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Summary of Food Digestion: ingestion and structure breakdown
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AJ: In vitro modelsDigestion of food is a chemical process by enzyme activity and mechanical process by the mixing.
The stomach has a flexible volume: 50 mL to 4 L. It is a receptable (vessel/container), grinder, mixer
and pump that controls the digestion process. The gastric juice is colourless, has a low pH (<2, breaks
down food and kills bacteria), contains pepsin (protein breakdown) and lipase (fat breakdown).
Typical disintegration profiles of foods are (y-axis: weight of product):
- Exponential: canned kidneys beans, ham, candy
o Disintegration is only induced by erosion from the surface by which the product
becomes smaller.
- Sigmoidal: fruits, raw carrot
o Gastric enzymes and juices should penetrate the food to make it softer before it can
be degraded. The cell walls of the food are degraded.
o The more the carrot is cooked, the faster the degradation. This may due to a loss of
hardness by the cooking.
- Delayed sigmoidal: dry foods such as peanuts, almonds, fried dough product (first a little
increase due to take up of gastric juice)
o At first there is an increase in weight (swelling) due to the uptake of gastric juice.
Thereafter the product can be degraded.
o The total weight of raw and roasted almonds first increase and then decrease
(delayed sigmoidal), but the dry mass retention only decreases. Roasted almonds
decrease weight faster.
So, the breakdown of foods is different. There is competition among surface erosion, texture
softening and absorption of gastric juice.
Different surface areas induce a different breakdown. Smaller particles take up more gastric juice as
they have more surface compared to their weight. After a certain time (e.g. 3 hours) it has less
weight retention (less % of weight less compared to start). Furthermore the type of preparation (raw,
boiled, roasted, fried) causes a different disintegration. In general processing increases the digestion.
Disintegration linear-exponential equation:
K is the increase in weight in time (min), ß defines the concavity of the time-weight retention
relationship (ß>0).
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,The strongest fluid motions are predicted in the lower part of the stomach. The rheological
properties of the gastric content has a significant effect on the behaviour of the antro-pyloric flow.
The food that is pushed through the pylorus, can be pushed back if it cannot go out (fully) of the
stomach.
A study is performed in pigs that consumed white and brown rice. After 300 minutes, the mixture is
liquid like for the white rice. The brown rice is much more intact. The digestion of brown rice takes
longer than the white rice because the brown rice fibers are harder to digest. It can be concluded
that the structure of the food effects the length of the digestion.
Main factors in physical and chemical food breakdown:
- Initial food properties
o Structure, physical properties, chemical composition
- Rate of breakdown
o Rate of softening, rate of hydrolysis
- Residence time in physical or chemical environment
o Time during mastication, time in antral region, time exposed to acid or enzyme
The food breakdown classification system focusses on processes of the physical (macro-structural)
breakdown of solid foods. The classification is based on initial hardness of food product and rate of
softening in physiological gastric conditions.
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,In the mouth there is chewing and mixing with saliva. The transit time is 10 seconds to 2 minutes.
The pH is 5-7 and salivary enzymes such as amylase and lingual lipase are present.
In the stomach there is mechanical and enzymatic processing of the ingested bolus. The transit time
is 15 minutes to 3 hours. The pH is 1-5 and components present are HCl, pepsin and lipase.
In the small intestine and colon the pH is higher and the transit times are longer.
When pH>4, the amylase of the mouth can still be active in the stomach. So, the starch degradation
can continue in the stomach. If the pH drops below 4 proteins and enzymes degrade. The pH in the
stomach not immediately very low, this takes some time to decrease.
There are several in vitro digestion models:
- Static mono-compartmental models
o In one vessel you add liquids, enzymes, etc.
o Simple and widespread
o They are useful to evaluate the influence of digestion conditions (e.g. pH) and to
carry out studies on the effect of food structure, food composition and food
processing.
- Dynamic mono-compartmental models (some food is entering and some leaving)
o Includes dynamic processes e.g. gastric emptying, continuous changes in pH, shear
and grinding forces, stepwise addition of enzymes, bile salts and electrolytes
phospholipids.
o Examples: DGM (Dynamic Gastric model), Human Gastric Simulator, Human
Duodenum model (it is hard to mimic peristalsis)
- Dynamic bi- and multi-compartmental models
o Includes dynamic systems e.g. gastric emptying, continuous changes in pH, shear and
grinding forces, stepwise addition of enzymes, bile salts and electrolytes
phospholipids.
o They have multiple compartments/vessels: bi-compartmental digestion model
(mimics e.g. stomach and duodenum, e.g. DIGID)
o Dynamic multi-compartmental model are very comprehensive and take a lot of time
(can mimic whole digestion tract e.g. TIM-1
To compare the research, a consensus method for stimulated gastro-intestinal digestion was
designed. This is a standardized method based on the digestion of an average adult.
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, The model assumes an average adult. However, elderly and infants have different compositions of
their gastric juices. Elderly have lower peristaltic movements, less pepsin and a higher pH. Elderly
have difficulties to get enough protein due to a reduced intake and digestion.
Infants have a relatively high pH (3.2-6.5) and have a limited pepsin activity. This also limits the
protein digestion of infants. This could cause a different allergen formation/breakdown.
Bioaccessibility: Fraction of a nutrient released from food matrix and available for intestinal
absorption (present in intestine). Bioaccessibility data is typically based on in vitro procedures.
Bioavailability: Fraction of ingested component available for utilization in normal physiological
functions (present in the blood). Bioavailability data is typically determined by in vivo assays
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