Abstract
This study investigated the structural and physicochemical characteristics of malic acid-treated sweet potato starch. Sweet potato starch mixed with various concentrations of malic acid solution underwent either thermal or nonthermal treatment. Observation of samples under a light microscope ensured the maintenance of granular shape and the Maltese cross. FT-IR spectra displayed a distinct carbonyl peak at 1722 cm−1, and analysis of the degree of substitution (DS) indicated an increase in the extent of ester bonds with increasing concentrations of malic acid. The DS of 2.0M-130 (0.214) was the highest and that of 0.5M-130 was the lowest (0.088) among the reacted starches. In vitro digestion test revealed an increased amount of resistant starch when a high concentration of malic acid was used. In addition, thermally treated samples maintained a higher content of resistant starch (RS) after 30 min of cooking at 100°C. After cooking, 2.0M-130 had an RS fraction of 53.4% which was reduced to 49.9% after cooking, revealing greater heat stability compared with nonthermally treated samples. The structure of malic acid-treated starch was investigated using a differential scanning calorimeter (DSC), an X-ray diffractometer, a rapid visco analyzer (RVA), and analysis of apparent amylose content. The results showed that thermal and malic acid treatment of starch caused not only partial hydrolysis but also rearrangement of the crystalline area and helix structure of starch by esterification. Analysis of malic acid-treated starch, using a rapid visco analyzer showed no pasting properties, due to lack of its swelling caused by the malic acid cross link.
Highlights
Starch is used in many kinds of food and serves as a major source of energy for humans
Photomicrographs. e granular shape of the various malic acid-treated starch samples was investigated using a light microscope (Figure 1). e shape of starches including the malic acid-treated samples had round, semioval, oval spherical, and round polygonal shapes, which was consistent with preivious studies [19, 22]. e malic acid-treated starch granules were not ruptured even at malic acid concentrations as high as 2.0 M
According to Hirashima et al [23], granules of starch were all broken at pH below 3.0, when more glucose chains were observed, compared to those in higher pH-treated samples
Summary
Starch is used in many kinds of food and serves as a major source of energy for humans. Starch can be classified into three categories based on the rate of its enzymatic digestion: rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) [1]. RS is defined as the sum of starch and degraded starch products that resist digestion in the small intestine of healthy people [2]. Depending on the cause of resistance, RS can be further divided into five categories [5, 6]: RS1, physically inaccessible starch due to entrapment; RS2, raw starch granules with crystallinity; RS3, retrograded starch; RS4, chemically modified starch; and RS5, amylose-lipid complexes. Chemical modification has been known to raise in vitro digestion resistance of starches, reducing postprandial glucose and insulin concentration, and maintain sensory attributes of the final foods [7].
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