1. Introduction

Ipomoea batatas L.) originated from Latin America, and is now cultivated worldwide, especially in Africa and Asia. China accounts for most of worldwide sweet potato production. The skin and flesh colors of sweet potato vary from white to yellow-orange and deep purple. Purple sweet potatoes contain a significant content of anthocyanins [ Sweet potato (L.) originated from Latin America, and is now cultivated worldwide, especially in Africa and Asia. China accounts for most of worldwide sweet potato production. The skin and flesh colors of sweet potato vary from white to yellow-orange and deep purple. Purple sweet potatoes contain a significant content of anthocyanins [ 1 ], which are attractive natural colorants exhibiting strong free radical scavenging activity [ 2 ].

Purple sweet potato is a promising source of anthocyanins for the food industry, however, a large amount of starch is wasted as by-products. Sweet potato starch is suitable for the production of resistant starch [ 3 4 ], which is a value added starch product with increasing market demand.

The resistant starch is the starch fraction which escapes digestion in the small intestine of healthy individuals, but is completely or partially fermented in the colon [ 5 ]. Resistant starch has been proved to exhibit potential health benefits, including reducing the glycemic response [ 6 ], lowering blood cholesterol [ 7 ], acting as a functional probiotic [ 8 ], and increasing the production of short chain fatty acids in the large intestine [ 9 ].

Recently, various methods for the production of resistant starches from different resources have been reported, including heat-moisture treatment (HMT) [ 10 ], enzyme debranching [ 11 ], chemical modification [ 12 ], and cross-linking [ 13 ]. Among these methods, the heat-moisture treatment and enzyme debranching are safe and cost-effective methods for the production of resistant starches.

14, Although the physicochemical characteristics of native and resistant starches from sweet potato have been extensively studied [ 4 15 ], a small number of works has been reported on the structure and properties of resistant starch from purple sweet potato and its probiotic effect.

In the present study, resistant purple sweet potato starches were produced by HMT and enzyme debranching combined heat-moisture treatment (EHMT). The physicochemical and crystallinity properties of native, HMT and EHMT starches from purple sweet potato were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier Transform infrared (FT-IR) spectroscopy. In addition, the effects of these starches on the proliferation of bifidobacteria were assessed and compared with those of glucose (GLU) and high amylose maize starch (HAMS). The physicochemical properties of resistant starches from purple sweet potato prepared by HMT and EHMT were assessed in order to evidence the potential use as probiotics.