Influence of Physical and Structural Aspects of Food On Starch Digestion
Influence of Physical and Structural Aspects of Food On Starch Digestion
Influence of Physical and Structural Aspects of Food On Starch Digestion
Physical and Structural
Aspects of Food on Starch Digestion
Ingrid Contardo and Pedro Bouchon
High consumption of foods rich in starch can be linked with some conditions
and diseases such as obesity and type II diabetes (Zimmet, Alberti, & Shaw, 2001).
The physical state of starch granules influences how they are digested while simple
sugars are absorbed in the gastrointestinal system. Even though the field of food
design is seeking to generate starchy foods with beneficial properties for health,
focusing on reducing or slowing the absorption of glucose in the blood, it is impor-
tant to understand how starch is hydrolyzed and absorbed in the body, and how the
design of novel foods can provide them with functionality through their structure
and disintegration.
The kinetics of starch digestion are influenced by the physical and chemical charac-
teristics of the starch granules and the interactions with physiological events occur-
ring within the gastrointestinal tract (Bjorck, Granfeldt, Liljeberg, Tovar, & Asp,
1994). The features of starch are also influenced by food composition and structure,
and food processing conditions. Extrinsic factors that influence starch digestion
include the nature of starch; its physical form; interactions with proteins, lipids, or
sugars; the presence of enzyme inhibitors; food processing; food structure (initial
hardness or porosity); and bolus hydration/disintegration, while intrinsic factors
Fig. 1 Extrinsic and intrinsic factors that influence the kinetics of starch digestion [based on
Bornhorst and Singh (2013), Englyst, Englyst, Hudson, Cole, and Cummings (1999), and Guyton
and Hall (2006)]
foods. The proposal involves the simulation of oral, gastric, and intestinal digestion;
and includes a standardized assay for enzymatic activity determination of the
enzymes that are added at each step.
During testing using static models, the oral, gastric, or intestinal phases are
reproduced by a mono-compartmental step and the specific test conditions
are adapted for each specific application. Typically, in experiments with static mod-
els, the foods are mixed with simulated fluids during a pre-determined time of
digestion while controlling the environment in temperature and pH and fixing the
concentration of enzymes. Static models lack the simulation of realistic enzyme to
substrate ratios, continued changes in pH, transit times or removal of digested prod-
ucts, in time and place. In contrast, dynamic in vitro models involve increased com-
plexity, with the addition of digestive fluids with control of flow rate and composition
of their secretions, realistic gastrointestinal profiles of pH, complex peristaltic
movements or mixing segmentation, diffusion, and gastric emptying cycles. One
dynamic model is the TNO Gastrointestinal Model (TIM), developed at The
Netherlands Organization for Applied Scientific Research (TNO) of the Nutrition
and Food Research Centre. This is a multicompartmental model designed to realisti-
cally simulate conditions in the lumen of the gastrointestinal tract. TIM combines a
reproducible and accurate simulation of digestion processes with detailed kinetic
elements. This model has been developed to study the fraction of a compound that
is available for absorption through the gut wall. Under this approach, the model has
been used to predict the glycemic response after intake of carbohydrates (Nalin
et al., 2015). The TIM-Carbo technology was validated against 21 different in vivo
plasma glucose response curves after the intake of carbohydrate food products
(R = 0.91). Another model is the Dynamic Gastric Model (DGM) developed by the
Institute of Food Research (IFR) (Norwich, UK), which can simulate both the bio-
chemical and mechanical aspects of gastric digestion (stomach and antrum) in a
realistic, time-dependent manner. Secretion rates can be adapted dynamically to the
changing conditions of acidification or fill state and the system allows for the use of
complex food matrices (comparable to in vivo studies) and emulates the peristaltic
movements of the stomach in amplitude, intensity, and frequency (Wickham,
Faulks, Mann, & Mandalari, 2012). Similarly, the Human Gastric Simulation (HGS)
is another dynamic in vitro stomach model, which consists of a latex vessel and a
series of rollers secured on belts that are driven by motor and pulleys to create a
continuous contraction of the latex wall in order to simulate the peristaltic move-
ments of stomach walls, with similar amplitude and frequency of contraction forces
as reported in vivo. It also incorporates gastric secretions, emptying systems, and
temperature control that enable accurate simulation of dynamic digestion processes
for detailed investigation of the changes in the physical chemical properties of
ingested foods. Kong and Singh (2010) used HGS during the digestion of rice. They
demonstrated that the amount of acid became a limiting factor for the acid hydroly-
sis as the solids content of emptied digesta was affected by the amount of rice avail-
able for digestion relative to the amount of acid present (see Fig. 2).
Some differences between in vivo and in vitro methods used to study starch
digestibility from solid foods exist, as in vitro methods cannot emulate all the com-
306 I. Contardo and P. Bouchon
Fig. 2 Profile of pH and change of solids content in emptied digesta during in vitro digestion of
rice using Human Gastric Simulation (HGS) (mean of n = 3) [extracted from Kong and Singh
(2010)]
Fig. 3 Representative overview of the processes during starch hydrolysis and absorption in the
gastrointestinal tract (mouth, stomach, small intestine, and large intestine), highlighting physical
and chemical aspects influencing starch digestion kinetics from solid food, and the key variables in
an engineering approach [adapted from Bornhorst and Singh (2014)]
(
C = C∞ l − e − kt ) (1)
308 I. Contardo and P. Bouchon
100
White bread Spaghetti Rice Biscuits
Lentils Chick peas Beans Peas
Boiled potatoes Crisp potatoes
80
Total starch hydrolysis (%)
60
40
20
0
0 30 60 90 120 150 180
Time (min)
Fig. 4 Total starch hydrolysis rate, using a first-order equation [extracted from Goñi et al. (1997)]
n
ds ds p
= exp ( − ki ( p − p0 ) ) 1 − (2)
dt dt p =0 pm
Influence of Physical and Structural Aspects of Food on Starch Digestion 309
Vmax S
v= (3)
Km + S
Where v is the reaction velocity (kg · L · min−1), vmax is the maximum reaction
−1
Breakdown behavior in the mouth and stomach of solid starchy products can deter-
mine the accessibility of starch granules during the hydrolysis process. In the mouth,
the physical and chemical transformations of starch-based foods by simultaneous
processes take place; food mastication, lubrication, mixing, and bolus formation.
The main role of mastication is the reduction of food in terms of particle size.
Usually, breakdown behavior is described in terms of the number of particles of dif-
ferent sizes present in the chewed/digested food, typically characterized by bimodal
behavior. For instance, such data are used to fit the cumulative distribution of par-
ticles in terms of either their numbers or surface areas to a Rosin–Rammler model
(unimodal Weibull distribution), and quantify their extent of breakdown based on
median and spread values; or use of the mixed Weibull distribution function
(Drechsler & Ferrua, 2016).
Oral digestion involves the enzymatic hydrolysis of starch, where salivary
α-amylases lightly hydrolyzes starch to generate fractions of maltose, maltotriose,
and α-limit dextrin (group of low-molecular-weight carbohydrates, 3–9 glucose
polymers) (Guyton & Hall, 2006). In addition, the mucin (glycoprotein) present in
the saliva acts as a lubricant in bolus formation, contributing to the hydration of
the fragmented food (Bornhorst & Singh, 2012). In this first step of digestion, it is
to be expected that starch hydrolysis is limited due to the fact that granules remain
in the mouth for a short period. However, studies of oral digestion show the impor-
tance of the time/intensity of mastication, viscosity, the flow rate/concentration of
enzyme and pH on the salivary α-amylase activity on starch digestibility
(Butterworth, Warren, & Ellis, 2011). Moreover, in solid starchy foods that require
significant physical breakdown during digestion, breakdown behavior is affected by
the initial hardness and the rate of softening. The rate of softening is a function of
the food structure (e.g., porosity, density) and the amount of acid and enzyme secre-
tions that have entered the food matrix (e.g., hydration in physiological gastric con-
ditions) (Bornhorst, Ferrua, & Singh, 2015). Texture changes fit the Weibull model:
Ht
= e( )
− kt β
(4)
H0
Where Ht (N) is the hardness at time t (min), H0 is the initial hardness (N), k is the
scale parameter (min−1); t is the digestion time (min); and β is the distribution shape
factor (non-dimensional).
Although the salivary enzymatic degradation of starch has been considered
insignificant (not more than 5% of all starches) in comparison to pancreatic amylase
in the small intestine, it may influence the final digestive process by affecting starch
granules and the food structure (Hoebler et al., 1998). Individuals with high salivary
amylase concentrations may be better adapted to ingest starches, whereas those
individuals with low amylase activity may be at greater risk of insulin resistance and
diabetes if chronically ingesting starch-rich diets (Mandel & Breslin, 2012). The
Influence of Physical and Structural Aspects of Food on Starch Digestion 311
salivary flow rate and its composition can be stimulated by differences in starch-
based foods. Engelen, Fontijn-Tekamp, and Van Der Bilt (2005) studied the influ-
ence of the characteristics of various starchy products on the swallowing threshold,
under the hypothesis that the urge to swallow food could be triggered by a threshold
level in both food particle size and lubrication of the food bolus. The influence of
oral physiology on the swallowing threshold was determined by measuring the sali-
vary flow rate, maximum bite force and masticatory performance. They used about
10 cm3 of bread, toast, Melba toast, breakfast cake, peanuts or cheese to determine
the influence on the swallowing threshold of various food characteristics (e.g., hard-
ness, moisture, and fat) and showed that salivary flow rates were significantly and
negatively correlated with the number of mastication (chewing) cycles of melba
toast and breakfast cake. Hence, subjects with more saliva needed less chewing
cycles for these dry foods.
Likewise, inactivation of salivary α-amylase may be influenced by low pH envi-
ronment or food composition, affecting the amount of hydrolyzed starch in the
mouth. Thus, the inhibition capacity to α-amylase by certain food components can
be used for its potential ability to modify the postprandial glycemic response. Some
α-amylase inhibitors are naturally present in cereals or legumes, or they can be
incorporated by formulation or food processing. Phenolic compounds, phytochemi-
cals in grains or crops, and bioactive compounds in seaweed have α-amylase inhibi-
tory properties. Heo et al. (2009) indicated that diphlorethohydroxycarmalol
(DPHC) isolated from a brown alga might be a potent inhibitor for α-glucosidase
and α-amylase. The increase of postprandial blood glucose levels was significantly
suppressed in the DPHC-administered group compared to the streptozotocin-
induced diabetic or normal mice.
After the food bolus is formed, it is swallowed and it travels through the esopha-
gus by muscle contractions (peristalsis) down to the stomach. The bolus transport
along the esophagus is influenced by the rheological behavior of starch-rich bolus
linked to viscosity, the swallowing rate, and the esophagus radius (Mackley et al.,
2013; Moritaka & Nakazawa, 2009).
In the stomach, gastric secretions (acids, enzymes, and electrolytes) are secreted
from the stomach wall. With the help of weak peristaltic contraction waves, they
come into contact with the bolus to generate the chyme (semifluid mixture), and
they inactivate the salivary α-amylase due to their low pH. Not all amylase is inac-
tivated at the same time as the profile of pH inactivation is affected by chyme char-
acteristics (e.g., flow rate or viscosity), food pH and composition, or the amount of
bolus. The pH is adjusted to the characteristics of the bolus and its behavior during
its gastric passage. As such, the buffering capacity of the food probably prevents its
immediate and homogenous acidification in the stomach. At the same time, the gas-
tric pH may influence the rate of bolus hydration/disintegration, uptake of
312 I. Contardo and P. Bouchon
Fig. 5 Acidic water (AW) penetration front profiles as a function of distance (μm) from the carrot
center (solid circles) and Edam cheese (white circles) soaked for 10 min at pH 1.5 (a) and at pH
7.0 (b) [extracted from Van Wey et al. (2014)]
Fig. 6 Available glucose (mg/mL) during 180 min in vitro small intestinal digestion of control (no
fibre), locust bean gum, guar gum, fenugreek gum, xanthan gum, flaxseed gum, and soy soluble
polysaccharide-fortified solutions [extracted from Fabek et al. (2014)]
Therefore, the mass transfer in gastric starch digestion may be affected by viscosity,
food components, and gastric movements.
Viscosity of the bolus can affect the bioaccessibility of starch granules and the
quantity of glucose available for absorption, and it may also influence the glucose
concentration in the bloodstream. A study developed by AlHasawi et al. (2017)
demonstrated that gastric viscosity differed considerably between commercially
available oat products, instant oats, steel cut oats, and oat bran, using TNO Intestinal
Model (TIM-1). Instant oat and oat bran viscosities were highest at the onset of
digestion and decreased over time, whereas the viscosity of steel cut oats at the
onset of digestion was the lowest level of viscosity observed, increasing over time,
and affecting the rate of starch digestion. Starch content was directly proportional to
total bioaccessible sugars and the rate of sugar release was slowest for steel cut oats
and quickest for oat bran. Thus, an increase in gastric viscosity may lead to reduc-
tion in the diffusion of hydrolyzed glucose. Likewise, cereals that are high in s oluble
fiber such as β-glucan may induce a high level of viscosity of the digesta once it has
reached the small intestine, where the absorption of glucose occurs. The viscosity of
soluble fibers depends on their ability to resist changes during gastrointestinal
digestion (Würsch & Pi-Sunyer, 1997). Fabek, Messerschmidt, Brulport, and Goff
(2014) demonstrated that dietary fiber influences the glucose diffusion of in vitro
small intestinal digestion in a simulated food model, which included protein and
starch (see Fig. 6). Villemejane, Wahl, Aymard, Denis, and Michin (2015) investi-
gated the effects of fiber in biscuit composition on the viscosity generated during
digestion using TIM-1 (stomach, duodenum, jejunum and ileum). The results
showed a significant effect of viscous soluble fibers on the chyme viscosity, up to
the ileal compartment. In the stomach, during the first hour of the digestion, fibers
were progressively liberated from the matrix and solubilized, which allowed main-
314 I. Contardo and P. Bouchon
tenance of the viscosity of the gastric content. These findings suggest that the inclu-
sion of more resistant to digestion ingredients, such as hydrocolloids (e.g., xanthan
gum, guar gum, or soy soluble polysaccharide) or soluble fibers (pectin, mucilages,
or β-glucan) in a food might be an effective strategy in lowering postprandial glyce-
mic responses in humans (Würsch & Pi-Sunyer, 1997).
The effect of viscosity on gastric emptying has shown diverse results. Increasing
bolus viscosity may delay gastric emptying rate, so it may increase satiety and mod-
ulate postprandial glycemic responses (Marciani et al., 2001; Zhu, Hsu, & Hollis,
2013). In contrast, Bornet et al. (1990) concluded that the α-amylase susceptibility
of test carbohydrates (25 g starch or equivalent glucose units) is a determining fac-
tor in the insulin response of healthy subjects, while viscosity of the test meals and
the gastric emptying rate have no effect.
The rate of bolus disintegration has also been shown to play a key role in gastric
emptying, as well as possibly influencing postprandial blood glucose levels. This is
conditioned by food composition or structure. Bornhorst and Singh (2013) demon-
strated that the disintegration rate and profile of bread boluses were significantly
influenced by bread composition in both static and shaking conditions. Each bread,
almond wheat, barley, rye, sourdough, wheat, or white, was characterized by its
moisture content, firmness, and water holding capacity. The initial moisture content
of breads also influenced the amount of gastric secretions absorbed. The total
amount of fluids absorbed by the bolus seemed to be inversely proportional to the
initial bread moisture content, while the firmness of bread and its water holding
capacity were found to be complementary food properties that must be considered
to explain the differences detected in mass retention profiles. Additionally, the gas-
tric disintegration of a bolus can be affected by the properties of the gastric fluids
(composition and rheology) and the gastric movements (stomach motility and antral
contractions) (Kong & Singh, 2008). The structural breakdown of food has a signifi-
cant influence on starch hydrolysis, both in terms of bolus formation and disintegra-
tion (Bornhorst & Singh, 2012). Various studies have shown that different foods will
produce different particle size distributions in the bolus during oral processing. For
example, the bolus produced after mastication of bread demonstrated a bimodal
distribution of particle sizes (30 mm, 500 mm) (Hoebler, Devaux, Karinthi,
Belleville, & Barry, 2000). Particle sizes in a bolus have been linked to the rate of
in vitro enzymatic degradation. Ranawana, Monro, Mishra, and Henry (2010)
reported that the degree of particle size, due to mastication, correlated with the rap-
idly available starch content (RDS) in chewed rice bolus. The quantity of undi-
gested material remaining at the end of the 120-min digestion correlated significantly
with the percentage of particles greater than 2000 μm in masticated rice.
After gastric digestion stomach emptying occurs, where the chyme is transported
from the stomach into the small intestine by intensive peristaltic contractions in the
antrum. The motion of the gastric fluids causes dramatic changes on the bolus struc-
ture, affecting both the glucose release and glucose diffusion during the digestion of
starch-based products. Kozu et al. (2010) investigated the effect of intra-gastric flow
on food digestion using computational fluid dynamics. They analyzed the flow phe-
nomena induced by gastric peristalsis with different fluid viscosities, focusing on
Influence of Physical and Structural Aspects of Food on Starch Digestion 315
The chyme is transported to the small intestine, where the starch is predominantly
hydrolyzed (~80%) to maltose, maltotriose, α-limit dextrins, or small glucose poly-
mers by pancreatic α-amylase from pancreatic secretions. These end-products are
further hydrolyzed to glucose by intestinal epithelial enzymes (Dhital, Warren,
Butterworth, Ellis, & Gidley, 2017). In addition, the intestinal motility process is
generated by three movement patterns: peristalsis, segmentation, and pendular
movements, which induce the mechanical digestion of starch. Peristalsis causes
propulsions that move the intestinal content in the anal direction, and segmentation
contractions cause mixing to promote absorption of nutrients and water. Finally, the
starch is absorbed in the form of glucose in the epithelium into the bloodstream,
through the brush border of the epithelial cells. In general terms, the absorption of
glucose can be represented by convection and diffusion processes across the intesti-
nal wall. Convective transport can be considered as a result of the flow induced by
the intestinal movements that transports and mixes chyme along the intestinal
lumen. Regarding the rate of movement of material from high to low concentration
by diffusion, it can be described mathematically by Fick’s laws of diffusion.
The small intestine can be divided into three parts: duodenum, jejunum, and
ileum. The entry of chyme into the upper portion of the duodenum triggers a key
event in the beginning of the intestinal phase of nutrient digestion. In this part, the
pancreatic secretions (enzymes, bicarbonate, and water) are secreted due to the
presence of chyme by pancreatic glands and epithelial cells. The characteristics of
pancreatic secretions related to viscosity, pH, and the flow rate/activity of enzymes
depend on the type and the amount of starch present in the chyme. Hence, again
when in vitro gastrointestinal methods are used to study starch digestion it is impor-
tant to understand mass transfer in the small intestine, as both biological and engi-
neering approaches are determining aspects to control and achieve realistic results
correlated with in vivo results. Regarding the enzymatic activity of pancreatic
α-amylase, some molecules present in foods have been shown to inhibit α-amylase
in the intestine. Studies on humans have shown that natural α-amylase inhibitors
isolated from wheat significantly reduced glucose absorption and the peak of post-
prandial glucose in healthy and type 2 diabetic subjects (Lankisch, Layer, Rizza, &
DiMagno, 1998; Slavin, 2004). The plant forming part of the starch-based foods
have chemical constituents with the potential to inhibit α-amylase activity. For
316 I. Contardo and P. Bouchon
example, the chemical structures of flavonoids and polyphenols have been shown to
inhibit α-amylase activity and can reduce blood glucose levels after starchy foods
have been eaten. This could be potential constituents for controlling type 2 diabetes
(De Sales, De Souza, Simeoni, Magalhães, & Silveira, 2012; Lo Piparo et al., 2008;
Nyambe-Silavwe & Williamson, 2016). Likewise, the characteristics of intestinal
motility have proven to be relevant to modulate starch digestibility (Jaime-Fonseca,
Gouseti, Fryer, Wickham, & Bakalis, 2016). Gouseti et al. (2014), developed in vitro
intestinal models to study the effect of gut motility on the accessibility of glucose
from model solutions, using a range of food hydrocolloids (guar gum, carboxy-
methyl cellulose, pectin), and showing how mass transfer has an influence on nutri-
ent bioaccessibility. The study showed that the presence of gum guar and pectin
have a significant effect in retarding simulated glucose accessibility, and these
results appear to be more pronounced at viscosities levels of around 0.01 Pa·s.
Systems with lower viscosities showed enhanced mass transfer levels. The data
were analyzed using engineering principles and dimensionless numbers that charac-
terize the fluid flow (Reynolds number) and mass transfer (Sherwood number) in
the gut. The flow behavior can be determined by the velocity of the peristaltic flow
and the physical properties of chyme. Sherwood numbers represent the ratio of
convective to diffusive mass transfer coefficient. The absorption of the glucose
involved transportation from the lumen (chyme) to the dialysis membrane, passing
through the membrane, and transfer to the recipient fluid. This three-stage process
was characterized by the luminal mass transfer coefficient, (Klumen, m/s), diffusion
(described by coefficient Dmembrane, m2/s) through the membrane of thickness Zmembrane
(m), and the recipient side’s mass transfer coefficient (Krec, m/s). The following
equation offers the relationship between the local and overall transfer coefficient.
1 1 Z 1 1 1
= + membrane + = + (5)
K overall K lumen Dmembrane K rec K lumen K system
In addition, it should not be forgotten that emotional stimuli intrinsic to the indi-
vidual also influence the flow of gastrointestinal secretions, and can therefore affect
starch digestion in different ways.
The nutritional quality of starch is associated with its rate of digestion and glucose
absorption. Starch bioaccessibility and glucose release may differ depending on
starch structure and the form in which the food structure is disintegrated, some with
starch being rapidly and others slowly digested. Three aspects of the food composi-
tion are important to highlight related to variability of starch digestion: source of
starch, interaction of starch with other components, and the presence of dietary fiber.
Influence of Physical and Structural Aspects of Food on Starch Digestion 317
100
Rapidly Digestible Starch content, RDS [%] 90
80
70
60
50
40
30
20
10
0
–1 +1 –1 +1 –1 +1
Feed moisture Screw speed Temperature
Fig. 7 Rapidly digestible starch, RDS (%) of high amylose maize starch (67% amylose content),
maize starch (28% amylose content), and waxy maize starch (6% amylose content). The distance
between two grid lines represents the least significant difference (LSD) [extracted from Robin
et al. (2016)]
The particle size of wheat grain is determinant forthe digestion rate of starch and
consequent glucose responses. Heaton, Marcus, Emmet, and Bolton (1988) demon-
strated that in vitro starch hydrolysis by pancreatic α-amylase was faster with
decreasing particle size, and the peak of postprandial plasma glucose was greater
for fine-flour wheat than that for cracked or whole grains.
Regarding potato starch, this has a large granular size (<100 μm) and a concen-
tration of covalently bound phosphate in the amylopectin molecules as phosphate
monoesters and phospholipids (Singh, Singh, Kaur, Sodhi, & Gill, 2003). Some
studies have provided evidence that raw potato starch shows a reduced susceptibil-
ity to the action of amylase, due in part to its large granular size, based on the idea
that it is only the surface of the granule which is available for initial hydrolysis
(Cottrell, Duffus, Paterson, & Mackay, 1995); and the presence of phosphate groups
with a high B polymorph content, although these can be modified by processing and
storage. Warren, Zhang, Waltzer, Gidley, and Dhital (2015) demonstrated that native
potato starch was digested slowly and required more enzymes than maize to achieve
complete digestion. Potato starch granules can be completely digested in vitro given
enough enzyme and time, demonstrating the likely dependence of in vivo resistant
starch levels on endogenous enzyme activity and the small intestinal passage rate,
either or both of which may vary between foods and/or between individuals. García-
Alonso and Goñi (2000) confirmed that boiled and mashed potatoes showed the
highest rate of digestion among raw flakes, oven-baked, French-fries, crisps, and
retrograded potato starch.
Legumes have acquired significant nutritional interest due to their rate of starch
digestion being lower in both in vitro and in vivo, compared to other starch sources
such as cereals. Their reduced bioavailability of starch has been attributed to the
presence of high levels of amylose (30–65%), a high content of viscous dietary fiber
components, the presence of antinutrients and B-type crystallites (Tharanathan &
Mahadevamma, 2003). These differences in their structural characteristics, as well
as their content of resistant starch, shows a slight reduction after processing in com-
parison to their raw form, which could allow them to be used as an alternative
source of resistant starch.
Most starch-based foods offer different metabolic responses even when they are
processed under similar conditions. These variances have been attributed to interac-
tions between starch with other food components such as proteins, lipids or sugars
during processing.
For instance, starch–gluten products such as bread are mainly processed by bak-
ing. In these types of product, gluten proteins (gliadins and glutenins) play a key
role in determining the baking quality of bread by conferring water absorption
capacity, cohesiveness, viscosity, and elasticity on dough (Wieser, 2007). During
processing, the proteins may physically become embedded in the starch, and inter-
Influence of Physical and Structural Aspects of Food on Starch Digestion 319
glucose occurs. This high viscosity delays glucose absorption. Some studies have
demonstrated that increased β-glucan intake improves glycemic control, indicating
it should be considered as a complementary mechanism in the treatment of patients
with type 2 diabetes (Jenkins, Jenkins, Zdravkovic, Würsch, & Vuksan, 2002). In
addition, Oh, Bae, and Lee (2014) established that under in vitro starch digestion,
decreasing levels of the inulin ratio in cakes resulted in a decrease in rapidly digest-
ible starch values. Interestingly, different types of soluble fiber have varying effects
on viscosity, and some studies have shown no correlation in all types of fiber
between high fiber content and reduced risk of diabetes, demonstrating that the
mechanics by which the hydrolysis of starch can be delayed are influenced by
intrinsic and extrinsic factors. Viscous fibers incorporated in food not only increase
the viscosity of the lumen but may also protect starch from enzymatic attack
(Gouseti et al., 2014). Furthermore, the presence of insoluble dietary fiber in com-
plex food systems has been associated with contributing to the control of diseases
such as obesity and diabetes, mainly due to the beneficial nutritional effects on
satiety and glycemic responses (Zhang & Hamaker, 2016). Thus, the compositional
differences link to white or whole-wheat flour influences the rate of starch digest-
ibility. Comparative studies between refined and whole grains (containing the outer
part of the bran) have demonstrated that whole grains of wheat with a high content
of dietary fibre helped lower the risk of diabetes mellitus (Liu et al., 2000).
Microstructural aspects of food can influence the digestion of starch. The bioacces-
sibility and bioavailability of starch is affected by the food matrix, influencing enzy-
matic functions and the residence time in the human stomach or intestine.
Accordingly, transformations of the food matrix and hormonal regulation mecha-
nisms can dominate the rate and extent of glucose release during gastrointestinal
transit (Parada & Aguilera, 2011a, b). Some microstructural aspects in solid starchy
foods and interrelated transformations are represented in Fig. 8, which involves
multiple reactions, mass transport, and glucose control mechanisms.
The main microstructural characteristics are linked to starch transformations,
particle size that can be obtained after the masticatory process, entrapment of starch
granules in the matrix, and viscosity and pH provided to the bolus by the compo-
nents forming the food matrix. Also, the physical texture (associated with the hard-
ness or density of the food) seems to have an impact on the availability of starch for
enzymatic digestion. As already discussed, the degree of starch gelatinization dras-
tically modifies the structure of materials in which the granules are entrapped, and
the digestion of starch and absorption of glucose within the digestive system.
Likewise, physical properties of the food bolus can be altered, providing a more/less
accessible structure, and so affecting the motility and susceptibility in the specific
activity of amylolytic enzymes. In addition, the hardness of a starchy food influ-
ences the particle size of its bolus. Chen, Khandelwal, Liu, and Funami (2013)
Influence of Physical and Structural Aspects of Food on Starch Digestion 321
Fig. 8 Microstructural aspects in starchy food and interrelated transformations involving multi-
ples reactions, mass transport, and glucose control mechanisms during digestion of starch [adapted
from Parada and Aguilera (2011a, b)]
studied the physical properties of food boluses, in particular the bolus particle size
distribution in relation to the hardness of the food. It was observed that bolus parti-
cle size decreased with the increase of food hardness (in cheese, peanuts, or cashew
nuts). The correlation between these two properties could be described by a power–
law relationship. Similarly, Alam et al. (2017) observed that the addition of 10% rye
bran had a significant effect on the structural, textural and mastication properties
both for puffs and flakes. The addition of rye bran increased hardness, decreased
crispiness, and increased the hydrolysis index of puffs and flakes to 89.7 and 94.5,
respectively, which was probably attributable to the increased number of particles in
the bolus. This was noticeable in the early phase of digestion, i.e. at 30 min, indicat-
ing that the disintegration process and consequently the particle size of the bolus
had an important role on the starch digestion rate. It is important to mention that
particle size is also influenced by inter-individual variability. Le Bleis, Chaunier,
Montigaud, and Della Valle (2016) proposed that bolus consistency could be
expressed by:
Qs (1+ n1)n 2
n2
K = K 0 exp −α β t (6)
W0
This equation provided a basic model to describe the disruption of bread at different
stages of mastication, where K0 is the initial consistency index of the bread (Pa sn);
α is the plasticization coefficient (non-dimensional); Qs is the stimulated salivary
322 I. Contardo and P. Bouchon
flow (L min−1); t is the chewing time (min); w0, represents the median particle size
(mm); and the other coefficients [β, adjusted coefficient for salivation (non-
dimensional); n1 and n2, adjusted exponents for fragmentation and salivation,
respectively (non-dimensional)] were obtained through fitting of experimental
results for breads enriched with fibers and examined in this study. The study showed
that bolus consistency decreased with chewing time, and this decrease was linked to
bolus moisture by a plasticization coefficient, which varied according to each indi-
vidual. Thus, the consistency of the bread directly influenced bolus disruption
assessed by changes in viscosity.
Starch granules entrapped in the food matrix (e.g., plant cell, gluten network or
encapsulation), seem to be another mechanism that hinders the physical accessibil-
ity of starch and the diffusion of amylolytic enzymes in the starchy products.
Bhattarai, Dhital, Wu, and Gidley (2017) observed that the rate and extent of hydro-
lysis of starch and protein were greatly increased when the cell wall physical barrier
was removed by either mechanical or enzymatic processes. The authors used an
in-vitro dynamic model to observe that isolated legume cells have sufficient
mechanical strength to survive mixing conditions in a simulated rat stomach–duo-
denum. Also, the cell wall could limit digestibility by restricting starch gelatiniza-
tion during cooking, as water transfer (amount of liquid water molecules) into the
cell restricts the swelling of starch granules. Therefore, the use of whole grains (e.g.,
wheat, oat, barley, rye) in starchy products may result in low glycemic responses
due to the preservation of food particles in the gastrointestinal tract. This is because
hard solid foods are emptied more slowly from the stomach than soft foods.
Pasta products have shown slow and progressive starch breakdown and release of
sugar in the body, leading to low postprandial blood glucose and insulin responses
(Bjorck, Liljeberg, & Ostman, 2000). These wheat-based products vary in flour
variety, shape, type of drying, and proportion of protein added in their formulation,
promoting low glycemic responses. Pastas are prepared using durum wheat flour,
however it is also possible to incorporate dietary fiber ingredients and hydrocolloids
to increase their nutritional value. Sheeting, extrusion, drying, and cooking pro-
cesses confer the formation of different pasta structures by successive structural
changes of two main components, that is, starch and proteins, which provide a
potential to regulate the glycemic response of cereal foods. Thus, major structural
transformations occur during the cooking stage. Fardet, Hoebler, Bouchet, Gallant,
and Barry (1998) established that the presence of a structured and continuous pro-
tein network is an important factor in explaining the slow degradation of starch in
pasta. The authors proposed that the action of α-amylase may be limited at various
levels: (1) by the restricted accessibility and porosity of food structure; (2) by the
tortuosity of the protein matrix; (3) by the possible interactions of the α-amylase
with the protein matrix; and (4) by the structure of the starch granules in pasta;
demonstrating that the physical texture of starch-based food is a determinant factor
for bioavailability of starch in human digestion.
Furthermore, the modification of food structures with the addition of hydrocol-
loids (in order to modify rheological and textural aspects) may also have an effect.
Hydrocolloids influence the digestion and absorption of available carbohydrates in
Influence of Physical and Structural Aspects of Food on Starch Digestion 323
30000 100
y = 9.7493x+12163
20000 R2=0.8144 80
10000 y = –0.019x+63.882 60
R2=0.9796
0 40
0 200 400 600 800 1000
Crust hardness (g)
Fig. 9 Correlation plots between total peak area, mass fraction of large particles, and crust hard-
ness [extracted from Gao et al. (2015)]
various ways. For instance, oat β-glucan in breads reduces the glycemic index (GI)
and glucose peak by 32–37% compared to a white wheat reference bread, and is
suitable for use in the baking of bread products (Ekström, Henningsson Bok, Sjöö,
& Östman, 2017). The addition of β-glucan into sugar cookies increased their attri-
bute of hardness, while affecting biscuit texture in turn decreased the carbohydrate
degradation and the rate of glucose absorption (Brennan, Samyue, & Abbot, 2004).
β-Glucan increased intestinal viscosity and delayed gastric emptying, which resulted
in a reduced rate of α-amylase action and reduced intestinal nutrient uptake
(Thondre, Shafat, & Clegg, 2013). Likewise, the use of viscous soluble fiber as
Psyllium improves glycemic control in patients with type-2 diabetes mellitus
(Feinglos, Gibb, Ramsey, Surwit, & McRorie, 2013).
On the other hand, bread with various structures and textures provides different
chewing behavior and bolus characteristics, affecting the release of glucose. It has
been observed that there is an inverse relationship between food moisture content
and saliva secretion. Also, in the case of harder bread, the swallowing threshold of
particle size is smaller (see Fig. 9). The larger force and longer time for bread with
hard and dry crust during oral processing, resulted in turn in an extensive break-
down of the bread structure, which may contribute to the higher digestibility of
bread with a lower moisture content (Gao, Wong, Lim, Henry, & Zhou, 2015).
4.1 F
ormation of Resistant Starch During Processing and Its
Relationship with Slow Starch Digestion
Resistant starch (RS) is a physiological concept that was initially defined as the
fraction of starch that was not hydrolyzed after 120 min of incubation with α-amylase
(Englyst et al., 1992; Sajilata, Singhal, & Kulkarni, 2006). However, it is now con-
sidered to be the fraction of starch and products of starch degradation that escape
digestion in the small intestine of healthy individuals. Five types of RS have been
established from RS1 (type I) to RS5 (type V) (Birt et al., 2013; Fuentes-Zaragoza,
Riquelme-Navarrete, Sánchez-Zapata, & Pérez-Álvarez, 2010).
In RS1, the starch granule is physically inaccessible to digestion due to its entrap-
ment in a matrix (e.g., grains, seeds, or food structure). In RS2, the resistance to
digestion is because the starch granule is in a granular form (e.g., compact structure
of granules such as ungelatinized resistant granules with type B- or C-polymorphism
of crystallinity). Ungelatinized granules are tightly packed in a radial pattern,
which limits the accessibility to digestive enzymes during hydrolysis. RS3, repre-
sents retrograded amylose formed during the cooling of gelatinized starch. It can be
formed when starch-based foods are thermally processed with enough water and
then cooled. Starch polymer chains begin to reassociate as double helices and can
form tightly packed structures stabilized by hydrogen bonding. In RS4, the resis-
tance to digestion is given by the formation of novel chemical bonds (e.g., cross-
linking with chemical agents). This type of RS includes chemically modified
starches. The last is RS5, which represents amylose–lipid complexes. Resistant
starches added to food matrices for health benefits are classified as functional fiber
by AACC (American Association of Cereal Chemists, 2001). In contrast to RS that
is naturally found in foods, it is considered dietary fiber. The dietary fiber definition
committee also reported that RS as a constituent of dietary fiber should be resistant
to digestion in humans and this should be assessed using methods that include
gelatinization steps to simulate cooking and processing (American Association of
Cereal Chemists, 2001).
Influence of Physical and Structural Aspects of Food on Starch Digestion 325
is broader. Furthermore, the addition of an alkali component has been found to sig-
nificantly enhance the swelling of starch granules and expedites the gelatinization
process (Wang et al., 2014).
Therefore, processing can offer alternatives for modifying the final nutritional
characteristics of foods. Also, limiting gelatinization of starch appears to be a suit-
able option to modulate starch hydrolysis and the glycemic responses of starch-
based foods. Studies applied to both temperature and pressure have demonstrated
that the gelatinization temperature may be lowered by reducing the processing pres-
sure. Thus, the extent of swelling and granule disintegration, as well as leaching of
amylose, can be controlled. Various authors have shown that varying the pressure
conditions in traditional food processing such as boiling, drying, or frying, allow
some specific properties to be maintained such as color, antioxidant capacity, the
stability of specific compounds, or incomplete gelatinization. During the process of
high-pressure technology, the gelatinization of the starch granules occurs differ-
ently from damage by high temperatures, although by applying enough high pres-
sure it is possible to obtain complete starch gelatinization (Baks, Bruins, Janssen, &
Boom, 2008). In contrast, in low-pressure processing there is less damage of the
granules.
Thermal driving force = ∆T = Toil − Twater boiling point at working pressure (7)
Processing conditions under low pressure may affect the capacity of food building
blocks to develop an adequate structure during processing, to provide the required
quality attributes. This may be less relevant in raw materials that are already struc-
tured by nature, such as tubers, but still important. In formulated products, this may
be critical, since specific changes are needed to create a structure. In starchy
328 I. Contardo and P. Bouchon
l
qcore qcrust qoil
x=0 x = d (t ) x = L /2
matrices, gelatinization is one of these critical steps, which requires the simultane-
ous presence of liquid water and temperature (above 55–60 °C). Frying is a complex
unit operation that involves simultaneous heat and mass transfer, resulting in
counter-flows of water vapor (bubbles) and oil at the surface of the piece. After
immersion in the hot oil, the surface of the product is heated to the boiling point of
water and the crust begins to form. As frying progresses, the evaporation front,
which is at the boiling point of the interstitial liquid, will move towards the interior
(moving front), delimiting two very well-defined zones: the crust and the core
(Ziaiifar, Achir, Courtois, Trezzani, & Trystram, 2008). The crust is the result of
several alterations that mainly occur at the cellular and subcellular level, where the
temperature exceeds the boiling point of water (Bouchon & Aguilera, 2001). The
temperature within the core, on the other hand, cannot exceed the boiling point of
water, and thus holds liquid water. A diagram that reflects the aforementioned con-
ditions, showing the temperature profiles in each region (red lines), the heat fluxes
and the moving front, is presented in Fig. 10.
Accordingly, during vacuum frying, if the operating pressure defines a water
boiling point that is lower to the one required for starch gelatinization (e.g., <
55 °C), starch gelatinization may be impaired. In fact, the crust region will be able
to attain temperatures that are higher than the gelatinization temperature, but no
liquid water will be left for gelatinization to occur. Conversely, the core region will
have enough liquid water, but will be below the required temperature to induce
starch gelatinization. Ovalle, Cortés, and Bouchon (2013) demonstrated that when
the operational pressure was reduced up to 6.5 kPa, at a water boiling point of 38 °C,
no starch gelatinization was observed during heating in water and oil, in situ and in
real time, using vacuum hot-stage microscopy (see Fig. 11). This result was attrib-
Influence of Physical and Structural Aspects of Food on Starch Digestion 329
Fig. 11 Representation of vacuum hot-stage microscopy used for vacuum and atmospheric heat-
ing miniaturization [extracted from Ovalle et al. (2013)]
uted to the rapid evaporation of water before gelatinization was reached. In addition,
when the amount of water was reduced the gelatinization process occurred in a
broader range of temperatures.
Contardo, Parada, Leiva, and Bouchon (2016) assessed the effect of vacuum fry-
ing on starch gelatinization and in vitro digestibility of starch, in terms of the frac-
tions of rapidly available glucose (RAG), slowly available glucose (SAG), and
unavailable glucose (UG) fractions. The authors demonstrated that dough samples
in the form of sheets, made of wheat starch (88% d.b.), gluten (12% d.b.), and water,
and fried under vacuum (6.5 kPa, Twater-boiling-point = 38 °C), showed less starch gelati-
nization (28%), less rapidly available glucose (27%), and more unavailable glucose
(70%) than their atmospheric counterparts (which presented 99% starch gelatiniza-
tion, 40% rapidly available glucose, and 46% unavailable glucose, respectively),
and that the values were close to those of the raw dough. Recently, they comple-
mented their study, by assessing in vivo starch digestibility, after feeding Sprague-
Dawley rats (Contardo, Villalón, & Bouchon, 2018). Results showed that
vacuum-fried dough had a maximal blood glucose level at 60 min, indicating a
slower glycemic response than that of samples fried under atmospheric counterparts
(maximal blood glucose level at 30 min), as shown in Fig. 12.
On the whole, both in vivo and in vitro studies were consistent and suggest that
starch digestibility can be altered through processing conditions by reducing the
operating pressure.
330 I. Contardo and P. Bouchon
195
Glucose
Blood glucose concentration (mg/dL)
180 AF (tep)
VF (tep)
165
Basal
150
135
120
105
90
75
0 15 30 45 60 75 90 105 120 135 150 165 180
Post-prandial time (min)
Fig. 12 In vivo starch digestibility expressed in terms of blood glucose concentration at various
postprandial times in rats given matrices with different degree of starch gelatinization, from
vacuum-fried (VF, 9.9 kPa) and atmospheric-fried (AF) dough after frying up to bubble-end point,
as well as Glucose solution (1.2 g/Kg animal) as Control, and physiological serum as Basal
[extracted from Contardo et al. (2018)]
5 Conclusions
The food composition and the structure of processed starchy products, as discussed
in this chapter, may influence starch digestibility. The physical–chemical character-
istics of starch ingested (interactions with other components of the food matrix, as
well as transformations during processing) have a relevant impact on starch diges-
tion, and the associated glycemic response. Starch interactions with other compo-
nents may induce changes in the starch molecule (e.g., interactions between starch
and lipids), reducing starch digestibility. Also, limitations of free water availability
during processing can hinder the gelatinization process, precluding starch digest-
ibility. Overall, the principles highlighted here may help in the development of strat-
egies to modify starch-rich foods so as to reduce glycemic impact and improve the
impact of consuming such foods on health.
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