The formation and in vitro enzymatic digestibility of starch-lipid complexes in steamed bread free from and supplemented with different fatty acids: Effect on textural and retrogradation properties during storage

Highlights

• Starch-lipid inclusion complex was formed in steamed bread (SBr).

• The starch-lipid complex inhibited the in vitro enzymatic digestibility of SBr.

• Starch-lipid complex inhibited the staling of SBr.

• The differences in crystalline density of SBr samples are proved by microCT.

Abstract

Herein, the formation of starch-lipid complexes in steamed bread (SBr) free from and supplemented with fatty acids of varying chain lengths, including lauric acid (LA), glycerol monolaurate (GML), stearic acid (SA), and glycerol monostearate (GMS) and their effects on in vitro enzymatic digestibility were investigated. The enthalpy value of SBr samples (1.86–3.46 J/g) was significantly decreased (P < 0.05) compared to wheat starch samples (5.64–7.17 J/g) fortified with fatty acids. The relative crystallinity (16.5%–32.8%) of SBr corresponds to the content of starch-lipid complexes. SBr supplemented with fatty acids exhibited softer texture than lipid-free SBr stored at 4 °C for 0, 1, 4, and 7 days. Higher enzyme resistance was observed in SBr samples supplemented with fatty acids and the content of resistant starch (RS) was increased from 7.54% to 23.13% in SBr supplemented with LA. As demonstrated by microscopic computed tomography (mCT), the crystalline structure of SBr samples supplemented with LA and GML have a higher density than SBr fortified with SA and GMS; the findings which are in line with thermal properties and X-ray diffraction analysis. In sum, the formation of starch-lipid complexes could be considered as a new way to improve the SBr textural features during storage.

Introduction

Over the past few years, novel processing technologies have been successfully applied to create healthy food that can reduce the risk of chronic diseases [1]. Gut microbiota ferment resistant starch (RS), which is widely used as functional ingredient, to short chain fatty acids [2]. In recent years, there has been growing interest to evaluate the effects of adding raw materials on the in vitro digestion of starch [[3], [4], [5], [6]]. Sorghum flour [3], dietary fiber [4], purple yam [5], and sourdough [6] were used in breads and cakes in favor of increasing their RS content. Targeting weight as well as Diabetes management has spurred a remarkable growth in the methods of increasing RS content in starch and starchy foods [6]. Many research groups have focused on preparation of RS using physical, chemical, enzymatic, and genetic modifications, along with other methods [7,8]. So far, five classes of RS (RS1–5) have been identified in starch [9].

In the presence of lipid, amylose tends to wind up into a rather single left-handed helix with inner cavity, forming a hydrophobic amylose-lipid complexes; referred as a V-type amylose-inclusion complexes [10]. Notably, previous studies had focused on preparation of RS5 from pure starch systems using different methods [[11], [12], [13], [14]]. On the other hand, production of RS through hydrothermal or high-pressure treatment [11], a high value of amylose-amylopectin ratio [12], lipids with relevant carbon chain lengths and structures [13,14], and ultrasound treatment, have been recorded. In this context, Eliasson reported the mechanism underlying the formation of amylose-lipid complexes and the cause of increased enzyme resistance [15]. However; details regarding the impact of food processing on RS5 formation, texture, and nutritional values are scarce.

Steamed bread (SBr), one of the traditional fermented staple foods consumed across China, is made from a simple formula of yeast, water, and wheat flour [16]. Over the past years, many reports have addressed the relationship between processing methods and the shelf-life of SBr [17,18]. Owing to its high-glycemic index, the intake of SBr to diabetic and patients with cardiovascular disease is limited [19]. Hence, it is of interest to reveal if lower blood glucose level of SBr was achieved in association with RS5 formation when making SBr. Additionally, it is worth mentioning that amylose-lipid inclusion complexes could inhibit the formation of double-helix structures (between amylose and the recrystallization of amylopectin), which play a key role in preventing starch retrogradation in SBr [20]. Hence, this study was conducted to investigate the formation of wheat starch-lipid complexes during steaming and analyze their effects on blood glucose response through in vitro enzymatic digestibility. Additionally, the formation of starch-lipid complexes in SBr and its changes during storage were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and microscopic computed tomography (mCT). The texture analysis and in vitro digestion were used to illustrate the effect of starch-lipid complexes on SBr properties. Notably, the effects of other ingredients in wheat flour (such as proteins) on starch-lipid complexes were not considered in this study.