R: Concise Reviews in Food Science JFS R: Concise Reviews and Hypotheses in Food Science Disintegration of Solid Foods in Human Stomach F. KONG AND R.P. SINGH ABSTRACT: Knowledge of the disintegration of solid foods in human stomach is essential to assess the bioavail- ability of nutrients in the gastrointestinal (GI) tract. A comprehensive review of food gastric digestion, focusing on disintegration of solid foods, is presented. Most of the research reviewed in this paper is contained in the medical, pharmaceutical, food, and nutritional literature. Stomach physiology is briefly introduced, including composition and rheological properties of gastric contents, stomach wall motility in fed/fasted states, and hydrodynamic and me- chanical forces that act on the ingested food. In vivo and in vitro methods used for studying food and drug digestion in GI are summarized. Stomach emptying rate, which controls the rate of absorption of nutrients, is highly related to the disintegration of foods. This topic is highlighted with focus on the important mechanisms and the influence of chemical and physical properties of foods. Future research in this area is identified to increase our fundamental understanding of the food digestion process in the stomach as related to the food composition, material proper- ties such as texture and microstructure, and chemical characteristics. This information is necessary to develop new guidelines for seeking innovative processing methods to manufacture foods specifically targeted for health. Keywords: controlled release, digestion, food disintegration, gastric emptying, stomach Introduction Food digestion in the GI tract In the human digestive process, foods undergo major size re- duction to help release embedded nutrients so that they may easily pass into the bloodstream for eventual absorption by the body cells. Mouth and stomach are the major compartments where foods are disintegrated into small size, whereas small intestines are the major site of nutrient absorption. In the digestive tract, both mechanical forces and chemical reactions break down ingested food into small molecules. The rate kinetics of digestion depends on the chemi- cal and physical characteristics of food and their interaction with the physiological events occurring within the gastrointestinal (GI) tract. Digestion of foods begins with chewing in the mouth. The oral step is rapid but plays an important role in digestion. Mouth se- cretes saliva containing mucus and the enzyme amylase. Masti- cation reduces the particle size, and hydrates and lubricates the food by mixing it with saliva. Mastication also reduces viscosity of starchy food by the rapid action of salivary amylase (Hoebler and others 2002). Food bolus is formed and transported through the esophagus to the stomach by the mechanism of peristalsis. Peri- stalsis is an advancing wave of contraction of the walls of a flexible conduit, forcing the contents forward (Siddiqui and others 1991). The stomach is divided into 4 major regions: fundus, body, antrum, and pylorus (Figure 1). The stomach has 3 main motor functions: storage, mixing, and emptying. The proximal part made of fundus and body acts as a reservoir for undigested material, re- sponsible for the emptying of liquids, whereas the distal stomach (antrum) is the grinder, mixer, and siever of solid food, and acts as a pump for gastric emptying of solids by propelling actions (Urbain and others 1989; Arora and others 2005). The reservoir function of MS 20070949 Submitted 12/20/2007, Accepted 2/23/2008. Authors are with Dept. of Biological and Agricultural Engineering, Univ. of California, Davis, CA 95616, U.S.A., and Riddet Institute, Massey University, New Zealand. Di- rect inquiries to author Singh (E-mail: rpsingh@ucdavis.edu). stomach is achieved through the flexible volume of the stomach, which can expand to accommodate food up to a volume of about 4 L. Mixing and homogenizing function is achieved through the secretion of gastric juice and stomach contraction that produces grinding and crushing of foods. Gastric juice secreted from glands lining the stomach contains gastric acid, bile salts, and digestive en- zymes. The gastric juices penetrate and dilute the food bolus. Peri- staltic waves originate from the stomach wall and spread toward the antrum, mixing and forcing the antral contents toward the py- lorus. The pylorus contracts to slow gastric emptying and results in further mixing of gastric contents. During this time, the stomach transforms its contents into multiphase slurry called chyme, which is a combination of separate phases of aqueous solutions, fats, and solids. The more intense peristaltic waves promote antral empty- ing, which allows gastric contents, mainly fluid mixed with small particles, to pass through the pylorus and enter the duodenum. The particle size of the food emptied through the pylorus is less than 1 to 2 mm during the fed state (Thomas 2006). Final stages of digestion and most of the nutrient absorption oc- cur in the small intestine, where the food is dissolved into the juices from the pancreas, liver, and intestine. All of the digested nutrients are absorbed through the intestinal walls. The waste products are propelled into the colon for excretion. Digestion of solid foods may be considered as a 2-step pro- cess: disintegration and dissolution. Disintegration indicates how fast a food particulate can break into small fragments so that any entrapped nutrient ingredients can dissolve into the gastric juice. Dissolution indicates how fast nutrient ingredients can dis- solve into solution for absorption. It is hypothesized that both these steps, disintegration and dissolution, can be affected or con- trolled by the food-processing conditions used at the manufactur- ing/preparation stage. Chemical and physical transformation of solid foods in mouth Oral mastication is the initial step in food digestion. From a physiological point of view, the main role of mastication is to C  2008 Institute of Food Technologists Vol. 73, Nr. 5, 2008—JOURNAL OF FOOD SCIENCE R67 doi: 10.1111/j.1750-3841.2008.00766.x Further reproduction without permission is prohibited