Elsevier

Carbohydrate Polymers

Volume 173, 1 October 2017, Pages 77-83
Carbohydrate Polymers

Glass transition of anhydrous starch by fast scanning calorimetry

https://doi.org/10.1016/j.carbpol.2017.05.042Get rights and content

Highlights

  • The glass transition of anhydrous starch can be measured by Fast Scanning Calorimetry.

  • The value measured at 312 °C is in agreement with extrapolated value of 316 °C.

  • Structural relaxation of anhydrous starch can be observed by Fast Scanning Calorimetry.

Abstract

By means of fast scanning calorimetry, the glass transition of anhydrous amorphous starch has been measured. With a scanning rate of 2000 K s−1, thermal degradation of starch prior to the glass transition has been inhibited. To certify the glass transition measurement, structural relaxation of the glassy state has been investigated through physical aging as well as the concept of limiting fictive temperature. In both cases, characteristic enthalpy recovery peaks related to the structural relaxation of the glass have been observed. Thermal lag corrections based on the comparison of glass transition temperatures measured by means of differential and fast scanning calorimetry have been proposed. The complementary investigations give an anhydrous amorphous starch glass transition temperature of 312 ± 7 °C. This estimation correlates with previous extrapolation performed on hydrated starches.

Introduction

Starch, as a storage polysaccharide in plants, is one of the most abundant natural polymers found on a wide range of crops worldwide. Starch is a polyanhydroglucose that consists of two homologous polymers: the linear amylose and the hyper-branched amylopectin, whose molecular weights (Mw) range from 104 to 106 g mol−1 and from 107 to 109 g mol−1, respectively. Depending on its botanical origin, the amylose/amylopectin ratio can vary: 20/80 for potato starch, 25/75 for normal maize starch, 70/30 for amylomaize (high amylose maize). In its native form, starch is stored as granules and presents a semi-crystalline structure which is usually transformed in amorphous state for the most of applications. It is the major component of many foods, as cereal foods, and it is also widely utilized for the development of biodegradable materials for non-food uses. Starch presents a glass transition temperature which has a great impact on mechanical properties, i.e. the texture of food, and aging of products. The importance of the glass transition temperature, in understanding the physical state and physicochemical properties of food materials, has been extensively discussed at the end of the last century (Levine & Slade, 1988; Noel, Ring, & Whittam, 1991; Roos, 1995). The glass transition temperature of starch depends on water content, which has a very efficient plasticizing effect (Bizot et al., 1997). The modeling of glass transition temperature dependence of water concentration is very useful in order to predict the behavior of starch materials and it has been the subject of numerous studies.

Couchman (Couchman, 1987) proposed a relation based on the thermodynamic, which describes the dependence of the glass transition of a mixture as a function of pure component properties:Tg(mixture)=w1ΔCp1Tg1+w2ΔCp2Tg2w1ΔCp1+w2ΔCp2where ΔCpi is the heat capacity change of component i, wi is the weight fraction of component i, and Tgi the glass transition of component i.

The Gordon Taylor formula (Gordon & Taylor, 1952) is often preferred. It is equivalent to the Couchman equation but uses the ratio of pure component heat capacity changes as an adjustable variable. Using the same notations as above, the equation becomes:Tg(mixture)=w1Tg1+kw2Tg2w1+kw2withk=ΔCp2ΔCp1

Because the degradation temperature is below the glass transition temperature of anhydrous starch, its glass transition temperature cannot be measured by « classic » thermal analysis method such as Differential Scanning Calorimetry (DSC). In fact, Orford et al. (Orford, Parker, Ring, & Smith, 1989) have shown that Tg of gluco-polymers increases from 29 °C for anhydrous glucose (DP1) to 167 °C for the anhydrous hexamer, maltohexaose (DP6), with a ΔCp which decreases from 0.88 J g−1 K−1 to 0.49 J g−1 K−1, respectively. The measure for higher chain length is not possible because thermal degradation occurs at 177 °C. Until now, the glass transition temperature of anhydrous starch has only been evaluated by using previous relations from the glass transition temperatures measured on hydrated starch (Bizot et al., 1997). The objective of this work is to study the feasibility of such a measure by fast scanning calorimetry (FSC).

The growing interest of the fast scanning calorimetry has opened new possibilities in the field of polymer science, either for crystallization (Furushima et al., 2017; Toda, Androsch, & Schick, 2016) or amorphous (Cebe et al., 2015; Koh, Gao, & Simon, 2016) properties. Such technique allows to heat up and cool down few nano-grams of sample through high scanning rates as fast as thousand Kelvin per second (Schawe, 2015). One of the interesting point of this technique is the possibility to inhibit and/or shift thermal events through the high scanning rates used. For example, inhibition of the nucleation or the crystallization have been reported for polymers such as poly(lactide) (Androsch, Iqbal, & Schick, 2015; Androsch, Di Lorenzo, & Schick, 2016) or poly(butylene terephtalate) (Androsch, Rhoades, Stolte, & Schick, 2015) by using high scanning rates. More specifically, Furushima et al. have succeeded to measure glass transition and melting for semi-crystalline poly(acrylonitrile) (PAN) (Furushima, Nakada, Takahashi, & Ishikiriyama, 2014). Through scanning rates above 250 K s−1, exothermic reactions happening in the temperature range of the melting have been successfully inhibited. In addition, with regards to amorphous properties, FSC leads to shift in the glass transition (Schick & Mathot, 2016). Termed thermal lag, this shift is due to the heat transfer delay between the heater and the sample. It is caused by the high scanning rates used. Thus, the temperature gradient leads to smearing effect. The measured thermal events appear to occur at higher temperature. As FSC has reported inhibitions or shift of various thermal events, the thermal degradation of anhydrous amorphous starch happening below its glass transition might be successfully inhibited despite the shift of glass transition. However, the thermal lag induced by the technique has to be assessed in order to compare the values of glass transition temperature extrapolated by means of DSC from hydrated starches. Classic thermal lag corrections proposed by Schawe (Schawe, 2015), related to the static and the dynamic ones, cannot be performed due to the thermal degradation of anhydrous starch. Consequently, a novel thermal lag corrections is proposed by using two polymers, considered as standard in this work: poly(lactide) and poly(bisphenol A carbonate). Those polymers present different values of fragility index m. The fragility index m measures the rapidity with which glass-forming liquid properties change as the glassy state is approached (Angell, 1991). As heat transfer delay exists between the heater and the sample due to the high scanning rate, different shifts might be expected with regards to the fragility index. Consequently, the two polymers, which are well known glass-forming liquids, have been chosen in order to assess the differences of glass transition temperatures measured between FSC and standard DSC. By means of FSC, the aim is to figure out the correct glass transition temperature of the anhydrous amorphous starch that would be measured by DSC. Despite a large molar distribution due to its amylose and amylopectin composition, leading to a wide glass transition domain, potato starch exhibits a clear ΔCp perfectly detectable and values available for hydrated system (Bizot et al., 1997). It is also the purest botanical origin. For these reasons potato starch has been chosen as a model for this study.

Section snippets

Sample preparation

Potato starch was obtained from Roquette Frères (Lestrem, France). The amylose/amylopectine ratio was 22/78 and the molar weight Mw was 113.106 g mol−1 (Rolland-Sabaté, Guilois, Jaillais, & Colonna, 2011). Starch films were obtained by the casting method. Native granular starch was solubilized in a high-pressure reactor at 130 °C for 20 min, using a 4% suspension in ultrapure water. The procedure was performed under a nitrogen atmosphere to avoid any risk of degradation. The solution was evenly

Thermal degradation

Fig. 1 shows thermogravimetric and differential scanning calorimetry analyses of starch films. As observed in Fig. 1(A), dehydration takes place in the starch from ambient temperature up to 130 °C. The weight loss during this phase is related to the moisture content. After the isothermal of 30 min to complete the dehydration of starch, the weight loss suddenly drops at a temperature of 290 °C, which corresponds to the thermal degradation of the starch. Prior to the DSC analysis, an isothermal at

Conclusion

To our knowledge, it is the first time that glass transition of anhydrous amorphous starch has been measured. Thanks to the FSC, a scanning rate of 2000 K s−1 has permitted to observe the heat capacity change at the glass transition on duration short enough to avoid the thermal degradation of the biopolymer. In addition, structural relaxation has been performed through physical aging and the concept of limiting fictive temperature. Both cases have depicted enthalpy recovery peaks on the

Acknowledgment

The authors gratefully acknowledge the Normandy Region, France, for the financial support to Xavier Monnier thesis as well as for FSC equipment.

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