物理改性对甘薯皮膳食纤维含量、多糖组成及其结构的影响

为进一步探究物理改性对甘薯皮可溶性膳食纤维(SDF)含量、多糖主成分与形貌结构的影响,采用超声、亚临界水和微波改性处理甘薯皮膳食纤维,测定SDF和甘薯皮膳食纤维中性多糖和酸性多糖的含量,并使用透射电镜、傅里叶红外光谱和X-射线衍射对改性前后的甘薯皮膳食纤维进行结构分析。结果显示:亚临界水改性对甘薯皮SDF得率、总膳食纤维(TDF)得率和中性多糖占比的影响显著(P<0.05)。SDF得率和中性多糖占比与膳食纤维被破坏的程度有关。酸性多糖中,0.1、0.2、0.3、0.5 mol·L^-1 NaCl洗脱出的部分占比最高的是未改性样品,分别占8.48%、19.52%、28.44%和2.32%;0.4 mol·L^-1 NaCl洗脱出的部分占比最高的是亚临界水改性样品,为10.99%;0.6 mol·L^-1 NaCl洗脱出的部分占比最高的是超声改性样品,为38.74%。透射电镜观察发现,亚临界水改性对甘薯皮TDF形态结构影响最明显,其比表面积增加,呈现出更复杂的空间结构。红外光谱分析可知,改性后的膳食纤维仍具备多糖的官能团,物理改性前后均存在果胶的多聚半乳糖醛酸结构,且物理改性增加了膳食纤维中果胶的总含量。X-射线衍射结果可知,亚临界水改性和微波改性均将甘薯皮TDF的纤维素晶型由Ⅰ型转化为Ⅱ型,其中,亚临界水改性对晶型的破坏最为严重。计算结晶度后可知,亚临界水改性样品的结晶度(21.33%)分别低于未改性、超声改性和微波改性样品15.12、7.85和4.41百分点。

Effects of physical modification on content,polysaccharide composition and structure of dietary fiber in sweet potato peels

In order to further explore the influence of physical modification on soluble dietary fiber (SDF) content, polysaccharide component, and morphological structure of sweet potato peels, ultrasonic, subcritical water and microwave modifications were used to treat the dietary fiber of sweet potato peels. The contents of SDF and neutral and acidic polysaccharides in the dietary fiber of sweet potato peels were determined. The transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were used to characterize the shape and structure of the dietary fiber. The results showed that the effect of subcritical water modification on the yield of SDF, total dietary fiber (TDF) and neutral polysaccharide was significant (P<0.05). The SDF yield and the proportion of neutral polysaccharide were related to the degree of dietary fiber destruction. The highest proportion of acid polysaccharides eluted by 0.1, 0.2, 0.3 and 0.5 mol·L^-1 NaCl were found in unmodified samples, which were 8.48%, 19.52%, 28.44% and 2.32%, respectively. The highest proportion of acid polysaccharides eluted by 0.4 mol·L^-1 NaCl was found after subcritical water modification (10.99%), and the highest proportion of acid polysaccharides eluted by 0.6 mol·L^-1 NaCl was found after ultrasonic modification. TEM showed that the subcritical water modification had the most obvious structural damage to TDF of sweet potato peels, as its specific surface area increased, which showed a more complex spatial structure. FTIR analysis showed that the modified dietary fiber still had functional groups of polysaccharides. The polygalacturonic acid structure of pectin was found in unmodified and physically modified dietary fiber, and the total content of pectin in dietary fiber of sweet potato peels increased after physical modification. XRD results showed that both subcritical water and microwave modification converted the cellulose crystal form of TDF in sweet potato peels from type I to type II. Among them, the subcritical water modification exhibited the most serious damage to crystal shape. It was calculated that the crystallinity after subcritical water modification was 21.33%, which was 15.12, 7.85 and 4.41 percent lower than that of unmodified sample, and samples after ultrasonic and microwave modification, respectively.