Relationship between the molecular structure of duckweed starch and its in vitro enzymatic degradation kinetics

Abstract

Starch molecular structural effects in duckweed (Lemna minor and Landoltia punctata) controlling in vitro enzymatic degradation kinetics was studied. The molecular size distributions of fully-branched starches and the chain length distributions (CLDs) of enzymatically debranched duckweed starches were obtained using size-exclusion-chromatography (SEC). The CLDs of both debranched amylose and amylopectin were fitted with models using biologically-meaningful parameters. While there were no significant correlations between amylose content and starch degradation rate, the total amounts of amylose with shorter chain length negatively correlated with undigested starch content, and the amount of amylopectin long chains negatively correlated with the degradation rate coefficient. This provides new knowledge for the utilization of duckweed starches in bioethanol production.

Introduction

Bioethanol can be produced from a variety of feedstocks used as carbon source, including first-generation feedstocks (such as sugarcane, corn, triticale, wheat and cassava) and second-generation feedstocks (agricultural and municipal waste, grasses and trees) [1,2]. However, there is a concern about the use of first-generation feedstocks for ethanol production, since they inevitably compete for cropland used for food/feed production and biodiverse landscapes, such as rainforests [3].

Duckweed, a floating aquatic plant of the Lamnaceae family, is a potential alternative to resolve this problem since it is used for the decontamination of wastewater by absorbing problematic minerals [4]. This usage however means that its starch cannot be used in the food or animal feed industries, but, by the same token, duckweed is a more promising raw material than corn for ethanol production. Duckweed starch content varies with species and growth conditions, ranging from 3% to 75% of dry weight [3]. Duckweed may produce biomass up to 64 g/g-week [5], which is higher than generally seen in larger aquatic or terrestrial plants. The starch production rate from duckweed can be as much as 28 t/ha-year, which is considerably greater than from corn (about 5 t/ha-year) [6].

Previous studies on duckweed starch have focused on growth conditions, starch accumulation and application as feedstock and/or for ethanol production [1,3,[7], [8], [9], [10]]. There has been no systematic research on how the duckweed starch molecular structural influences starch enzymatic degradation rate (see e.g. [11]), which is of importance in determining the production of glucose, a key precursor in ethanol production. A study of the correlation between duckweed starch structural parameters and its degradation kinetics would help understand the role played by molecular structure in starch hydrolysis, and reveal structural features linked to higher yields and production rates of fermentable sugar.

To understand this, starch molecular structures, specifically the molecular size distributions of the fully branched molecules and the chain length distributions (CLDs) of enzymatically debranched starches, were obtained using size exclusion chromatography (SEC, a type of gel permeation chromatography, GPC). The CLDs of both amylopectin and amylose were fitted by biosynthetic models [[11], [12], [13]]. The kinetics of the cooked duckweed starch degradation rates were estimated using logarithm-of-slope analysis (LOS) combined with a modified nonlinear least-square (NLLS) method, as reported previously [11]. This yields the values of k (the digestion rate coefficient) and C_∞ (the residual undigested starch). Correlations between the structural parameters obtained from the model fitting and the digestion properties were then obtained using standard statistical methods.