Non-starch polysaccharides (NSPs) are a general term for all carbohydrates in plant tissues except starch, which are regarded as the largest source of soluble and insoluble dietary fiber [1]. As natural ingredients, they have received widespread attention for physiological functions, such as anti-obesity, anti-fatigue, anti-diabetic and anti-hypertensive effects [2]. Nowadays, a variety of NSPs have been studied to prepare high-fiber products with low calorie and high nutrition, which are very popular among consumers. Several studies indicated that polysaccharide structure has a significant effect on flour processing properties by interacting with gluten and starch in dough system [3]. Molecular weight, a modifiable structural feature, is often considered as one of the main factors affecting the processing adaptability and biological activity of polysaccharides [4]. Polysaccharides with different molecular weight could alter the dough properties and product quality during processing and storing stages. Commonly, polysaccharides with higher molecular weight possess the more complex structures with the higher viscosity, which are easier to cross-link and entangle with others to form complex spatial structures. And the variation in molecular weights of polysaccharides impacted the pasting process of starch by modifying the spatial arrangement of starch chains [5]. Polysaccharides with high molecular weight from Pachyrhizus erosus appeared to protect the integrity of starch granules and reduce their contact with water molecules so as to inhibit the starch pasting process [6]. Such structural changes should inevitably cause the remarkable quality differences of pasta products. The presence of high molecular dextran has been proven to strengthen the dough structure and lower the hardness of bread [7]. Garlic polysaccharide fraction with high molecular weight was also reported to enhance the structure stability and gas retention capacity of dough by intertwining with gluten protein [8].

Pullulan is an extracellular polysaccharide secreted by Aureobasidium pullulans. As a linear polysaccharide with no branching structure, pullulan is constituted by backbones of consecutive α-1,6 glycosidic bonds with maltose trisaccharide as repeating unit [9]. It has some reliably and excellently physicochemical properties, such as colorless, odorless, easily soluble in water and stable in nature without any toxicity or carcinogenicity [10]. In practice, pullulan has exhibited a serial of promising applications in food processing field, extensively serving as low-calorie food ingredient for pastry and staple food as well as coating and thickening agent for improving food quality [9,11]. It could not only inhibit the recrystallizing and aging process of rice starch, but also improve the water retention capacity of the gel system in the refrigerated rice [12]. Due to its high water binding performance, the presence of pullulan might limit the water migration and impede the polymer depolymerization in frozen cooked noodles (FCNs) during storage [10]. However, few studies focused on its interaction changes with major components in dough system induced by regulating pullulan molecular weight, which would make differences in dough structure and application properties.

In this study, we compared the effects of pullulan varying with molecular weights on structural and technological properties of dough, product quality and in vitro digestion properties of biscuits. It is hypothesized that pullulan molecular weight will make a difference in its interaction with multiple components in dough and then can bring changes to the dough structure. Scanning electron microscopy (SEM), confocal laser scanning microscope (CLSM), Fourier transform infrared spectroscopy (FTIR) and low-field nuclear magnetic resonance (NMR) were used to observe structural changes in dough. The processing adaptability of dough system involving rheological, textural and pasting properties was investigated and the product quality involving physical and digestive properties of biscuits were systematically evaluated. The study dedicated to achieve a better understanding of pullulan function in dough system and guide the fractionation applications of pullulan in food field.