P ROCESSING E FFECTS ON P HYSICAL P ROPERTIES

1.7.3 P ROCESSING E FFECTS ON P HYSICAL P ROPERTIES

The physiological benefits and better technological performance of soluble, highly viscous fibers over their insoluble counterparts increased the interest in investigating processing factors that have an impact on solubility, extractability, and viscosity and molecular weight of β-glucan in isolates, concentrates, or cereal products and grains. 183

Although some β-glucan preparations obtained from cereal grains by dry- or wet-milling procedures are products with high content of total β-glucans, they might not be the best source of soluble or high-viscosity polysaccharides. Jaskari et al. 144 found the amount of soluble β-glucans (at 37˚C for 2 h) to be greater (34%) in a dry-milling preparation containing lower β-glucans than in a wet-milling product (24%) containing significantly more β-glucans. Heating increased the amount of soluble β-glucan, on average, from 29 to 84%. The molecular weight of β-glucans

was 8.4 × 10 5 from oat bran obtained by dry milling and 6 × 10 5 from fiber- concentrated oat bran obtained by wet milling, and remained unchanged during hydrothermal treatment; however, the later preparations developed much higher viscosity than the former as a result of the hydrothermal treatment. Knuckles et al. 56 reported 59% β-glucan solubility for low-fiber oat brans and 55% for higher-fiber oat brans. Similarly, Malkki et al. 136 found the highest β-glucan extractability at 40˚C in an untreated oat bran (90.8%), whereas for the preparations from this bran enriched in β-glucans by wet milling with neutral cold water, acidic cold water, hot- mixture ethanol–water, and cold-mixture ethanol–water, the respective values were

48.0, 76.7, 40.4, and 44.0%. The viscosity and molecular weight of β-glucans were reduced at acidic conditions under concentration and trypsin treatment. β-Glucan from the cold-water, wet-milled concentrate had the highest molecular weight and

a higher hydrodynamic volume than β-glucan isolated from the other concentrates. 136 Since it is postulated that solubility and viscosity of β-glucans are responsible for the biological activity of these polymers, several thermal, enzymic, and physical treatments of cereal grains were assessed as potential strategies for increasing the level and viscosity of β-glucans in cereal grains or extracts. Processing and storage, which involve structural breakdown and restructuring, may therefore increase their availability and solubility. Prolonged storage of baked corn bread at 20, 4, and –20˚C appeared to increase extractability and measurability of β-glucans in corn bread; β-glucan content increased from ~0.35 to ~0.5 after 7 days of storage. 184 Freezing and frozen storage for 3 weeks of soup preparations containing β-glucans did not affect the molecular weight or concentration values of these polysaccharides. The effects of freezing on high and low molecular weight oat β-glucan preparations have been compared. Freezing decreased the viscosity of very high molecular weight

β-glucan (M w = 1.7 to 2 × 10 6 ) slightly, had no effect on the viscosity of intermediate molecular weight β-glucans (M w = 0.9 to 1.4 × 10 6 ), and increased the viscosity of the low molecular weight products (M w = 60 to 160 × 10 3 ). Molecular size and pH have a notable impact on viscosity of β-glucans during freezing and frozen storage.

38 Functional Food Carbohydrates

Noticeable shear-thinning effects were also observed when β-glucan dispersions were first subjected to freeze–thaw cycling, implying formation of network struc-

tures, a process known as cryogelation, 19,20 as discussed in detail in the previous section. Overall, the thermal history and structural features of β-glucan preparations seem to significantly affect the rheological properties of these polysaccharides.

Physical treatments such as stirring and sonication were found to increase extract- ability of β-glucans from barley, but a reduction of the molecular size of the polysac- charide was observed. In alkaline solutions, both treatments also decreased the molec- ular weight of β-glucans. The decrease in molecular weight is related to the energy

applied. 70 On the other hand, a study of flow behavior of barley β-glucans under acidic solutions (pH 2.0) after a high-shear treatment (24,000 rpm, 4 to 30 min at 25˚C, at

3 and 1% w/v concentrations) showed no pronounced changes in viscosity compared with the control solutions, where no high-shear treatment was applied, implying a stable viscosity of β-glucans under high-shear homogenization. 185

Response surface methodology has been used 20 for the expression of viscosity variations occurring during thermal processing of acidic media containing barley β-glucans of two different molecular weights (apparent peak molecular weight =

140 and 250 × 10 3 ). The pH range (~2.7 to 4.3) used in this study was typical of the pH usually met in acidic food products like fruit and tomato juices, while the temperature (~70 to 95˚C) and time (~4 to 75 min) ranges reflected well the conditions under which the pasteurization process of such products occurs in most of the food industries. The viscosity reductions, as a consequence of acid hydrolysis, were dependent on pH, temperature, and time, factors that were well fitted in second-order polynomial equations. Acid hydrolysis effects were more pronounced for the high molecular weight sample, indicating potential differences in the flow behavior of liquid products containing β-glucans of different molecular size, during

thermal processing of acidic products. 20 A similar experimental design was adopted to examine the effects of β-glucan concentration (1 to 6% w/w), sucrose (10 to 40% w/w), and salt (3 to 12% w/w) on the viscosity of the aqueous phase of a mayonnaise- type dispersion, in which β-glucans can be used as a stabilizer of the oil-in-water dispersion, containing a fixed vinegar concentration 2% (v/w). All the factors exerted strong influence on the steady shear viscosity (significant positive linear effects), while their interaction effects were negative, indicating that the viscosity increases were smaller when the factors were combined at high levels. 20

Moisture and heat processing were found to influence β-glucan availability in sorghum grain. The β-glucan content from 0.12% in unprocessed sorghum increased

up to 0.55, 1.14, or 3.05% by processing the grains for various periods with soaking at 37˚C or autoclaving at either 130 or 120˚C, respectively. 91 Zhang et al. 83 found

that steaming of oat groats reduced the amount of β-glucans that could be extracted, compared with raw or roasted grain, but the extracts from steamed grain had much greater viscosity, despite the fact that the average relative molecular mass values of β-glucans among raw, roasted, and steamed oat samples were equivalent. These authors assumed that dry heat treatment (roasting) might increase intramolecular associations in the polymeric chain, whereas moist heat (steaming) might disrupt intramolecular cross-linking in native β-glucan, replacing intramolecular bonding with water binding, and therefore allowing a linear chain configuration to generate

Cereal β -Glucans: Structures, Physical Properties, and Physiological Functions 39

enhanced rheological properties. An increase of solubility and β-glucan content when barley was extruded has also been reported. 27,186 The physical properties of extruded preparations from barley meal and their extracts have been investigated and com-

pared with an autoclaved product. 68 The amount of extracted β-glucan at 40˚C was more influenced by the feed moisture than by the mass temperature during process-

ing. Using a KCl/HCl solution (pH 1.5) for acidic extraction, less than 20% of the β-glucans present in barley meal and approximately 60% of the β-glucans from extruded and autoclaved products could be extracted. Extrusion was found to be connected with a partial depolymerization of β-glucans. Despite the decrease of molecular size of β-glucans during hydrothermal treatments, the macromolecular state of β-glucans was preserved during extrusion; the molecular weight of β-glucans

extracted from barley meal was 160 × 10 3 , whereas from the autoclaved product it was less than 100 × 10 3 and from extrudates, depending on the extrusion temperature, it was between 80 (170˚C) and 125 × 10 3 (150˚C). The rheological properties of the examined products were essentially affected by the amount and molecular weight of β-glucans. The highest acid extract viscosity (AEV) was found for the untreated

barley meal. On the other hand, Izydorczyk et al. 70 reported that hydrothermal treatments, such as autoclaving and steaming of barley, had no significant effect on

extractability of β-glucans, but prevented enzymic hydrolysis of β-glucans, and thereby substantially improved their molecular weight and the viscosity development of barley slurries. Therefore, hydrothermal treatments of barley grains were sug- gested as a potential pretreatment before incorporation of the grain into food systems. In contrast, roasting barley grains at 100˚C did not result in any increase of viscos- ity. 70

Enzymic rather than physical treatments proved to be more important in account- ing for the increased extractability of β-glucans. 70,125,144 Enzymic treatments with a phytase preparation on oat bran showed an increase in β-glucan extractability, and

simultaneously a partial degradation of the polysaccharide (M w = 600 × 10 3 ) to a product of M w = 40 × 10 3 , which resulted in reduction of the viscosity of the oat bran slurry. 144 On the other hand, Izydorczyk et al. 70 found that the addition of enzymes (protease and esterase) in combination by hydrothermal treatments (auto- claving and steaming) on barley grain resulted not only in increased β-glucan solubilization, but also in improved molecular weight of β-glucans. Such treatments, therefore, might have a potential to positively affect the physiological responses to barley β-glucans in human diets.

Robertson et al. 125 found that barley β-glucan extractability increased to around 50% during cooking at 100 to 190˚C, from about 30% in the raw barley flour. Distinct

differences were observed between two cooking methods of oatmeal, gradually from room temperature or rapidly by addition to boiling water of rolled oats. The gradually cooked oatmeal had more solubilized β-glucan and gave a more viscous supernatant; the solubilized β-glucan was the major contributor to the viscosity. Microscopic examination revealed differences in the appearance of the two differently cooked samples; the disruption of the endospermic cell walls of the gradually cooked samples was greater than for the rapidly cooked samples. 187 In a latter study, 188 rolled-oat porridge was prepared by conventional cooking and microwave cooking to determine and compare the effects of these cooking methods on the texture,

40 Functional Food Carbohydrates

viscosity, and amount of solubilized β-glucans. The porridge prepared by microwave cooking was more grainy, less viscous, and with less solubilized β-glucans than conventional cooking. The difference was slight with short cooking duration (1 min) but increased drastically with prolonged cooking (20 min); on average, β-glucan content doubled when conventional cooking was extended to 20 min. Thicker oat flakes (regular-cooking rolled oats) released less solubilized β-glucan than did thin- ner oat flakes (quick-cooking rolled oats) upon cooking. Moreover, microscopic examination revealed that the cell walls of the rolled oats prepared by microwave cooking were less disrupted than those of conventional cooking. These differences were attributed to the effect of stirring; the microwave-heated rolled oats were stirred relatively less often than the conventionally cooked samples. 188

The extractability and molecular weight of β-glucan in oat bran, oat bran muffins, and oat porridge, and the changes taking place during processing and storage, have been studied. 117,189 The original brans used, prior to mixing with other ingredients and cooking, showed about 25 to 30% β-glucan extractability and a M w of 1 to 2 ×

10 6 . Cooking of oat bran muffins resulted in reduction of the M w to 600 to 950,000, but increased the percentage of β-glucan solubilized about threefold, to 55 to 85%. During frozen storage, extractable β-glucans decreased by >50% in all muffins, but no change in the peak M w of β-glucans was detected; the decline in solubility of the β-glucan possibly reflects changes in molecular organization during frozen storage. 117

β-Glucans in ready-to-eat breakfast cereals (oat bran, multigrain flakes) were found to range in molecular weight from 600 to 2930 × 10 3 . 116 Aman et al. ˚ 155 also

reported large differences in the average molecular weight and the molecular weight distributions of various oat products, which are very likely to be of nutritional importance; oats, different oat fractions, and experimental and commercial oat-based foods were examined in this study. Oats, rolled oats, oat bran, and oat bran concen-

trates all had high average molecular weights (2060 to 2300 × 10 3 g/mol) and essential monomodal distributions. The M w for the oat-containing experimental foods ranged from 450 to 1920 × 10 3 , with extruded flakes, macaroni, and muffins having high molecular weights and pasteurized apple juice, fresh pasta, and tea cakes containing degraded β-glucans. Also, the average molecular weight was found to

vary from 240 to 1670 × 10 3 in different types of oat bran-based breads baked with almost the same ingredients. The large particle size of the bran and short fermentation time limited the β-glucan degradation during baking and in yeast-leavened bread; the polymodal distribution of β-glucan in these breads implied the enzymatic nature of this degradation. Of the commercial oat foods studied, porridge made of rolled oats, a breakfast cereal product, and an extruded oat product showed high M w (1930

to 2010 × 10 3 ); the crisp bread (950 × 10 3 ) and the yogurt-like product (830 × 10 3 ) contained medium degraded β-glucans; and bread loaf, fried pancakes, pancake batter, and fermented oat soup had highly degraded β-glucans (190 to 630 × 10 3 ). In general, processing such as baking, including a fermentation step, fresh pasta preparation, and production of fermented soup and pancake batter all seem to result in extensive degradation of oat β-glucan. It is evident from this study that heat treatment is relatively lenient for the β-glucan, while prolonged treatment at lower temperatures may result in extensive enzymatic degradation. 155 Another study 190 also

Cereal β -Glucans: Structures, Physical Properties, and Physiological Functions 41

reported a slight decrease in molecular weight of oat β-glucans (from 200 × 10 3 to

80 to 100 × 10 3 ), during production of cereal β-glucan beverages containing oat β-glucan concentrates, whereas production of beverages with a low molecular weight (40 × 10 3 ) barley β-glucan concentrate did not cause any further decrease in the molecular weight of β-glucans. On the other hand, the viscosity of the barley concentrate was higher than the viscosity of the oat concentrate as a result of the higher gelling capacity of the barley β-glucan. 190