Water-soluble polymers in agriculture: xanthan gum as eco-friendly alternative to synthetics

Abstract

Water-soluble polymers (WSPs) are a versatile group of chemicals used across industries for different purposes such as thickening, stabilizing, adhesion and gelation. Synthetic polymers have tailored characteristics and are chemically homogeneous, whereas plant-derived biopolymers vary more widely in their specifications and are chemically heterogeneous. Between both sources, microbial polysaccharides are an advantageous compromise. They combine naturalness with defined material properties, precisely controlled by optimizing strain selection, fermentation operational parameters and downstream processes. The relevance of such bio-based and biodegradable materials is rising due to increasing environmental awareness of consumers and a tightening regulatory framework, causing both solid and water-soluble synthetic polymers, also termed 'microplastics', to have come under scrutiny. Xanthan gum is the most important microbial polysaccharide in terms of production volume and diversity of applications, and available as different grades with specific properties. In this review, we will focus on the applicability of xanthan gum in agriculture (drift control, encapsulation and soil improvement), considering its potential to replace traditionally used synthetic WSPs. As a spray adjuvant, xanthan gum prevents the formation of driftable fine droplets and shows particular resistance to mechanical shear. Xanthan gum as a component in encapsulated formulations modifies release properties or provides additional protection to encapsulated agents. In geotechnical engineering, soil amended with xanthan gum has proven to increase water retention, reduce water evaporation, percolation and soil erosion - topics of high relevance in the agriculture of the 21st century. Finally, hands-on formulation tips are provided to facilitate exploiting the full potential of xanthan gum in diverse agricultural applications and thus providing sustainable solutions.

Fig. 1

Haworth formula of xanthan gum. The backbone consists of β‐1,4‐linked glucose units with a trisaccharide side chain whose terminal mannose unit is linked fifty–fifty to a pyruvate group and the non‐terminal residue usually carries an acetyl group. Redrawn from (Sworn, 2009).

Fig. 2

Xanthan gum production steps: fermentation, pasteurization, precipitation with IPA (isopropanol), decantation, dehydration, drying, milling/sifting, packaging.

Fig. 3

Model curves of viscosity build‐up of xanthan gum of two different particle structures, added to distilled H₂O at 0.3% [w/v]. The agglomerated grade reaches 90% of the maximum viscosity after approx. 115 s. The standard grade reaches 90% of the maximum viscosity after approx. 810 s.

Fig. 4

Model flow curves of solutions of 0.1% [w/v] different xanthan gum grades in standard tap water (1 g l^‐1 NaCl, 0.15 g l^‐1 CaCl₂·2H₂O). Typical application‐related shear rates are denoted. Differences in apparent viscosity are particularly pronounced in the low shear regime with implications for grade selection depending on desired behaviour.