Determination of starch gelatinization temperatures by an automated headspace gas chromatography

Highlights

• Determination of starch gelatinization temperatures by a headspace GC technique.

• Measuring the tracer release from liquid during an automated temperature ramping.

• The present method is simple, accurate and suitable for the batch sample testing.

Abstract

This paper reports on an automated temperature-programmed headspace GC technique for the determination of gelatinization temperature of starch. 1-butanol was used as a tracer and added to the starch-solution for the test. Based on measuring the GC signal of 1-butanol in the headspace with the temperature increasing, two transition points (corresponding to the onset temperature and the ending temperature of starch gelatinization) were observed by plotting the GC signal of 1-butanol vs. the temperature. The results showed that the method has a good measurement precision (the standard deviation < 1 °C) and accuracy (the average relative differences = 3.9%, compared to a standard reference method). The present method is simple and suitable for the gelatinization temperature testing for starch samples.

Introduction

Starch is a carbohydrate that consists of amylose (α-1,4- D-glucopyranosyl units) and amylopectin (the short chains linked together at their reducing end side by α-(1/6) linkage) [1]. Starch has been widely used to many applications, e.g., as a staple food, food thicken agent for making soups, bakery and meat products [2,3]. It is also used as the additive to improve the thermal and mechanical properties of paper and related packaging materials [4,5]. Among the properties of starch (swelling, gelatinization and retrogradation), gelatinization is an important parameter that affects its performance in the application. For example, once starch slurry is heated and reached to its gelatinization temperature, the chains of the molecules will extend and separate. As a result, the viscosity of emulsion will increase and a three-dimensional gel structure will appear, from which the chains of starch can be easily hydrolyzed by digestive enzymes [6]. Therefore, a method that can easily and accurately determine the starch gelatinization temperatures, i.e., the onset temperature (T_o) and ending temperature (T_e), is important to the product quality and process optimization.

Traditionally, the gelatinization temperatures of starch can be determined by three approaches, i.e., the microscopic, viscosity, and thermal analysis based measurement. In the microscopic method [7], the native starch aqueous solution is heated from ambient temperature to a higher temperature, the starch solution dropped on glass slide during the heating process is tested by a polarized light microscopy. By observing the change and disappearance of polarization, T_o and T_e of gelatinization can be determined, respectively. The disadvantage of this method is that the large collimation error in the polarized light observation caused by the operator’s judgment. Brabender viscograph (BV) technique was also applied to the gelatinization temperature measurement, based on the viscosity change of a starch slurry during a continuing temperature ramping [8]. Although this method can provide the information about temperature-dependent gelatinization rate during the sampling heating, a very larger sample quantity (e.g., 20–40 g of starch) and longer testing time are required [9]. The main problem of the BV method is that there are larger variations in both T_o and T_e measurement. This is because the granule swelling of starch suspension did not account for the rapid viscosity rise during a continuing temperature ramping [10], which can be noted from its viscosity-temperature curve. Currently, the thermal analysis based method, i.e., differential scanning calorimetry (DSC) is widely used as a test method for the efficient determination of starch gelatinization temperatures [11,12]. However, because the tiny crucible placed in DCS instrument cannot be agitated, there is a larger uncertainty for the sedimentation of starch granules added in water in each test. As a result, a significant error can be observed in the replicate measurement [13,14](Karapantsios, 2010 #677;Karapantsios, 2010 #677). Moreover, only very small sample size (˜ 6 mg) is allowed to be used in the measurement, which creates the problem associated with the poor representative, especially for the industrial sample test.

Headspace gas chromatography (HS-GC) is a powerful tool to determine volatile organic compounds (VOCs) in liquid or solid samples with a complicated matrix [15,16]. In a previous study [17], we used a volatile tracer assisted HS-GC technique to determine the viscosity of concentrated pulping spent liquors based on the rate of the trace release from the medium. Since the starch gelatinization increases the viscosity in its solution and there is an automated temperature-ramping program in most of commercial headspace auto-sampler systems, e.g. Agilent and Thermo Fisher, the above concept of the volatile tracer assisted HS-GC technique could be a new approach to realize the starch gelatinization temperature measurement.

In this work, we developed a simple and automated HS-GC technique for the determination of gelatinization temperature of starch. The main focuses of this research were on the establishment of the methodology and optimization of the conditions in the sample preparation and headspace equilibration and measurement. The performance (precision and accuracy) of the method were also evaluated.