Structural changes of starch-lipid complexes during post-processing

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Food and Beverage Chemistry/Biochemistry

Structural changes of starch-lipid complexes during postprocessing and their effect on in vitro enzymatic digestibility Renbing Qin, Jinglin Yu, Yufang Li, Les Copeland, Shuo Wang, and Shujun Wang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b06371 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Journal of Agricultural and Food Chemistry

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Structural changes of starch-lipid complexes during post-processing

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and their effect on in vitro enzymatic digestibility

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Renbing Qinab, Jinglin Yua, Yufang Liab, Les Copelandc, Shuo Wangad*, Shujun Wangab*

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a State

Key Laboratory of Food Nutrition and Safety, Tianjin University of Science &

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Technology, Tianjin 300457, China

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b School

of Food Engineering and Biotechnology, Tianjin University of Science &

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Technology, 300457, China

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c

The University of Sydney, Sydney Institute of Agriculture, School of Life and

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Environmental Sciences, NSW Australia 2006

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d Tianjin

Key Laboratory of Food Science and Human Health, School of Medicine, Nankai

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University, Tianjin, 300071, China

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* Corresponding authors: Dr. S Wang

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Mailing address: No 29, 13th Avenue, Tianjin Economic and Developmental Area (TEDA),

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Tianjin 300457, China

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Phone: 86-22-60912486

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E-mail address: [email protected]

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ACS Paragon Plus Environment


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Abstract: The effects of cooking and storage on the structure and in vitro enzymatic

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digestibility of complexes formed between fatty acids and debranched Hi-amylose

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starch (DHA7-FA) were investigated for the first time. Cooking decreased greatly the

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crystallinity of DHA7-lauric acid (LA) and DHA7-myristic acid (MA) complexes, but

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had little effect on the crystallinity of DHA7-palmitic acid (PA) and DHA7-stearic

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acid (SA) complexes. Cooking increased enthalpy change (ΔH) values and

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short-range molecular order of DHA7-FA complexes. Cooking decreased in vitro

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enzymatic digestibility of DHA7-FA complexes, with the extent of the effect

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decreasing with increasing fatty acid chain length. Holding the samples 4 oC for 24 h

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after cooking did not affect greatly the long- and short-range molecular order nor in

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vitro enzymatic digestibility of DHA7-FA complexes. From this study, we conclude

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that cooking disrupted the long-range crystalline structure of DHA7-LA and

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DHA7-MA complexes, but enhanced the short-range molecular order of all of the

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DHA7-FA complexes. The latter effect accounted mainly for the reduced in vitro

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enzymatic digestibility of DHA7-FA complexes.

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Keywords: Hi-amylose starch; amylose-fatty acid complexes; cooking; structure; in

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vitro digestibility


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INTRODUCTION

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Starch and lipids are major components of foods that play important roles in their

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texture, flavor and nutritional quality. Amylose-lipid complexes can be formed during

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food processing or on subsequent cooling and storage.1 The formation of these

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complexes has attracted much attention over many years because of their impact on

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starch rheological properties,2-3 retrogradation,4 digestibility in vivo5 and enzymatic

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digestibility in vitro.6-7 Complexes between amylose and lipids are also now classified

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as a type of resistant starch (RS), namely RS5.8-10 RS escapes digestion in the small

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intestine and enters the large intestine, where it is fermented by the gut microbiota to

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produce short-chain fatty acids (SCFAs),11-16 which play an important role in

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intestinal health and the general well-being of the host.17-25

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Due to its many supposed human health benefits, increasing the RS content in foods is

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the subject of much research26-28. Previous studies have focused mainly on the

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preparation of RS by different methods, or its formation during processing of

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ingredients into foods.29-34 Little information is available on the effect of further

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handling (i.e., “post-processing”) of foods on the properties of RS, and in turn the

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textural and nutritional value of these food products. Many cooked starchy foods,

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such as Western oven-baked breads, Chinese steamed breads, some frozen foods or

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rice dishes, are reheated before being consumed by humans. Hence, it is of interest to

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increase our understanding of the changes that RS undergoes during this

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“post-processing” stage. Amylose-lipid complexes, which are now considered an



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important resistant starch form (RS5) that can occur in processed foods, and that can

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be formed experimentally under controlled conditions, provides a good model system

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to investigate structural and functional changes of RS after reheating. Thus, the

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objective of this study was to investigate the effects of cooking and storage on

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structure-function relationships of RS5 relevant to its in vitro enzymatic digestibility.

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The results will be of significance for better understanding the changes of RS5 during

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post-processing and for better control of starch digestion during food processing.

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MATERIALS AND METHODS

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Materials

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High-amylose maize starch VII (HA7, amylose content 68.7%) was a gift of

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Ingredion Inc. (Westchester, IL, USA). Lauric acid (C12:0, LA), myristic acid (C14:0,

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MA), palmitic acid (C16:0, PA), stearic acid (C18:0, SA), pullulanase (EC 3.2.1.41,

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activity of pullulanase was 1498 NPUN/mL (New Pullulanase Unit Novo).) and

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α-amylase (EC 3.2.1.1, type VI-B from porcine pancreas, 13 U/mg) were purchased

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from Sigma Chemical Co. (St. Louis, MO, USA). The glucose oxidase/peroxidase kit

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(GOPOD format) and Aspergillus niger amyloglucosidase (3260 units/mL) were

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purchased from Megazyme International Ireland, Ltd. (Bray Co., Wicklow, Ireland).

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All other chemicals were of analytical grade and were from Sigma-Aldrich Chemical

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Corporation (Shanghai, China).

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Preparation of RS5


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RS5 was prepared according to the method of Hasjim et al.8 with minor modifications

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as follows. The HA7 slurry (10% w/v) in 0.2 M sodium acetate buffer (pH 5.0) was

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heated at 130 °C for 60 min to completely gelatinize the starch. The starch paste was

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cooled to 60 °C and debranched by pullulanase (80 NPUN /g starch) for 12 h with

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agitation. The debranched starch suspension was re-heated at 130 °C for 30 min, after

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which the respective fatty acid (FA; 10% w/w, dry starch basis) was added to the

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suspension and maintained at 90 °C for an additional 1 h. The mixtures were cooled

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to 25 °C with stirring for 2 h. The resulting starch-lipid complex was recovered by

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centrifugation (2000 g, 20 min) and washed with 50% ethanol. This step was repeated

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three times to remove uncomplexed FAs and to obtain salt-free complexes. The

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amylose-FA complexes, referred to generically as DHA7-FA, were freeze-dried,

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ground using a mortar and pestle, and stored at 4 °C until further analysis.

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Post-processing of RS5

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Post-processing of RS5 was performed according to a method described previously35

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with modifications. RS5 (1 g) was weighed accurately into a polypropylene bag and

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mixed thoroughly with 3 ml of distilled water. The bags were sealed and allowed to

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stand at room temperature for 2 h before heating in a boiling water bath for 10 min.

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Some of the heated samples (designated DHA7-FA-100) were frozen immediately in

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liquid nitrogen for about 10 min, whereas others were kept at 4 °C for 24 h

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(DHA7-FA-100-4) before freezing. Both sets of samples were freeze-dried, ground

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into powders, and passed through a 100 μm sieve.



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X-ray Diffraction Analysis (XRD)

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Relative crystallinity of the RS5 samples was determined by an X-ray diffractometer

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(D8 Advance, Bruker, Germany) operating at 40 kV and 40 mA with Cu-Kα radiation

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(λ= 0.15406 nm). The samples were equilibrated over a saturated NaCl solution at

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room temperature for 7 days before analysis. The XRD spectra were obtained from 4

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to 35° (2θ) at a scanning rate of 2°/min and a step size of 0.02°. The relative

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crystallinity was calculated as the ratio of the crystalline area over the total area under

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the X-ray diffractograms using the software of TOPAS 5.0 (Bruker, Germany).

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V-type crystallinity and B-type crystallinity were calculated respectively as the ratio

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of area of V-crystal peaks (7.5, 12.9 and 19.8°) and B-crystal peaks (5.6, 16.9, and

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22.6°) to the total area under XRD curves.

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Laser Confocal Micro-Raman Spectroscopy

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Raman spectra were recorded using a Renishaw Invia Raman microscope system

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(Renishaw, Gloucestershire, U.K.) equipped with a Leica microscope (Leica

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Biosystems, Wetzlar, Germany), and a 785 nm green diode laser source was used.

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Each spectrum of starch samples (4000-400 cm-1) was collected from at least five

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different spots with a resolution of approximately 7 cm-1. The full width at half

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maximum (FWHM) of the band at 480 cm-1 was calculated using the software of

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WIRE 2.0, which is taken as an indicator of the short-range ordered structure in

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starch.36


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Differential Scanning Calorimetry (DSC)

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Thermal properties of samples were measured using a Differential Scanning

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Calorimeter (200F3, Netzsch, Germany) equipped with a thermal analysis data

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station. RS5 samples (3 mg) were weighed accurately into 40 μL aluminum pans, and

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deionized water was added to give a water/starch ratio of 3:1 (w/w). The RS5-water

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mixtures were allowed to stand overnight at room temperature before DSC

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measurement. The samples were heated from 20 to 135 °C at a heating rate of

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10 °C/min. An empty aluminum pan was used as the reference. The values of the

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thermal transition temperatures (onset, To; peak, Tp; conclusion, Tc) and enthalpy

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change (H) of RS5 were determined from the data recording software.37

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In Vitro Enzymatic Digestibility of RS5

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The in vitro digestibility of RS5 samples was analyzed according to the procedure of

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Wang et al.38 Starch (100 mg, dry weight basis) was suspended in 4.0 mL of 0.1 M

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sodium acetate buffer (pH 5.2) and 1.0 mL of freshly prepared enzyme solution

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containing 1645 units of amylase and 41 units of amyloglucosidase was added. The

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starch/enzyme mixtures were incubated at 37 °C with stirring at 260 rpm for 3 h. At

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specific time points (from 0 to 180 min), an aliquot (0.05 mL) of the hydrolysate was

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withdrawn and mixed with 0.95 mL of 95% ethanol to deactivate the enzymes. The

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amount of glucose released was determined using the Megazyme GOPOD kit. The

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percentage of hydrolyzed starch was calculated by multiplying the glucose content



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with a factor of 0.9. Starch classifications based on the rate of hydrolysis were:

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rapidly digested starch (RDS, digested within 20 min), slowly digested starch (SDS,

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digested between 20 and 120 min) and resistant starch (RS, undigested starch after

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120 min). The hydrolysis index (HI) was derived from the ratio between the area

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under the hydrolysis curve (0~180 min) of each RS5 samples and the corresponding

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area of white bread expressed as a percentage over the same period. From the HI

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obtained in vitro, the estimated glycemic index (eGI) value was then calculated using

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the empiric formula proposed by Granfeldt et al.39

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eGI = 8.198 + 0.862 × HI.

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Statistical Analysis

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All analyses were performed at least in triplicate, and the results are reported as the

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mean values and standard deviations. In the case of XRD, only one measurement was

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performed. One way analysis of variance (ANOVA) followed by post-hoc Duncan’s

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multiple range tests (p 0.05). ND, not detected To: transition onset temperature, Tp: transition peak temperature, ∆H: transition enthalpy *indicates type I complex, +indicates type II complex.


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Table 3. In vitro digestibility and estimated glycemic index of RS5 samples before and after cooking or followed by storage Samples

RDS (%)

SDS (%)

RS (%)

HI

DHA7-LA

25.9±1.0f

27.6±1.3f

46.4±0.5b

51.4±0.5i

52.5±0.4i

DHA7-LA-100

22.9±0.3a

18.8±0.8b

58.4±1.1fg

37.6±0.9a

40.6±0.8a

DHA7-LA-100-4

23.3±0.6ab

17.4±0.1ab

59.4±0.6g

38.0±0.3ab

40.9±0.3ab

DHA7-MA

25.2±0.6def

31.0±1.6g

43.9±1.0a

53.6±0.6j

54.4±0.5j

DHA7-MA-100

24.7±0.8cde

23.0±0.4d

52.3±0.5d

45.0±0.5f

47.0±0.4f

DHA7-MA-100-4

24.3±0.6bcd

21.0±0.6c

54.7±0.3e

43.7±0.2e

45.8±0.2e

DHA7-PA

25.8±0.3ef

25.3±0.5e

48.9±0.8c

50.3±0.6h

51.6±0.5h

DHA7-PA-100

24.2±1.1bcd

23.2±1.2d

52.6±0.3d

44.9±0.3f

46.9±0.2f

DHA7-PA-100-4

24.8±0.6cdef

23.3±1.2d

51.9±0.6d

46.1±0.9g

47.9±0.8g

DHA7-SA

24.6±0.2cde

17.9±0.3ab

57.5±0.1f

42.2±0.3d

44.6±0.3d

DHA7-SA-100

23.6±0.9abc

18.0±1.2ab

58.4±1.0fg

38.7±0.4bc

41.6±0.4bc

DHA7-SA-100-4

23.9±0.4abcd

16.4±1.4a

59.7±1.1g

39.4±0.5c

42.2±0.5c

eGI

Values are means ± SD. The letters a, b, c, d, e, f, g, h, i, j represent a significant difference between the data in the same column (p< 0.05);

570 571 572 573 574 575 576 577 578 579 580 581 33



582 583 584 585 586 587 588 589 590 591 592 593 594 595 596

Table of Contents Graphic

597

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Structural changes of starch-lipid complexes during post-processing

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