substrates is presented in Table 2
. Chemical composition of legumes varied widely, corroborating data of
Kamath et al. (1980
). In our experiment, DM
concentrations were similar among legumes, except for navy beans, which
were lower in DM. OM concentrations were similar among substrates. CP
content ranged from a low for navy beans to a high for lentils and
northern beans. The CP concentration of lentils agreed with the
reported range of 20.4–30.5% (Salunkhe et al. 1985
).
The fat concentration of legumes ranged from a low for navy beans to a
high for northern beans. Concentrations of TDF were high for the entire
legume group, with black beans having the highest TDF content and
black-eyed peas having the lowest. The legume group contained
mainly I (92.2–100% of TDF), whereas S values ranged from 0 to 7.8%
of the TDF.
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3 percentage units
for the cereal group. CP content varied from a low for white rice to a
high for wheat. Watson (1953
) and Juliano et al. (1964
) reported CP values of 11.8 and 11.0% for barley and
brown rice, respectively. These values agree with our values of 12.7%
for barley and 10.2% for brown rice. Cereal grain fat concentrations
ranged from a low for white rice to a high for oats. Total dietary
fiber concentrations varied widely, with oats containing 25 times
more TDF than white rice. Cereal grains contain the husk, pericarp
and/or bran in varying concentrations, thus providing components that
contain fiber, albeit in generally lower concentrations compared with
legumes. The majority of TDF found in the cereal group was I, although
cereals contained a much greater proportion of S than did legumes.
DM concentrations were similar among the grain and potato flour group.
OM concentrations ranged from 92.9% (soy) to 99.6% (sorghum). CP
content was highest for soy and lowest for brown rice. Soy flour
contained the highest concentration of fat in the flour group. Total
dietary fiber concentrations varied from a high for barley flour to a
low for sorghum flour. High concentrations of fiber in barley flour may
be a result of high concentrations of ß-glucans present in the grain
(Liljeberg et al. 1992
). I concentrations again were
higher than S concentrations in the flour group, although large
differences in S concentrations occurred among substrates. Overall,
grain flours contained less TDF compared with their cereal grain
counterparts.
DM concentrations of grain-based food products differed by only 3 percentage units. OM concentrations were similar among substrates except for rice bran, which contained more ash (>10%). CP content ranged from 7.7% (corn meal) to 16.9% (rice bran). Fat concentrations ranged from a high for rolled oats to a low for corn meal. Total dietary fiber varied widely among the prepared grain products, with rice bran being the highest, rolled oats and hominy grits being intermediate, and macaroni, spaghetti and corn meal being the lowest. I concentrations were highest for rice bran and lowest for macaroni. As a percentage of TDF, S concentrations were highest (mean 34.5%) for macaroni, spaghetti, rolled oats and hominy grits.
DM, OM and CP concentrations were similar among the reference substrates. Corn and potato starch contained low fat concentrations. Corn starch and potato starch contained no TDF. Although 5.3% TDF was detected in the amylomaize, this is probably RS rather than fiber.
Starch fractions.
Concentrations of starch fractions of substrates are presented in
Table 3
. FG concentrations were low for all substrates, as expected.
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) reported
that leguminous starches displayed a C-type pattern of
crystallinity. This type of starch is more resistant to hydrolysis than
that with an A-type crystallinity pattern and helps explain why
legumes have high amounts of RS.
Gee and Johnson (1985
) found that there was a
relationship between the "half-time starch hydrolysis" (time taken
to achieve 50% hydrolysis of the original starch of the substrate) and
the dietary fiber content of certain foods. Their data indicate that
legumes such as peas and red kidney beans all had higher half-time
hydrolysis rates (60.0 and 58.0 min, respectively), whereas white bread
and white rice had much lower values (19.5 and 2.1 min, respectively).
In their study, dietary fiber content averaged 27.8% for peas and red
kidney beans and only 3.1% for white bread and white rice.
McBurney et al. (1988
) also found that SCFA production
from ileal effluent was significantly correlated with dietary fiber
isolates but not whole foods. The authors concluded that dietary fiber
isolates, rather than whole foods, could provide the closest estimation
of colonic SCFA production. Another possible reason for the higher RS
concentrations in legumes could be the relationship between starch and
protein. Tovar et al. (1990
) found that when red kidney
beans were preincubated with pepsin, there was an increase in their
susceptibility to amylolytic attack.
TS values of legumes obtained by adding FG, RDS, SDS and RS components
closely paralleled those reported from the determination of starch
using the method of Thivend et al. (1972
), attesting to
the accuracy of the Muir and O’Dea (1993
) method for
quantifying starch fractions. A possible explanation for the higher
concentration of TS for substrates such as black beans and
black-eyed peas using the Thivend et al. (1972
)
method may be the inclusion of sucrose in the measurement. This method
enzymatically converts sucrose into monosaccharides and allows for
their recovery in the supernatant. The Muir and O’Dea (1993
) method does not account for this conversion, so sucrose
is not part of the starch value.
Cereal grains varied widely in percentage starch found in each of the
starch fractions. RDS and SDS concentrations represented the majority
of the TS in the cereal group. Cereal grains have an A-type
crystalline form, which is the starch structure least resistant to
hydrolysis (Ring et al. 1988
). This crystalline form
leads to more of the starch being categorized as RDS and SDS. RDS
concentrations as a percentage of TS varied from 32.5 to 82.5%. White
rice contained the highest concentration of SDS (51.4%) as a
percentage of TS. Raw cereals are partially inaccessible to digestion
due to the physical form of the cereal itself (Englyst et al. 1992b
). Structures like the pericarp and seed coat may impede
the efficiency of amylase digestion of starch in cereal grains.
RS concentrations were highest for sorghum and lowest for oats. Four
categories of RS have been defined (Brown 1996
). The
first category (RS1) includes starch granules that are physically
inaccessible and can be found in whole or partially milled grains and
legumes. The second category (RS2) refers to native starch granules,
whereas the third category (RS3) refers to retrograded starch that is
formed during processing. The fourth category of RS (RS4) was only
recently described and includes chemically modified starches resistant
to enzymatic hydrolysis to some degree.
Flours also varied widely in percentage starch found in each of the
starch fractions. Approximately 95% of the TS in flours is RDS and SDS
combined. RS concentrations were highest for corn and soy and lowest
for barley. Englyst et al. (1992a) reported that white
wheat flour contained 49% RDS, 48% SDS and 3% RS as a percentage of
TS. Our wheat flour contained 55.4% RDS, 42.2% SDS and 2.5% RS as a
percentage of TS, agreeing closely with the values of Englyst et al. (1992a
). RS concentrations were low for the flour group as
a whole. Cereal flours display an A-type crystalline pattern, which
is more readily hydrolyzed than raw cereals that are not as highly
processed as flours. Therefore, cereal flours contain more RDS and SDS
than RS.
The nutrient profile of cereal grains and their corresponding flours
varied considerably. Grain flours are made up primarily of two
components: protein and starch. Cereal grains, in contrast, contain the
pericarp, aleurone layers and germ portions of the grain that provide
lipid and fiber (Hoseney 1994
). Cereal grains are
processed and milled to flours, thereby altering the chemical
composition of the flour compared with the cereal grain. Even DM
concentrations varied when cereal grains were compared with their flour
counterparts. Except for barley, flours were numerically higher in OM.
CP concentrations of flours were 1–3 percentage units lower than that
for ground grains. Total dietary fiber concentrations of flours, except
for barley, were numerically lower compared with their ground grain
counterparts. This reduction in TDF points to how the processing of
grains alters their fiber content through removal of the pericarp,
aleurone layers and germ. Of interest is how the processing of cereals
to flours affects the starch fraction profile. The combined RDS and SDS
concentrations of cereal grains were 74% of TS versus, flours,
which were 95% of TS. The RS concentrations were, on average, five
times higher in the cereal grains than in the flours.
For the grain-based food products, RDS concentrations, expressed as
a percentage of TS, were highest for macaroni and rolled oats;
intermediate for spaghetti, corn meal and hominy grits; and lowest for
rice bran. SDS concentrations varied, with corn meal having the highest
concentration and rolled oats having the lowest. Prepared grain
products contained moderate levels of RS (mean 9.6% as a percentage of
TS). Hermansen et al. (1986
) postulated that starch in
foods like spaghetti is more slowly digested because of the densely
packed starch in the food. During pasta production, pasta is kneaded
and extruded, leading to a tight, entrapped starch granule
(Colonna et al. 1990
). Again, food ingredients like rice
bran with high TDF (28.0%) may experience a lower amount of starch
hydrolysis as a result of its fiber content. Corn meal contained the
highest concentration of TS, whereas rice bran contained the lowest.
The reference substrates varied widely in their starch fractions. RDS
values were highest for corn starch and similar for potato starch and
amylomaize. SDS values were similar for corn starch and amylomaize and
lower for potato starch. As a percentage of TS, potato starch had the
highest RS concentration and corn starch had the lowest. Englyst et al. (1992a
) found that raw potato starch contained 75% RS
as a percentage of TS. Starches from tubers such as potatoes tend to
exhibit B-type crystallinity patterns that are highly resistant to
digestion (Englyst et al. 1992a
). Amylomaize contains
mostly amylose, which has been shown to lower not only digestibility
but also blood insulin and glucose values in humans (Behall et al. 1995
).
A common characteristic of all foods and feeds studied is that RS is a component of each. This starch fraction is not hydrolyzed and enzymatically digested in the small intestine but rather serves as a substrate for fermentation by microflora either in the ileum and/or large bowel.
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In vitro experiment 1 |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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Fermentative events in the nonruminant occur as a result of bacterial
activity in the colon and possibly in the ileum of the small intestine.
Ruseler-van Embden et al. (1992
) found >25 different
species of bacteria residing in the small intestine of dogs.
Murray et al. (2000
) found the following
colony-forming units (CFU)/mL of ileal effluent after isolation and
plating: 4.2 x 108 total anaerobes, 7.1
x 105 total aerobes, 1.3 x 106 Escherichia coli, 1.7 x 108 Clostridium perfringens, 1.8
x 108 Bifidobacteria and 3.3
x 106 Lactobacillus.
Finegold et al. (1970
) found that in the human
ileostomate, there were
107–108 colonies/g of
ileal contents. These values confirm that there may be a substantial
bacterial population residing in the small intestine of both dogs and
humans. It is uncertain whether ileal microbes are indigenous to this
site or whether they emanate from the cecum, finding their way via the
ileocecal valve into the small intestine. The contents of the small
intestine normally flow rapidly, possibly becoming static for an
appreciable period only in the distal small intestine (Drasar and Hill 1974
).
IDS and TDS concentrations.
IDS concentrations for the legume group were statistically highest
(P < 0.05) for black-eyed peas and split peas,
next highest for lentils and navy beans and lowest for northern beans,
black beans and red kidney beans (Table 4
). Bjorck et al. (1992
) reported that the small
intestinal digestibility by rats of a cooked and canned pea product was
70%. As a percentage of TS (i.e., IDS/TS), our ileal digestibility
value for split peas (45.7%) was lower and may be due to the raw,
unprocessed nature and high RS content of this substrate. Key et al. (1995
) also found that as the concentration of cooked
haricot beans in the diet of rats increased from 0 to 450 g/kg, ileal
digestibility decreased from 87 to 69% for the haricot
bean–containing diet.
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) found that wrinkled peas contained 55.4% amylose
as a percentage of TS, whereas beans contained higher amounts of
amylose (mean 59.2%). Higher amylose concentrations in starches are
believed to lower digestibility (Borchers 1962
). This
could explain why the peas displayed higher TDS concentrations and the
beans displayed lower TDS concentrations. Goodlad et al. (1992
) found that as the proportion of peas doubled from 250 to
500 g/kg in the diet of rats, total tract digestibility was reduced
(P < 0.05) from 94 to 91%. The TDS concentrations
were low for the bean group as a whole, with a possible explanation
being the physical entrapment of the starch within fibrous,
thick-walled parenchyma cells (Wursch et al. 1986
).
Also, antinutrients (e.g., enzyme inhibitors, lectins and tannins) have
been found in legumes, and that could reduce the digestibility of
legume starches. RS and TDF concentrations also were high for the
legume group and could affect starch digestion in these substrates. Starch utilized by the microflora in the large bowel (percent TDS - percent IDS) was numerically highest for split peas and lowest for northern beans.
For the cereal grains group, IDS concentrations were highest
(P < 0.05) for brewer’s rice and lowest for oats. As
a percentage of TS, starch in oats and barley was completely digested
at the ileum. Englyst and Cummings (1985
), using human
ileostomates, found that raw oat starch hydrolysis was complete in the
small intestine. Barriers to amylase digestion apparently do not impede
the starch in oats. Of interest is how white rice and brown rice differ
with respect to IDS values. O’Dea et al. (1981
) found
that relative rates of starch hydrolysis in an in vitro system
correlated very closely with in vivo peak glucose responses in humans.
In vitro rates of starch hydrolysis (percent hydrolyzed/30 min) for
ground brown rice and white rice were 68.2 and 71.8%, respectively.
O’Dea et al. (1981
) suggest that fiber might act
indirectly to slow carbohydrate absorption, restricting access of
hydrolytic enzymes to starch from an unrefined source like brown rice.
Our brown rice source was higher in TDF content and lower in IDS than
white rice. Two other grains have been shown to have structural
differences that may contribute to resistance to digestion.
Hosney (1994
) suggested that the protein-starch
matrix of sorghum and corn grains was quite strong, making hydrolysis
and digestion more difficult.
TDS concentrations for cereals varied from a high (P < 0.05) for millet to a low (P < 0.05) for oats and
barley. TDS concentrations were high, indicating continued digestion
of starch by the microflora once it reached the large bowel.
Moore et al. (1980
) fed plant-based diets containing
one of three grain sources (rice, oats or corn) to dogs. Total tract
starch digestibility for the uncooked oat diet (93.8%) was lowest,
intermediate for the uncooked corn diet (94.3%) and highest for the
uncooked rice diet (98.6%). Our values were 92.6% for corn, 86.0%
for white rice and 100.0% for oats. The dog diets in the Moore et al. (1980
) study were extruded; this leads to increased
susceptibility to amylase and greater starch digestion.
Cereal starch utilization by microflora in the large bowel varied.
Millet had the highest (P < 0.05) and wheat had the
lowest (P < 0.05) digestibility values. Of interest is
how fermentable each substrate is if any appreciable amount reaches the
large bowel. According to Hosney (1994
), millet, sorghum
and corn starch granules appear to be similar. In our study, their
fermentative capabilities were similar, with millet having the highest
value.
IDS concentrations for flours were lowest (P < 0.05)
for soy, which has very low starch concentrations, but higher for
wheat, barley, potato and brown rice. The highest (P < 0.05) concentrations were noted for corn and sorghum. IDS
concentrations were high for most flours. With low TDF and RS
concentrations in the flours, there appears to be less of a barrier to
digestion of starch. Our wheat flour had a lower IDS concentration
compared with all other flours with the exception of soy. (Snow and O’Dea 1981)
assayed different flours (rice, barley, rye,
white [bleached wheat flour] and wheat) to determine their in vitro
starch hydrolysis capacity. After 30 min of hydrolysis, all flours were
similar in percent starch hydrolyzed (mean 16.1%) except for wheat
flour. The authors postulated that an amylase inhibitor may have
affected the hydrolysis rate of the wheat flour. Also, wheat starch can
contain nonstarch polysaccharides (Topping et al. 1993
).
The flour that had the lowest (P < 0.05) TDS
concentration was soy, whereas corn had the highest (P
< 0.05) TDS value. All flours were virtually completely digested
when TDS concentrations were compared. The flours used were primarily
composed of RDS and SDS (mean 95.1%), and as a result of processing,
most barriers to digestion are overcome. Murray et al. (1999
) found that the starch component of canine diets
containing high-starch flours as the main source of carbohydrate
was nearly completely digested (>99%).
Starch utilization by microflora (percent TDS - percent IDS)
varied numerically in the flour group from a low for sorghum to a high
for wheat. Microflora fermented virtually all available remaining
starch. Even though the wheat flour IDS concentration was relatively
low, large bowel microflora appeared to ferment the remaining starch
well. The wheat amylase inhibitor mentioned by Snow and O’Dea (1981
) appeared to have no effect on the microflora once the
wheat starch was placed in an environment simulating the large bowel.
For the grain-based food products, IDS was lowest for rice bran and
highest (P < 0.05) for corn meal. Expressed as a
percentage of TS, rice bran, rolled oat and hominy grit starches were
completely digested at or before the ileum. Macaroni and spaghetti were
well digested at the ileum (95.5 and 91.5% as a percentage of TS,
respectively), but certain factors can reduce their susceptibility to
amylolytic attack. Colonna et al. (1990
) found that
high-temperature drying of pasta may result in high levels of
protein cross-linking, leading to a greater encapsulation of starch
and thus decreasing its susceptibility to amylase. There also can be
differences (P < 0.05) between the digestion of
macaroni and spaghetti, as noted in our study. Granfeldt and Bjorck (1991
) tested macaroni and spaghetti glucose
responses in 10 human subjects. Spaghetti resulted in a glycemic index
score of 60.5, whereas macaroni resulted in a score of 78.0. Macaroni
had a lower product thickness and a greater surface area that allowed
easier access to amylase. Rolled oats were completely digested at the
ileum. This corroborates the results of Heaton et al. (1988
), where insulin responses were measured in humans fed
certain cereal products (corn, wheat or rolled oats). Rolled oats
resulted in a higher peak insulin response compared with oat flour.
Decreasing the particle size of both corn and wheat seemed to increase
digestion rate, but this was not the case for oat products.
When grain-based food products were compared, TDS concentrations were different (P < 0.05) among substrates. The highest (P < 0.05) TDS value was found for corn meal. Rice bran, rolled oats and hominy grits were completely digested proximal to the terminal ileum. The processing and cooking of rice bran and rolled oats affect their digestion. As mentioned previously, rolling oats appeared to disrupt the structural integrity of the grain, leaving it accessible to enzymatic attack. Processing of the rice kernel through a milling machine produces rice bran and polished rice. The compositions of rice and rice bran vary greatly due to this processing. Rice bran is composed of the aleurone layer and some parts of the endosperm and germ of the rice kernel after milling.
Starch utilization by microflora (percent TDS - percent IDS) again varied for the grain-based food products. Corn meal was highest (P < 0.05) compared with all other substrates. A larger percentage of starch was fermented in the large bowel for spaghetti compared with macaroni. This relates to the greater amount of starch escaping digestion in the small intestine, making spaghetti more efficacious if the goal is to supply the large bowel with more starch.
Of the reference substrates, IDS concentrations were lowest
(P < 0.05) for potato starch, intermediate for
amylomaize and highest (P < 0.05) for corn starch.
High concentrations of RS in potato starch cause its digestion to be
limited in the small intestine. Mathers et al. (1997
)
fed either a raw potato or corn starch diet to rats and found that the
digestibility of the corn starch diet was 99% at the ileum, whereas
only 28% of the potato starch diet was digested at the ileum. Native
potato starch granules are composed of a B-type crystalline
pattern. These granules exist as a layer of large blocklets that appear
to confer resistance to enzymatic hydrolysis (Gallant et al. 1992
). Amylomaize was more digestible than potato starch,
possibly due to its lower concentration of RS.
TDS concentrations were lowest (P < 0.05) for
amylomaize, intermediate for potato starch and highest (P
< 0.05) for corn starch. Total tract digestibility of potato
starch fed to rats at 240 g/kg of the diet was 80%, whereas corn
starch at 240 g/kg was 100% (Mathers et al. 1997
).
Starch utilization by microflora in the large bowel (percent TDS
- percent IDS) was greatest (P < 0.05) for
potato starch, pointing to its high fermentative capacity. Of interest
is that although potato starch was lower in IDS, it was higher in TDS
compared with amylomaize. Lajvardi et al. (1993
) fed
rats either a cooked potato starch, arrowroot starch, high amylose corn
starch or raw potato starch diet. Raw potato starch was found to be the
most fermentable starch of the four tested. Only raw potato starch was
found to significantly prolong gastrointestinal transit time, possibly
allowing this substrate a longer time to ferment in the large bowel.
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In vitro experiment 2 |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1: Quantification of...Experiment 2: Determination of...RESULTS AND DISCUSSIONIn vitro experiment 1In vitro experiment 2REFERENCES |
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This experiment was conducted to determine whether ileal fermentation events, independent of hydrolytic digestion events, affected the disappearance of OM for a widely divergent group of substrates. Although data were collected at 0-, 2.5-, 5- and 7.5-h time periods, only those collected at 7.5 h are reported because they were judged to be most relevant from a biological perspective (i.e., data at the 5- and 7.5-h time points were similar; 7.5 h is about the length of time chyme would be available to ileal microbes).
OMD of substrates is reported in Table 5
. All substrate x time interactions were significant at
P < 0.05. After correction for solubility, OMD was
very low for the legume group as a whole. Solubilization of the
substrates at the 0-h fermentation time was high (13–17%), resulting
in lower corrected OMD values. Red kidney beans and black beans had the
lowest (P < 0.05) OMD values of all legumes tested.
Schweizer et al. (1990
) found, using ileostomates fed a
white kidney bean flake–containing diet, that 10% of the bean
starch was not absorbed from the small intestine. Tovar et al. (1992
) postulated that the high amylose-to-amylopectin ratio,
the physical insulation of starch by thick-walled cells and the
presence of amylase inhibitors resulted in a reduction in digestibility
of leguminous starches. These physicochemical characteristics of
legumes act as direct inhibitors of -amylase and, thus, starch
breakdown.
|
). Cell walls and encapsulation of
starch in a protein matrix of whole grains greatly affect digestion by
reducing access of amylase to the starch itself.
Flours as a group were very digestible by ileal microbes. Potato and
soy flours had extremely high solubility values. Rice and corn had the
highest (P < 0.05) OMD values compared with other
flours, and wheat flour had the lowest (P < 0.05).
Interestingly, the two former flours had among the lowest solubility
values. Wheat flour was two to three times lower in OMD than all other
flours. Murray et al. (2000
) also reported a low OMD at
7.5 h for wheat flour (1.9%). In this case, protein may
encapsulate the starch granules, thereby reducing the digestibility of
the starch (Annison and Topping 1994
).
Of particular interest is how processing affects substrate
disappearance. For example, barley flour was approximately five times
more digestible compared with barley grain. Heaton et al. (1988
) compared particle size effects of wheat, corn and oats
on human in vivo plasma insulin responses and on in vitro rate of
starch digestion by pancreatic amylase. Insulin responses were as
follows: whole grains < cracked grains < coarse flour
< fine flour. In vitro starch hydrolysis by amylase was faster
for grains of smaller particle size. Larger food particles have a lower
surface-to-volume ratio, and this might reduce the access of enzymes to
the interior of the particle as might the presence of intact cell
walls. Processing affects the physical nature of cereals, causing the
disruption of the cell matrix and increasing starch digestion.
Grain-based food products ranged in OMD from a low for macaroni to a
high for rolled oats. Knudsen et al. (1993
) stated that
oat bran, a rich source of dietary fiber containing ß-glucans, is an
easily fermentable energy source for microflora. The process of rolling
would make the fiber more accessible to microbial enzymes during the
fermentation process.
OMD was greatest (P < 0.05) among reference substrates for corn starch, intermediate for amylomaize and lowest (P < 0.05) for potato starch. Potato starch contained the highest concentrations of RS, which influenced its digestion.
Data indicate that small intestinal bacteria ferment cereal grains and flours differently. The flour group had relatively high OMD values (mean 27.5%), whereas the cereal group had relatively low OMD values (mean 8.8%). This relates to the greater amount of processing that resulted in production of the flours. The cereal grain, as a result of this processing, loses TDF and RS components, as was found in this study. The lower concentrations of TDF and RS in flours lead to increased susceptibility to both enzymatic and microbial digestion.
Organic acid production.
SCFA and lactate production data at the 7.5-h fermentation time are
reported in Table 5
. All substrate x time interactions
were significant at P < 0.05.
Among leguminous substrates, the greatest (P < 0.05)
total production of SCFA was for split peas, and the lowest was for
navy beans. The high concentrations of total SCFA as a result of pea
fermentation point to the ability of this substrate to be more rapidly
fermented than beans. Bjorck and Siljestrom (1992
) found
that 90% of a pea product that reached the large bowel of a rat was
fermented. The lower amylose content of peas could lead to higher
fermentability by microflora, whether ileal or large bowel in origin.
Tovar et al. (1992
) reported that lentils contained more
potentially available starch than did red kidney beans, corroborating
the higher total SCFA concentration.
Lactate production was similar for all legumes. The largest amount of lactate produced was for split peas and black-eyed peas, whereas the lowest lactate production was for navy beans.
Among cereals, barley and oat fermentation resulted in the greatest (P < 0.05) total SCFA concentrations. The lowest (P < 0.05) total SCFA concentrations were for corn, white rice, brown rice and sorghum. Lactate production was greatest (P < 0.05) for barley compared with all other cereal grains.
Butyrate concentrations found in oats and barley (data not shown) were numerically higher compared with the cereal grains group (mean 0.66 mmol/g OM) as a whole. The presence of ß-glucans, a soluble dietary fiber found in both oats and barley, may have stimulated butyrate production by ileal microflora.
Potato flour resulted in the highest (P < 0.05) total
SCFA production compared with all other flours. Murray et al. (2000
) also found that potato flour was numerically highest in
total SCFA production when comparing six different flours incubated in
inoculum containing ileal microorganisms. Processing was suggested as
responsible for the increased susceptibility of potato flour to
fermentation. The lowest (P < 0.05) total SCFA
production was for sorghum and corn flours.
Flour fermentation resulted in generally higher lactate concentrations
than for the other groups. Average lactate production for flours was
0.23 mmol/g OM. Zentek (1995
) performed in vitro studies
using canine ileal chyme to measure the fermentative capabilities of
different substrates. He postulated that ileal fermentation of
carbohydrates favored the growth of lactobacilli, which produce lactate
as a major metabolic end-product. The high starch levels resulting
from extensive processing of flours may have created a favorable
environment for the selection of lactobacilli and subsequent production
of lactate.
Rolled oats resulted in the highest (P < 0.05) total
SCFA production compared with all other grain-based food products.
Yiu et al. (1987
) found raw oat starch to be highly
digestible because of the disruption of starch granules due to oat
processing. Rolling the oats leads to this disruption of the starch
granules in the oat grain. Also, lactate production was highest
(P < 0.05) for rolled oats, again relating to the high
degree of processing and subsequent fermentative capacity of rolled
oats.
Total SCFA production for the reference substrates was highest
(P < 0.05) for corn starch compared with all other
substrates. Zentek (1995
), using canine ileal chyme,
found that after 24 h of in vitro fermentation, corn starch
resulted in higher concentrations of total SCFA compared with potato
starch (7.11 versus 5.80 µmol/mL of fermentation broth,
respectively). This is comparable to our SCFA and lactate data, in
which corn starch had the highest (P < 0.05)
concentrations and potato starch had the lowest. Although both are
composed of starch, potato starch contains a much higher concentration
of RS (66.9%) than corn starch (8.1%), possibly leading to a
reduction in the fermentation of potato starch.
The response criteria used in this experiment to test differences among substrates included OMD and organic acid production. Organic acid production appears to be the more accurate criterion for the determination of fermentative activity, because OMD values are obtained using a gravimetric method with its attendant difficulties. High solubility values in relation to OMD do not appear to equate to high total SCFA concentrations. For example, the average solubility value for the flour group was 19.8% and total SCFA concentrations were only 4.99 mmol/g OM. The cereal grains group, on the other hand, averaged 4.5% solubility but had a total SCFA concentration of 5.92 mmol/g OM. Likewise, there were no statistically significant correlations between OMD and total SCFA concentrations (data not shown).
Of interest to many researchers is the potential fermentation of RS.
Although starch is fermentable and believed to favor butyrate
production, the data are not entirely consistent (Topping and Clifton 2000
). Our data do not point to increased
concentrations of butyrate from the fermentation of RS. For example,
legumes had high concentrations of RS (mean 24.7%), whereas butyrate
concentrations averaged 0.77 mmol/g OM. Flours, low in RS
concentrations (mean 2.8%), had similar butyrate concentrations (mean
0.67 mmol/g OM) as the legume group (data not shown).
What is the contribution of ileal bacteria to starch disappearance
compared with that resulting from any residual digestive enzymes
present in ileal chyme? Using the same ileal in vitro model,
Murray et al. (2000
) found that fermenting substrates in
the presence of sodium azide–treated ileal bacteria resulted in no
total SCFA for the first 5 h and minimal amounts at 7.5 h.
This points to the minimal effect of residual digestive enzymes on
starch disappearance using this in vitro model.
In conclusion, starch and fiber fractions in foods and feeds affect starch digestion in the gastrointestinal tract as assessed using in vitro models. It should be noted that the emphasis of this work was the effect of starch and fiber fractions on intestinal microbial digestion. Gut motility, digestive enzymes and other aspects of gut function will affect digestion in vivo. Greater knowledge of the precise chemical composition and digestive capabilities of starch fractions in foods and feeds will allow for more precise dietary formulations for both humans and companion animals, with implications in both performance and health arenas.
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FOOTNOTES |
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2 The authors acknowledge the Council on Food and Agricultural Research (C-FAR) for their support of this research.
4 Abbreviations used: CP, crude protein; DM, dry matter; FG, free glucose; I, insoluble fiber; IDS, ileal digestible starch; OM, organic matter; OMD, organic matter disappearance; RDS, rapidly digestible starch; RS, resistant starch; SCFA, short-chain fatty acids; SDS, slowly digestible starch; S, soluble fiber; TDF, total dietary fiber; TDS, total digestible starch; TS, total starch.
Manuscript received July 24, 2000. Initial review completed September 3, 2000. Revision accepted November 10, 2000.
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G. Bosch, W. F. Pellikaan, P. G. P. Rutten, A. F. B. van der Poel, M. W. A. Verstegen, and W. H. Hendriks Comparative in vitro fermentation activity in the canine distal gastrointestinal tract and fermentation kinetics of fiber sources J Anim Sci, November 1, 2008; 86(11): 2979 - 2989. [Abstract] [Full Text] [PDF]
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L. D. de-Oliveira, A. C. Carciofi, M. C. C. Oliveira, R. S. Vasconcellos, R. S. Bazolli, G. T. Pereira, and F. Prada Effects of six carbohydrate sources on diet digestibility and postprandial glucose and insulin responses in cats J Anim Sci, September 1, 2008; 86(9): 2237 - 2246. [Abstract] [Full Text] [PDF]
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