Rice is a major cereal crop grown in the world, which contain the starch content is as high as about 75 % [1]. Starch is the most common energy source in human nutrition and comprises amylose and amylopectin. Based on the digestibility rate, starch can be classified as rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) [2,3]. RDS is a type of starch that is enzymatically digested into glucose molecules within 20 min. SDS is a type of starch that is enzymatically digested into glucose molecules within 120 min [4]. Compared to RDS and SDS, RS is fermented in the colon only after 120 min of consumption. Wang et al. [5] reported that RS has good physicochemical and sensory properties, can promote insulin secretion, and can suppress the glucose response. Hence, RS is an important functional food ingredient.

A high RS content can be obtained by modifying the raw materials through physical, chemical, and enzymatic methods [6]. Physical methods, such as ultrasound, ultra-high pressure, and extrusion processing [[7], [8], [9], [10]], are widely used to prepare RS because of their environmental friendliness. Among them, extrusion has been used as a potential method to improve the RS level in cereals [11]. Because of its multifunctionality and high productivity, extrusion can efficiently modify gelatinized and degraded starch, causing amylopectin breakage at the branch points [12,13]. In recent years, several reports have focused on optimizing the extrusion conditions, conducting characterizations, or studying the correlations among the process, system, and product parameters. Yu et al. [14] reported the effect of different screw speeds on the structure and physicochemical properties of reconstituted rice composed of rice starch and glutelin. Wang et al. [15] reported the effect of different moisture contents on the structure and properties of corn starch during extrusion. These studies concluded that optimization of the extrusion conditions and significant changes in the structures of the extrudate were necessary. In addition to exploring extrusion conditions, the adjustment of extrusion product parameters can also change the composition of starch. Zhu et al. [16] reported that the retrogradation properties and digestibility of chestnut starch were slowed down with xanthan gum/hydroxypropyl methylcellulose addition under extrusion, resulting in increasing RS content. And Kim et al. [17] reported that the different extrusion conditions to treat wheat flour slurry, the RS content increased compared with the unextruded pastry wheat flour. Additionally, Zhang et al. [18] found that a dead-stop operation of the extruder could be intentionally halted after reaching a steady state. This operation was used to collect the sample from the extruder barrel and the die. However, the extrusion process is still considered a “black box” because the operation is complicated. Extrusion requires an input and an output, and proceeds under a combination of temperature, shear, and pressure. The comprehensive effect of these parameters leads to a dynamic state that causes complex physicochemical changes in the constituents of the material. Thus far, the functional zones in the extrusion process for modified starch remain unclear. The morphology and structural changes in starch granules are also indefinite, necessitating the current study to reveal the material changes in the extrusion zones.

RS is an important functional food ingredient, which is considered as prebiotics to improve the food quality. RS can also be used as a functional ingredient in foods with a low glycemic index (GI ≤ 55), and hence, has practical significance in reducing the risk of type II diabetes. Wolever [19] reported that the consumption of yogurt was associated with a decreased risk of diabetes. However, there could be a rapid increase in the blood glucose of the stomach or intestines during yogurt digestion [20]. Several researchers have been attempted to use polysaccharides [21], soluble dietary fibers [22], or other anti-digestive ingredients to delay the rapid increase in the blood glucose level [2]. Wan et al. [23] proposed the use of inulin as a sugar substitute in yogurt. Inulin could not only improve the sweetness but also promoted intestinal health as a soluble dietary fiber. Tian et al. [24] found that insoluble dietary fibers from modified okara added to yogurt had an enhanced hypoglycemic effect. RS can be used as an anti-digestive ingredient and has received widespread attention for its applications in food. However, the information on the use of RS in skimmed yogurt after extrusion is limited, and the procedure to extrude the product from the raw material remains to be clarified.

Therefore, the aim of this study was to investigate the formation of RS by twin-screw extrusion in different zones, and then explore the effects of RS application to skimmed yogurt to improve its quality and reduce the GI. This study may provide insights into the preparation of RS and its application as an anti-digestive ingredient to prepare low-GI yogurt.