Paper No. 16. Paper: Advances in Food Science & Technology. Module No. 24. Module: Beta Glucan: Properties and Health Benefits

Transcription

1 Paper No. 16 Paper: Advances in Food Science & Technology Module No. 24 Module: Beta Glucan: Properties and Health Benefits 24.1 Introduction Among many known and tested immunomodulators, a polysaccharide beta-glucans occupy the prominent position. These glucans are extensively used as food additive, for the treatment of metabolic syndrome due to its numerous functional and bioactive properties. Glucans are glucose polymers, classified according to their interchain linkage as being either α- or β-linked. Found in foods such as oats, barley, mushrooms, and yeasts and also to a lesser extent in rye and wheat Occurrence of Beta-glucans Beta-glucans are predominantly found in the internal aleurone and subaleurone cell walls. The content of β-glucan varies with environmental conditions during endosperm development and is regulated by (1-3, 1-4)-β glucan endohydrolase (EC also known as licheninase or 1-3, 1-4, β-glucanase) to facilitate endosperm cell-wall degradation during germination. Table 24.1: Cereals g /100 g (dry weight) % water-soluble fraction Barley Oats Sorghum Rye Maize Wheat Among cereals, the highest content of β-glucan has been reported for barley (2 20 g) and for oats (3 8 g). Other cereals also contain β-glucan but in much lower amounts as presented in the above table. Other sources of β-glucan include some types of seaweed and various species of mushrooms such as Reishi, Shiitake, and Maitake. Fractions rich in β-glucans are readily obtained from cereal grains by dry milling followed by sieving and air classification processes or by wet milling

2 followed by sieving and solvent extractions. These approaches result in concentrates or isolates containing 8 30% and 95% β-glucans, respectively Structure of Beta-glucans Beta-glucans are the group of polysaccharides, consisting of D-glucose monomers linked by β- glycosidic bonds. The simplest glucan is the linear and unbranched β-(1,3)-d-glucan, found among prokaryotes and eukaryotes. Another simple structural type occurs mostly in the nonlignified cell walls of cereal grains, and consist of linear β- (1,3;1,4)-D-glucans. Branched structures of β-glucans consist of β-(1,3)- or β-(1,4)-glucan backbone with either (1,2) - or (1,6)- linked β-glucopyranosyl side branches. They are major structural components of the cell walls of yeast, fungi, and some bacteria. The side branched β- (1,3;1,2)-D-glucan is only present in the type 37 capsule of the bacterium Streptococcus pneumonia. Branched β- (1,4;1,6)-D-glucan and β-(1,3;1,6)-d-glucan are found in different groups of yeast, fungi, and algae. In algae, β-glucans are present as storage polysaccharides or cell wall components. Some cyclic (1,2) and (1,3;1,6) β-glucans were also isolated from various bacteria. These glucans are important for plantmicrobe interactions, and act as signaling molecules during plant infection. Besides differences in type of linkage and branching, β-glucans can vary in terms of frequency and length of branching, degree of branching, molecular weight (from 10 to 10 daltons), polymer charge, and/or solution conformation (random coil or triple or single helix) as well as solubility.

3 All these factors play a role in shaping β-glucan-associated biological activities, and should be taken into consideration when discussing the physiological impacts of β-glucans Classification of Beta-glucans Most β glucans are insoluble. Solubility of β glucans is associated with the degree of polymerization (DP), being completely insoluble in water when DP > 100. They can be classified according to solubility properties: alkali insoluble, acetic acid insoluble; alkali soluble; and highly branched. A range of extraction and purification techniques are available for extraction of β-glucan. This may include hot water extraction, solvent extraction, enzymatic extraction, and alkali extraction Regulatory Claims on Beta-glucans The most documented nutritional benefit of β -glucan in foods is flattening of postprandial blood glucose and insulin rises. The Food and Drug Administration (FDA) has acknowledged nutritional claims that the use of dietary fibers reduces the glycemic and cholesterol responses of individuals. Current recommendations suggest an intake of g of dietary fiber per day. The FDA has certified a recommendation of 3 g /day of β glucan as having a nutritional effect. It has no known side effects even in very extreme doses. It has a Generally Recognized as Safe (GRAS) status from the FDA. Owing to its importance the FDA endorsed its use in food products and made it obligatory for labeling requirement to acquire health claims Properties of Beta-Glucans The physiological effects of β-glucan are mainly attributed to its physicochemical and structural properties interacting with the gastrointestinal tract, as reflected by its ability to generate viscous solutions at low concentrations in the upper part of the gastrointestinal tract and to undergo fermentation in the colon Many investigations have revealed that there is a direct correlation exists between dietary components with that of treatment of metabolic syndrome. Hence, dietary components are regarded as the potential first-line intervention in the health management practices. Dietary constituents are capable of controlling blood glucose, insulin and lipids, blood pressure, and food intake have been identified. There is growing evidence that high intakes of fruits, vegetables, legumes, and cereals are beneficial. Many of their benefits have been attributed to their low glycemic properties and their dietary fiber content. The focal point is on β-glucan. a. Viscosity and Gelling Properties

4 Beta-glucan is high molecular weight linear polysaccharides and has a high volume occupancy in solution (high intrinsic viscosity). The high volume occupancy leads to polymer coil overlap and entanglement at low concentrations ( %w/v) above which flow viscosities increase exponentially with concentration and molecular weight, to about the fourth power. The evidence strongly indicates that the primary factor responsible for a most of exhibiting the health benefits, (low glycemic response in foods is the viscosity of the β glucan). The viscosity properties of β-glucan (as that of other polysaccharides) depend upon concentration, their solubility, and molecular weight. In freshly prepared barley β-glucan solutions, attraction forces between molecules are less strong but after an induction period some β-glucan solutions/dispersions may begin to adopt gel-like behavior, thus, developing high viscosity at low concentrations. Shear thinning behavior of cereal β-glucans also exhibited at low concentration, but at higher concentration they tend to form gels and their gelling properties are influenced by molecular weights and molecular structure. Higher molecular weight β-glucan gel does not show this property any tendency to gel even after 200 hours storage. On the other hand, short chain molecules with low molecular weight show higher mobility and these short chains with low molecular weight β-glucan structures diffuse more readily, and hence have a greater possibility of forming junctions with neighboring chains b. Foaming Capacity and Foam Stability The foaming capacity (FC) and foaming stability (FS) properties are important when β -glucan is to be used as a functional ingredient and stabilizer in batters, often a high foaming capacity and stability is desirable in cakes and batters. A higher foaming capacity was observed when sample was extracted by hot water extraction procedure and this could be due to relatively higher protein contents that trapped more air to develop more foam. A negative correlation is can be seen between foaming capacity and viscosity, which shows that viscosity imparted by β -Glucan, makes the liquid to foam with difficulty. c. Water Binding Capacity The Water binding capacity (WBC) of a fibre measures the amount of water retained by the fibre after subject to stress. These hydration properties of β-glucan are important in many food applications and have an impact on shelf life of food product. β -glucan extracted by hot water treatment exhibits highest value of WBC followed by alkali and acid extracted β -glucan. The

5 high value of WBC for β -glucan suggested that this material could be used successfully as a functional ingredient to avoid syneresis problem in various food products such as jam, jellies, sauces and cheese Health Benefits of Beta-Glucans Nowadays, there is increased popularity in the use of β-glucan as food additive for the treatment of metabolic syndrome and as a disease modifying agent. For decades beta glucan has been studied for its most impressive biological effects on mammals. It has been common knowledge in the scientific community that β-glucan is the most powerful immune stimulant known. A large number of studies indicated the effectiveness of β- glucan against various diseases and disorders, and several applications reported in many scientific work are the tendency to reduce onset of colorectal cancer, increased stool bulk and provide assistance against constipation, reduction in glycemic index, flattening of the postprandial blood glucose levels and insulin rises, prevention of insulin resistance, lowering of blood lipids, especially serum total and LDL-cholesterol, prevention of coronary heart disease, production of short-chain fatty acids, prevention of hepatic damage by reducing taxol-induced hepatic damage, and promotion of the growth of beneficial gut microflora, heals and rejuvenates the skin, and has many various other benefits. Since β glucans tend not to be degraded by human enzymes, they have a natural nutritional fiber property. A high fiber diet has demonstrated protective hypocholesterolemic effect reducing the risk of chronic diseases such as arthritis and heart disease. β glucans do this by increasing intestinal viscosity and reducing cholesterol absorption, thereby promoting cholesterol excretion and imparting a hypocholesterolemic effect. This has prompted the U.S. Food and Drug Administration (FDA) to approve some patents of β glucans to be sold over the counter, for treatment of hypercholesterolemia. Beta-glucans help to lower blood cholesterol by preventing the absorption of cholesterol from food in the stomach and intestines, when taken by mouth. The proposed mechanisms include binding of bile acids, decreasing their uptake and increasing excretion and increased viscosity of small intestine content, creating a layer that blocks absorption of dietary cholesterol. In vitro, animal and human studies have shown that 1,3 β-glucans can enhance the responsiveness and function of immune cells, stimulating both humoral and cellular immunity. Β-glucans can enhance the functional activity of macrophages and activate the anti-microbial

6 activity of mononuclear cells and neutrophils. Several soluble fibers, including β-glucan, reduce postprandial glucose and insulin responses, and improve insulin sensitivity both in diabetic and non-diabetic individuals. Several mechanisms have been suggested to explain the glucose- and insulin-lowering effects of soluble fibers, more precisely β-glucan. One of the mechanisms includes the ability of soluble fibers to form viscous solutions. Delayed gastric emptying occurs with increased digesta viscosity, slowing subsequent digestion and absorption. High digesta viscosity decreases enzyme diffusion and stimulates the formation of the unstirred water layer, decreasing glucose transport to enterocytes. Its beneficial role in insulin resistance, dyslipidemia, hypertension, and obesity is being incessantly acknowledged. The fermentability of β-glucans and their ability to form highly viscous solutions in the human gut may also constitute the basis of their health benefits Effect of Processing on Characteristics of β-glucans Processing may affect the molecular (chemical structure and degree of polymerization), structural (molecular interactions) and functional properties (viscosity, water binding capacity and solubility) which, in turn, could affect the sensory, physiological and ultimately the health benefits of β-glucans. Changes in the properties of β-glucans may arise from shearing damage due to mechanical processing, or by excessive heat treatment of food products. Unfavorable structural changes may also occur during commercial purification, such as the depolymerisation of the linear structure, resulting in decreased molecular weight and reduced viscosity Application of β glucans in the Manufacture of Functional Dairy Products Incorporation of β -glucans, into low fat dairy products can make their mouth feel, scoopability and sensory properties resemble those of full-fat products. Similarly, β -glucan incorporation into low fat cheese curds has beneficial effects on their gelation and rheological characteristics. The addition of β -glucan solutions to milk modifies curd formation, including reducing curd cutting time and increasing curd yields. These effects appear to be related to the gelling capacity of β - glucans and their ability to form a highly structured and elastic casein protein glucan matrix Challenges of Beta -Glucan Fortification Development of β-glucan-fortified foods remains highly challenging as consumers are not willing to accept greater health benefits on the expense of deteriorations in the sensory characteristics of food products. Incorporating significant quantities of fiber into food products constitutes a technological challenge due to the possible deleterious effects on textural quality.

7 The addition of fibers may contribute to modifications in the texture, sensory characteristics, and shelf-life of foods due to their water-binding capacity, gel-forming ability, fat mimetic, antisticking, anti-clumping, texturising, and thickening effects. Adding β-glucan into milk and dairy products was reported to be problematic; first due to its viscosity that may alter the sensory characteristic of foods and second due to its typical slimy texture in the mouth.