Food structural effect

Food types and processing techniques are the main factors considered by researchers in the in vitro gastric digestion studies. These in vitro studies clearly show how food structure influences the gastric digestion of food, because there is only one variant in the system, i.e. food structure.

Recently, the effect of food structure on the disintegration of food during gastric digestion has received much attention. Both in vitro and in vivo studies have shown the structural effect of food on the disintegration of foods during gastric digestion.

Mastication greatly changes the structure of solid food, which is swallowed as a form of bolus. The properties of bolus (e.g. the particle size distribution) play an important role in the gastric digestion of food. However, few studies consider the effect of oral

processing on the gastric digestion of foods. Nevertheless, these studies about effect of original structure of food without oral processing on the disintegration of solid foods provide useful information for understanding food digestion in the human body. Kong

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& Singh (2009) studied the disintegration of a variety of foods including ham, fried dough, carrot, beef jerky, almond and peanut in a stomach model, using the mass loss method. Their stomach model simulated the biochemical environment and antral

shearing of the stomach. The rate of disintegration of the foods generally decreased with the increase of food hardness or toughness. For example, ham with very low hardness lost 50% weight within 5 min during gastric digestion whereas peanut with high hardness lost 50% weight using 715 min. A similar phenomenon, that different foods vary in disintegration rate during gastric digestion was also reported in other studies (Siegel et al., 1988). Processing of foods significantly affects their gastric digestion properties. Boiling, roasting and frying significantly change the structural properties of peanuts. The disintegration of peanuts represented by mass loss of food particles during gastric digestion varied with different processing methods. The time for loss of 50% weight was 10.7, 8.3, 6.7 and 3.6 hours for raw, boiled, roasted and fried peanuts, respectively (Kong et al., 2013). Furthermore, with the increase of boiling time, the disintegration rate of carrots in the stomach model increased because of the decreased food hardness.

Bornhorst & Singh (2013) examined the effect of the properties of bread bolus created by mechanical grinding and mixing with artificial saliva on the disintegration of bread bolus in the stomach model. It is believed that the original structure of various breads significantly influenced the disintegration of bread bolus in the stomach model via altering the water adsorption ability and cohesive force of bread bolus. In an in vivo

study using six mini-pigs, the heated rennet milk protein gel was emptied significantly more slowly than the raw rennet milk protein gel with the similar composition,

indicating the denser microstructure of heated rennet milk protein gel slowed down the gel disintegration (Barbé et al., 2013).

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Mixing meals in the stomach is very important to food disintegration because the antrum is the main site for food trituration. There is an interesting report about the meal mixing in the stomach (Bornhorst et al., 2014). The pigs were fed with two equal- sized portions of meals which were labeled with different colors; the first portion was consumed first and subsequently the pigs were fed with the second portions. The rigid meals (raw almonds and brown rice) underwent a slower mixing process than the soft meals (roasted almonds and white rice). The authors believed that gastric emptying was a controlling factor to the mixing of meals that was more important than the type of food matrix and pre-processing prior to consumption. This was because the gastric emptying showed almost linear correlation to percent mixed meals. However, Urbain et al. (1989) found that the mixing of a meal with solid particles retained in fundus and antrum was fast, with an efficient mixing occurring during the lag phase of gastric emptying.

The food breakdown during gastric digestion at a macroscopic scale is discussed above, but the effect of food structure on the gastric digestion at a molecular level is another important area for understanding food digestion. Structural properties of protein aggregates or protein 3-D network determine the hydrolysis rate and peptide species generated in the gastric digestion. Heated whey proteins formed above pI (~ pH 4 - 5) are more susceptible to gastric digestion than those formed below isoelectric point probably because the unfolding and aggregation of whey proteins at near neural pH exposed more hydrophobic residue and accessible peptide bond for enzyme reaction (Zhang & Vardhanabhuti, 2014). The study of Nyemb et al. (2014) provides an evidence for how the food structure at a molecular level affects digestion. The authors prepared ovalbumin aggregates with different morphologies of linear (~ 33 nm), linear-branched (~ 16 nm), spherical (30 μm) and spherical agglomerated (80 μm) by heating at pH

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9/ionic strength 0.03 M, 7/0.03 M, 7/0.3 M and 5/0.3 M, respectively. The results showed that linear aggregates hydrolyzed much faster than the spherical aggregates in the model stomach. Furthermore, the peptide bonds appeared to be specifically cleaved, depending on the morphology of the aggregates. Different surface area of aggregates and different cross-linked patterns of proteins during aggregate formation are likely to be the possible mechanisms.

The understanding of food breakdown in the stomach is crucial to maintaining the human health, and is also important when designing of new foods. However, there is a lack of knowledge of food disintegration in the human body because very few

techniques can be used to monitor the change in physicochemical forms of foods in the human body. In vitro methods may help in exploring the disintegration mechanisms of solid food in the stomach, especially when the function of regular mechanical grinding of human antrum is simulated in an in vitro gastric model.