Gastric Digestion of Foods—Challenges and Opportunities for Food ...

Gastric Digestion of
Foods—Challenges
and Opportunities for
Food Engineers
R. Paul Singh
Professor of Food Engineering
University of California, Davis
http://www.healinglightseries.com/tutorialdigestion.html

• Introduction
Outline
– Stomach structure, gastric juice, peristalsis
– Food and drug disintegration and stomach emptying
• Gastric digestion—disintegration of solid foods
– Development of a model stomach system
– In vitro studies on food disintegration
• Computational modeling of flow in human
stomach

Digestion system
• The overall function
– extract nutrients into
useable form
– absorb nutrients
– eliminate unneeded
materials
• Food takes between 24-36
hours to pass through the
gastrointestinal tract

Stomach
• Volume: 50ml to 4 liters of liquid
• From an engineering perspective, the human
stomach is a receptacle, a grinder, a mixer and a
pump that controls the digestion process
– Chemical digestion by enzyme activity
– Mechanical digestion by the mixing in the stomach
• Gastric juice: Colorless fluid
– 1.5 L secreted/d
• Hydrochloric acid
– breaks the food apart and kills most of the bacteria
that you swallow
• Mucus (~1.5 g/L)
– forms a gelatinous coating over the mucosal surface.
• Pepsin (~ 1 g/L)
– proteins broken down into smaller polypeptide chains
• Salt, Gastric Lipase
– fat digestion begins here

Antrum
• Pylorus: contracts to empty
materials from the stomach
into the small intestine.
• Pyloric sphincter: a
specialized valve that
selectively empties the small
particles and retains the large
• Fundus: begins digestion
of proteins and mixes
together stomach
contents.
• Body: digests proteins
and blends materials in
stomach and reduced to
a paste
• Antrum: Breaks down
large food material in to
small particles

Dynamic MRI image series showing propagating antral contraction waves (small
arrows) displayed in time intervals of 10 s. (Schwizer and others 2006)

Antral contraction of stomach
Propulsion, grinding, and
retropulsion of solids by peristaltic
contractions of distal stomach
(Kelly 1980)

Dilution of food bolus by elution
Color-coded dilution maps acquired at different times after a volunteer ingested
500 ml of viscous locust bean gum meal. A transverse EPI image is also shown
as an anatomic road map (L, left; R, right) (Marciani and others 2001a)

Measurement of stomach force
Author Force value Methods
Vassallo et
al. 1992
Kamba et
al. 2000
Marciani et
al. 2001b
The force per
contraction averaged
0.2 N
1.50 N for fasting
conditions and 1.89 N
for fed conditions
0.65 N exerted by the
antral walls
Vassallo MJ, et al. 1992.
a 1.8 cm balloon mounted on a tube and fixed a few
centimeters from the antrum of a human stomach
a press-coated tablet contained a marker drug with
brittle Teflon outer layer featured by a range of
fracture strengths.
agar gel beads (diameter 1.27 cm). MRI to directly
image the breakdown of the agar gel beads in the
antrum
Marciani et al.
2001b

Manometric tube and axial force catheter used by Marciani et al (2000)

In vivo methods to assess gastric
disintegration and emptying rate
• Feeding study
– acquiring the digesta samples using naso-gastric
• Intubation techniques: gastric barostat and intraluminal
manometry
– “gold standards” for assessing motility of the stomach
• Scintigraphic imaging: liquid barium sulphate, radioopaque
spheres
– standard method to measure gastric emptying
• Ultrasonography measures gastric volume or antral crosssection.
The information is used to estimate the rate of emptying
and evaluate antral motility.
• Magnetic resonance imaging (MRI)
• Indirect methods such as blood test and breath test

Scintigraphic
images show
capsule
disintegration and
gastric emptying of
its contents
www.bio-images.co.uk/AAPS2002.pdf

MRI images showing disintegration and
gastric emptying of drug tablet
MRI
www.pharmprofiles.co.uk/UserFiles/File/pdf/DepomedPresentation.pdf

Factors affecting stomach emptying
rate
• food caloric content, macronutrients and volume
– 2 – 4 kcal/min (8.4–16.8 kJ/min)
• Increasing the viscosity of liquid meals delays
gastric emptying and increases satiety
• size, density, texture and microstructure of the
food
• Biological factors
– body posture, physical activity, age, health, gender

TNO intestinal model (TIM)
Stomach
pH electrode
Hollow fiber membranes
simulating the absorption
Intestine
TNO Nutrition and Food
Research (Zeist, The
Netherlands)

Schematic diagram of the dynamic in vitro model of the stomach and
small intestine (TIM): (A) gastric compartment, (B) duodenal
compartment (C) jejunal compartment, (D) ileal compartment, (E) glass
jacket, (F) flexible wall, (G) rotary pump, (H) pyloric valve, (I) pH
electrode, (J) secretion pump, (K) pre-filter, (L) hollow fiber membrane,
(M) dialysis system, (N) ileal delivery valve.

The model gut
• The model gut is used
to investigate effect
of food structure on
the release of
allergens
Institute of Food Research,
Norwich Research Park, Colney,
Norwich NR4 7UA, UK

In Vitro Dissolution Testing of Oral
Dosage Forms: USP apparatus
• Apparatus 1 - Basket (37º)
• Apparatus 2 - Paddle (37º)
• Apparatus 3 - Reciprocating Cylinder (37º)
• Apparatus 4 - Flow-Through Cell (37º)
• 500 ml –1000 ml (900 ml)
• Agitation speed: 50-100 rpm for basket method, and 25-75 rpm for
paddle method.
• Aqueous dissolution medium composed of 0.1 N HCl (or pH 1.2)

Research Needs
• Influence of hydrodynamic and mechanical forces
present in stomach on food disintegration
• Changes in rheological properties of gastric juice
and the hydrodynamics of the fluid with ingested
meal and their implications on food digestion.
• In vitro digestion models developed for detailed
investigations to determine rate of food
disintegration
• Role of food material properties such as texture
and microstructure on kinetics of food
disintegration in gastric environment

Objectives
• Gain an understanding of the link between food
material properties obtained at the manufacturing
stage and the disintegration kinetics of the food
in the human stomach.
– Develop an in vitro stomach model for detailed
investigations of food disintegration kinetics
– Explore the relationships between food material
properties including texture and microstructure and the
disintegration kinetics of food products
• Develop a computational model of fluid flow and
solid disintegration in a human stomach

Model stomach system
• Custom-built turntable
• Jacketed glass chamber
• On-line Force measuring apparatus
Sample holder

Force (N)
0.25
0.2
0.15
0.1
0.05
0
Profile of continuous force and
0 1 2 3 4 5
Time (min)
periodic force
Force (N)
0.35
0.28
0.21
0.14
0.07
Vassallo MJ, et al. 1992.
0
0 5 10
Time (min)

Hardness (N)
50
40
30
20
10
0
0 3 6 9 12
Cooking Soaking time time (min)
Hardness of raw and cooked
carrot samples during soaking
in gastric juice
Hardness (N)
50
40
30
20
10
0
Carrot hardness during
cooking in boiling water
0 10 20 30 40 50
Soaking time (min)
Raw carrots
2-min-cooked
carrots
6-min-cooked
carrots

Wt/W0
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0 50 100
Time (min)
Raw carrot,
0.015 N
2-min-cooked
carrot, 0.017N
6-min-cooked
carrot, 0.017N
Mass retention curve
for raw and cooked
carrots
Emptying data with fitting of curves
by power exponential model
y(
t)
= 1−
( 1
−
e
−kt
β
)
y(t): the fractional meal retention, β >1.0
indicates an initial delay in emptying, β

Mass retention data of carrots fitted
with modified power exponential model
Wt/W0
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0 20 40 60 80
Time (min)
0.012 N
0.070 N
0.18 N
0.24 N
Raw carrots
• Factors influencing mass
retention curve
– hardness of the foods during
digestion
– and the extent of physical
force acting on the foods
Wt/W0
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
Wt/W0
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0 20 40 60
Time (min)
2 min-cooked carrots
0 10 20 30 40
Time (min)
6 min-cooked carrots
0.017 N
0.02 N
0.103 N
0.146 N
0.006 N
0.017 N
0.03 N
0.041 N

Exponential relationship between half
time (t 1/2 ) and mechanical force (F)
t1/2
70
60
50
40
30
20
10
0
Raw carrots 2-min-cooked carrots 6-min-cooked carrots
Raw carrots:
y = 60.821e -5.3434x R 2 = 0.9646
2-min-cooked carrots:
y = 40.501e -9.6003x R 2 = 0.8424
6-min-cooked carrots:
y = 25.326e -30.34x R 2 = 0.817
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Force (N)

Competition between surface erosion and
tenderization during food disintegration
Food
particulate
during
digestion
a)
b)
Tenderization front Soft layer
Wt/W0
Wt/W0
1
0.2
1
0.2
0 5 10 15
Time (min)
0 20 40 60
Time (min)

Wt/W0
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0 10 20 30 40 50
Time (min)
Hardness of ham sample during
soaking in gastric juice
0.013 N
0.023 N
0.075 N
0.14 N
Hardness (N)
6
5
4
3
2
Mass retention data of
ham fitted with model
0 20 40 60 80
Soaking time (min)

Penetration front
Carrot (Methylene blue)

Penetration front: microscopy images
of carrot during digestion

Influence of sample size of carrots on half-time for disintegration
Half time (t1/2)
70
60
50
40
30
20
10
0
y = 58.202e -5.7505x
R 2 = 0.8854
y = 40.948e -10.876x
R 2 = 0.8227
0 0.1 0.2 0.3
Force (N)
Small size (d 3mm
× L 4.5mm)
Big size (d 6mm × L
6mm)

Wt/W0
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0 20 40 60 80 100
Disintegration time (min)
37˚C optimal
temperature for
digestion
Half time t1/2 (min)
21 C
29 C
37 C
45 C
200
160
120
80
40
0
Disintegration rate of
carrots with
temperature of gastric
juice (0.14 N)
20 30 40 50
Temperature of gastric juice (°C)

Wt/W0
Disintegration rate of carrots
decreases with increase in
the viscosity of gastric juice
1
0.8
0.6
0.4
0 20 40 60 80
Disintegration time (min)
1% gum
0.5% gum
0.25%gum
no gum
Gastric emptying curves
for the 4 locust bean
gum meals: lowviscosity
nonnutrient
control (LVC), highviscosity
nonnutrient
control (HVC), lowviscosity
nutrient (LVN),
and high-viscosity
nutrient (HVN).
(Marciani et al. 2001a)

Wt/W0
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0 50 100 150 200 250
Disintegration time (min)
Almond
Wt/W0
1.2
1
0.8
0.6
0.4
raw
fried
roasted
Peanut
0 50 100 150 200 250
Time (min)
Roasting and frying increase disintegration
rates of peanut and almond (0.1 N)
raw
fried
roasted

1 mm
Changes in structure of
almond during digestion (30
min) static
100 um

Needs in Pharmaceutical research
• “... mechanical functions of stomach and
duodenum are well defined in terms of
viscoelastic properties, movement patterns of
their walls….flow phenomenon to digestion
remains to be established…contribution of
pressure forces, shear stresses, flow reversals
and vortical flow remains to be quantified.”
Schulze (2006)

Computational Model of a Human
Stomach
(a) Stomach geometry constructed with circular rings, (b) Tetrahedral grids
used to mesh half of the stomach geometry

Grids of Stomach geometry modified in FLUENT using user defined functions

Conclusions
• The custom-built model stomach system is useful for
studying the kinetics of food disintegration in the
stomach
• Kinetics of food disintegration is mainly affected by
surface erosion and texture softening during digestion
• Half-time (t 1/2 ) is an important parameter describing the
ability of foods to resist disintegration during digestion
– Higher mechanical force, smaller size, and longer cooking time
are all related with shorter half-time
• Stomach emptying profile might be largely affected by
the food disintegration process
– related to the changes in food texture and the forces acting on
the foods during the digestion process.
• Food processing steps such as boiling, roasting and
frying appear to improve solid disintegration rates