Structural modification on potato tissue and starch using ethanol pre-treatment and drying process

Highlights

• Ethanol modified potato structure and improved water flow.

• Drying time was reduced by 56%.

• Any change was observed on the starch molecular profile.

• Drying caused “scratches” on the starch granule surface.

• The pasting and gel profiles showed few changes.

Abstract

This work evaluated ethanol as a pre-treatment to the convective drying of potatoes, focused on obtaining their starch. Potato slices were treated by immersion in ethanol for up to 30 min and then dried by convection (40 °C, 1 m/s). Different treatments were conducted to evaluate the effect of pre-treatment with ethanol and/or drying on the potato tissue and starch structure and properties. Potato microstructure, rehydration capacity, and drying kinetics were evaluated. After pre-treatment, drying, and rehydration, the starch was extracted and evaluated in relation to both structure and properties. Ethanol pre-treatment promoted structural changes on potato tissues and cells: the cells became withered, with thinner walls, lose turgidity, and changed the initial shape becoming more distorted and compacted. These permanent structural modifications impacted the subsequent mass transfer: the drying time was reduced in ∼56%, and the initial rehydration rate increased ∼25%. There were no changes in the starch molecular profile and only minor imperfections in the surface of the granules. The starch pasting and gel properties were only slightly altered by the drying process. In conclusion, drying and its pre-treatment with ethanol can be a simple technology for preserving potatoes focused on starch extraction.

Introduction

Starches are natural biopolymers with great importance as ingredients for different industries, such as food, feed, chemical, petrochemical, textile, paper and cellulose, adhesives, biodegradable plastics, among others.

Commercial starches are obtained from sources such as seeds (mainly maize, wheat, barley, and rice), roots and tubers (mainly cassava and potato). Potato starch is a valued source in different industries, due to some special properties (Grommers & Krogt, 2009). For instance, potato starch is used in the food industry due to its high clarity/transparency and neutral flavor, and in the textile industry due to its film properties, adhesive power, and paste penetration depth. In addition, potato starch is preferred as a pre-coating of filters due to its large granules, as well as it is also applied in water treatment due to its high phosphate content. In the paper and cellulose industry, potato starch is preferred due to its film-forming properties, less moisture retention, and excellent bonding power (Mitch, 1984).

However, roots and tubers have high moisture content, making them highly perishable after harvesting, which can result in economic losses if they are not quickly processed. In fact, although starches from potato and cassava present interesting industrial applications, they also present high variability and heterogenicity due to the vegetable senescence and partial degradation (for example due to fermentation) – which limits their production and application.

In this context, drying can be a simple, inexpensive, and efficient way to assure potato preservation focused on starch extraction. Particularly, convective drying uses hot air to remove the water from the product, being efficient and simple.

However, this process can take many hours, which is undesirable from a production perspective, as well as it can negatively impact the product quality. For instance, the presence of water in starchy raw materials, the long drying time, combined with the high temperatures, can promote changes in the starch characteristics, such as its gelatinization properties. Therefore, new approaches are necessary to enhance potato drying. Moreover, simple approaches are important for application by small producers.

In fact, ethanol pre-treatment promotes structural modifications and physical mechanisms that enhance the subsequent drying (Llavata et al., 2020). In fact, there is growing interest in using the ethanol pre-treatment to improve drying processes, such as the convective drying of pumpkin (Carvalho, Massarioli et al., 2020, Carvalho, Rojas et al., 2020; Rojas, Silveira, & Augusto, 2020), pineapple (Carvalho et al., 2020; Santos & Silva, 2009; Silva, Braga, & Santos, 2012), bananas (Corrêa et al., 2012; Silva et al., 2012), carrot (Santos et al., 2021), melon (Da Cunha et al., 2020), foam mat of pitayas (Araújo et al., 2020) and guaco (Silva, Celeghini, & Silva, 2018). Moreover, recently the ethanol pre-treatment started to be studied to improve other drying techniques, such as the convective drying assisted by ultrasound of apple (Rojas, Augusto, & Cárcel, 2020), infrared drying of potato (Rojas & Augusto, 2018b; Rojas, Silveira, & Augusto, 2019) and garlic (Feng et al., 2019), infrared with hot air in scallion (Wang et al., 2019) and scallion stalk (Zhou et al., 2020), and vacuum drying of apple (Amanor-Atiemoh et al., 2020).

However, although the high economic and industrial relevance, the effect of pre-treatment with ethanol and drying on a raw material whose final product is starch was still not evaluated.

Therefore, the objective of this study was to evaluate the pre-treatment with ethanol on the convective drying of potatoes, focused on the subsequent starch extraction. To achieve it, we evaluated the structural changes in both potato tissues and starch (both granules and molecules), correlating them with starch properties (pasting and gel), focusing on structural characteristics and properties of the starch as an industrial ingredient.