Resistant Starch and Health

Starch is the major source of carbohydrates in the human diet. Starch is present in many different fruits, vegetables, roots, and grains. Starch and starch derivatives are a nutritive, abundant, and economical food source. Starch can be consumed unprocessed in the form of raw fruits and vegetables or in the form of more shelf-stable processed foods. Food starches contribute to the characteristic viscosity, texture, mouthfeel, and consistency of many food products. In the human body, starch is digested by a-amylases. First, salivary a-amylase in the mouth catalyzes the hydrolysis of amylose to maltose, maltotriose, and maltotetraose and the hydrolysis of amylopectin to the same products plus two a-limit dextrins. The partially digested starch then passes into the stomach. After some resident time in the low pH environment of the stomach, the partially hydrolyzed starch then passes into the small intestine where it is neutralized. The majority of hydrolysis of the starch is then accomplished by pancreatic a-amylase that is secreted from the pancreatic duct via a multiple attack mechanism. a-Amylases hydrolyze the starch into small mono-, di-, and oligosaccharides that then need to be further broken down to be absorbed as glucose. Two other enzymes are necessary to convert the hydrolysis products of the a-amylases into glucose, which can be actively transported across the small intestine membrane. These two enzymes are the brush-border glucogenic enzymes maltase– glucoamylase and sucrase–isomaltase. The resulting D-glucose is then actively transported across the luminal membrane of the small intestine and passes into the blood via the sodium– glucose cotransporter, which is located at the luminal surface of enterocytes. Several factors influence the rate and extent of starch digestion. The main hydrolysis of starch is performed by the a-amylases. Starch digestion by a-amylases requires a series of steps. First, the enzymes diffuse into the starch matrix of the food. In the second step, the enzymes bind to the substrate, and finally, the enzymes cleave the a-1,4-glycosidic linkages of the substrate (starch). The structure of the food matrix and the structure of the starch itself influence the kinetics of the amylase hydrolysis. Resistant starch (RS) has been defined as “the starch and products of starch digestion that are not absorbed in the small intestine of healthy individuals.” Based on the source of the enzyme resistance, RS has been classified into five different types (Table 1). In type 2, type 3, and type 5 RS, the enzyme resistance is due to the physical structure of the starch molecules. Type 2 RS is found in raw starch granules and the enzyme resistance is due to the natural organization of the starch within the starch granules. Significant amounts of type 2 RS can be found in green banana, potato, and high-amylose maize. In type 3 RS, the enzyme resistance is due to the physical structure of starch chains that have undergone some type of restructuring due to retrogradation or heat treatment. Different factors, such as amylose/amylopectin ratio, chain length, lipid content, and processing conditions, have been shown to influence the amount and quality of type 3 RS. Annealing and heat moisture treatment are two types of heat treatments that are often used to create RS. Both of these treatments, as well as gelatinization, either fully or partially melt crystalline structure present in the native starch granules. After heat treatment, linear amylose molecules and linear regions of amylopectin molecules can organize into a mix of amorphous and crystalline areas with varying degrees of enzyme resistance. Formation of the crystalline structures usually takes place above the glass transition temperature and below the melting temperature, and any components present that influence the glass transition temperature can therefore be expected to influence the formation (yield and quality) of the formed type 3 RS. The amylose content of starch has been positively correlated with RS yield and the formation of type 3 RS is strongly related to the crystallization of amylose. The amount of RS formed is also dependent on the water content and temperature used during the heat treatment. Water does act as a plasticizer in the system and a minimum water content is necessary to achieve the chain mobility needed to form crystalline structure resistant to enzyme digestion. At high-starch concentrations, the starch chains interact more easily, leading to increased crystal and RS formation. The presence of lipids has been shown to decrease the formation of type 3 RS due to formation of amylose–lipid complexes (type 5 RS). The enzyme resistance in type 5 RS is due to the molecular structure of amylose–lipid complexes that canbe either present in the native starch or formed by controlled reactions using nongranular starch and lipids to form the resistant amylose–lipid complex. The enzyme resistance in type 4 RS is due to chemical modification of the starch. Chemical modification of