Effect of hydrothermal treatments on the physicochemical properties and resistant starch content of buckwheat starch

Buckwheat, the annual plant, has arised and been widely consumed in Central Asia region for ages. It belongs to Polygonaceae family and Fagopyrium genus. Buckwheat named as pseudocereal due to its similarities and differences from cereals like wheat, barley and rice. Buckwheat is known as an important source of carbohydrate, protein, lipid, vitamin, dietary fiber and minerals. Because of its high nutritious value, it seems to be a good source of food components. The buckwheat used in this study was obtained from local market and it was milled and sieved. Native buckwheat starch has been either modified with heat-moisture treatment or annealed after starch has been isolated. Native buckwheat starch was used as control for determining the effects of hydrothermal treatments on starch properties. Effects of hydrothermal treatment on digestibility of starch were also investigated. In addition, solubility, water absorption and fat binding capacities, swelling power, emulsifying capacity, syneresis and turbidity of starch samples were evaluated. The impact of hydrothermal treatments on pasting, morphological, thermal properties and X-ray diffraction patterns were also investigated. Firstly, basic compositions of samples were determined. According to this evaluations, total moisture contents of buckwheat and native starch were found as 12,09% and 12,48%; ash contents as 1,83% and 0,24%; protein contents as 10,37% and 1,78%; lipid contents as 2,41% and 0,36%; total starch contents as 63,78% and 81,81%, respectively. Resistant, digestible and total starch contents of native and treated starches were found significantly different from each other (p<0,05). HMT and ANN increased RS (resistant starch) and slowly digestible starch (SDS) content, but decreased rapidly digestible starch (RDS) content of starches. RS content of starch doubled after HMT and ANN treatments. RS and SDS contents of all starches were significantly different (p<0,05). Besides RDS content of native buckwheat starch was significantly different than treated starches (p<0,05), while RDS content of heat moisture treated starch was not significantly different than annealed starch (p>0,05). Hydrothermally treated starches have lower calorific value than native starch due to has higher resistant and slowly digestible starch content than native buckwheat starch, so that these samples may be used for diabetic food formulations. RS content of buckwheat was 1,33% and this value was similar to that of other cereals. HMT and ANN treatments significantly reduced solubility, water binding capacity and swelling power of starches (p<0,05). Water binding capacity, solubility and swelling power of native starch was higher than treated starches. Solubility index and swelling power of HMT starch was similar to that of ANN starch (p>0,05) while water absorption capacity of HMT starch was lower than that of ANN starch (p<0,05). Crystallites were formed with interactions between amylose-amylose and amyloseamylopectin molecules. These crystallites, which has hard structure limited swelling and solubility of granules. Hydrothermally treated starches have low water absorption capacity due to crystallites formed. Crystallites have low water absorption ability compare with amorphous region. Besides, HMT reduced fat binding capacity while ANN reduced emulsifying capacity which were not found to be statistically significant (p>0,05). The highest fat binding and emulsifying capacities were determined for native buckwheat starch when compared to treated starches. Fat binding capacity of heat moisture treated starch was not different than native starch (p>0,05), while the emulsifying capacity of starches were significantly different from each other (p<0,05). ANN reduced the fat binding and emulsifying capacity of starch. Fat binding capacity of ANN treated starch was different from native starch (p<0,05), while the emulsifying capacity of starhes were not significantly different (p>0,05). Fat binding capacity of heat treated starch was higher than ANN treated starch while emulsifying capacity was lower. Fat binding property is a measurement of emulsifying capacity of starch. Starch was granular form so not acted as an emulsifying agent. Starch may affect the emulsifying property of proteins. HMT and ANN increased gelatinization temperature while gelatinization enthalpy of the starhes were reduced. The highest gelatinization temperature were determined for HMT treated starch. Gelatinization temperature range narrowed with HMT treatment while broadened with ANN treatment. Starch granules reorganizated during hydrothermal treatment and interactions between amylose-amylose or amyloseamylopectin increased. Crystallites formed, which has hard structure limited swelling of granule so that gelatinization of starch delayed and gelatinization temperature increased. At the same time hydrothermal treatments reduced pasting viscosity significantly (p<0,05). The highest viscosity were determined for native starch while the lowest pasting viscosity determined after HMT. Pasting property of buckwheat was found low due to its low viscosity values. Buckwheat and native starch possessed A-type diffraction pattern like other cereals. X-ray diffraction pattern remained unchanged after hydrothermal treatments. In addition, HMT increased peak intensity when compared with native starch at the same angle. Whereas, ANN both increased and decreased peak viscosity depending on the angle. Buckwheat flour presented main peaks at about 12,90 and 19,80 (2ϴ) while native and hydrothermally treated starches presented main peaks at about 12,90, 16,80 and 19,80 (2ϴ). In addition, all of starches presented weak peaks at about 15,00 and 22,20 (2ϴ). Peak intensity of buckwheat flour was lower than native starch. Degree of starch crystallinity of native starch, HMT starch and ANN starch were 65,4%; 71,1%; 67,3%, respectively. The buckwheat granules were polygonal-irregular shaped and the average size of the granules were 4,3-5,42µm. Similarly, native buckwheat starch granules were irregular polygonal shaped or partial spherical with average size of granules such as 6,08-8,5 µm. Granule structure remained unchanged after HMT and ANN treatments. ANN and HMT caused collapses on the surface of the granules therefore granules showed as lamella bulk. Granule structures were clear after HMT but some deformations occurred on the surface of granules. Turbidity of starch samples were formed due to interactions between molecules at the early stage of retrogradation. Turbidity of native and treated starches increased as the storage time increased. Turbidity of all starches increased quickly within 48 hours, after 72 hours turbidity values seemed to be stabilized. Turbidity values of all starches were close to each other, therefore hydrothermal treatments have no important effects on turbidity values of starches. Syneresis, identified as squeezed water content from starch gel during cold storage. Syneresis is a measurement of retrogradation and an undesired property for food industry.When the amylose content increased syneresis ratio also increased. In particular, HMT treated starch showed highest syneresis due to its highest amylose content. Water absorption capacity is one of the main factor affected syneresis ratio. Syneresis ratio increased as water absorption capacity decreased. Heat moisture treated starch showed highest syneresis ratio due to has lowest water absorption capacity. In addition native buckwheat starch showed lowest syneresis ratio due to has highest water absorption capacity. In this thesis; buckwheat starch obtained was found to have higher viscosities with respect to rice and wheat starches that are commonly used. So that it can be used as thickening agents in many food formulations and especially for the products suitable for celiac patients. In addition, hydrothermally treated starches have high RS content as compare with native starch so that these samples may be used in low calorific and diabetic food formulations. Hydrothermal applications applied also resulted in higher RS contents in starches so that they seem easy and promising applications for increasing the RS contents of starch sources.