Ascorbic acid mediated hydrolysis of galactomannans

Abstract

Ascorbic acid (AA) is an antioxidant widely used in the food industry to prevent colour fade and spoilage. This study assesses the effect of 0.02 wt% AA on the rheology of common food thickeners – galactomannans (GM). GMs immediately exhibit a significant reduction in solution viscosity upon AA addition: guar gum (-67% ± 7%), locust bean gum (-47% ± 5%), and cassia gum (-58% ± 4%). Other food acids at 0.02 wt% showed no decline in viscosity, nor did another reducing agent, potassium iodide. GMs were then mixed with xanthan gum (XG) +/- low acyl gellan gum (LAG) and AA’s impact was assessed using small amplitude oscillatory shear rheology. As the temperature decreased, the storage modulus decreased in the presence of AA compared to without. The molecular weight (MW) of the GMs +/- AA, was assessed using size exclusion chromatography – multi angle light scattering. The reduction in MW, was between 6-8 times for each GM, and was supported with analytical ultracentrifugation. This established the hydrolytic decomposition of GMs by AA, leading to a decrease in function due to a reduction in MW. This hydrolytic effect was observed regardless of pH, showing that acid hydrolysis isn’t the primary mechanism. This study shows, for the first time, that AA causes extensive degradation of galactomannans, affecting their viscoelastic characteristics. These findings could affect many products; informing decisions on their quality and shelf life, as well as their cost-effectiveness and environmental life cycle.

Highlights

Introduction

Ascorbic acid (AA) is widely used across various industries including food, pharmaceuticals, agriculture and microbiology. Also known as vitamin C, it is a water-soluble, naturally occurring compound that cannot be synthesised by humans owing to the absence of L-gulonolactone oxidase an enzyme used in its biosynthesis (Smirnoff, 2001). This makes AA essential to the healthy functioning of the human body, and without continuous ingestion poor health and eventually death will occur. The primary uses of ascorbic acid are related to its antioxidant capacity, and thus ability to scavenge reactive oxidant species (Ames, Shigenaga, & Hagen, 1993). Since these species can cause degradation of DNA, lipids, proteins and carbohydrates, as well as other types of materials, it is important to prevent them from acting on these materials (Choe & Min, 2006). In the food industry AA is widely deployed to fortify foods with an essential vitamin, as well as a preservative to prevent colour fade, spoilage and acting as a radiation protector (Aliste & Del Mastro, 2004). AA is a preferred ingredient in the food industry owing to a relatively low cost, ready availability and ‘clean label’ status, with many synthetic antioxidants starting to fall out of favour due to issues with consumer perception (Mesías, Martín, & Hernández, 2021). While AA is known to act as a reducing agent, under certain circumstances such as the presence of transition metals, AA can actually act as pro-oxidant (Buettner & Jurkiewicz, 1996; Yen, Duh, & Tsai, 2002). AA has been shown to cause the breakdown of starches (Majzoobi, Radi, Farahnaky, & Tongdang, 2012; Vallès-Pàmies, et al., 1997) and other polysaccharides such as xyloglucan and pectin (Zou, Nie, Yin, & Xie, 2020).

Galactomannans are a group of plant-derived polysaccharides that consist of a mannose backbone with galactose side chains. The mannose backbone, owing to its similarity to cellulose, is insoluble in water but the galactose side chains interfere with the interchain mannose association and impart a degree of water solubility (Prajapati, et al., 2013). The most commonly used galactomannans are locust bean gum (LBG), guar gum (GG), cassia gum (CG) and tara seed gum. The differences between them are based on the ratio of galactose to mannose, and a lot of their function is derived from their molecular weight and side chain interactions (Prajapati, et al., 2013). Galactomannans have multiple applications in food systems including as thickening agents to replace fat in low-fat products or to thicken sauces and dressings to prevent sedimentation and separation (Singh, Singh, & Arya, 2018). Galactomannans undergo synergistic interactions with other polysaccharides including xanthan gum (XG), kappa-carrageenan and agar. With XG, galactomannans see an increase in viscosity greater than either of the two gums can achieve alone. In addition to this, while neither forms a self-supporting gel alone, together they form a soft, elastic gel (Copetti, Grassi, Lapasin, & Pricl, 1997). With agar and kappa-carrageenan, galactomannans cause an increase in gel strength, and a decrease in Young’s modulus, indicating a reduction in brittleness (Dunstan, et al., 2001).

In this study, the effect of AA on the structure and function of three galactomannans is examined: LBG, CG and GG. AA was added at 0.02 wt% to single galactomannan systems at 2 wt% and the subsequent changes in viscosity were measured via rotational rheometery. This was compared to other food acids at the same concentration, and potassium iodide, another small species reducing agent. Following this, systems containing a galactomannan, XG and low acyl gellan gum (LAG) were examined by small amplitude oscillatory shear rheology in order to assess whether AA affected the function of galactomannans in combination with other hydrocolloid gelling agents. XG and LAG were tested alone with AA, to demonstrate that it is specifically the galactomannan degradation that was responsible for the observed rheological changes. Back extrusion texture profile analysis was used to examine the effects of AA on the bulk structure of the gel systems consisting of CG, XG and LAG. Then to investigate the mechanism of action, the molecular weight of the galactomannans with and without AA was scrutinised by size exclusion chromatography – multiple angle laser light scattering (SEC-MALLS) and this was confirmed with analytical ultracentrifugation (AUC).

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Materials

Cassia gum and GG were purchased from Daymer ingredients (UK). AA, XG, LAG, Potassium iodide, malic acid and fumaric acid were purchased from VWR (UK). Citric acid and tri-potassium citrate were purchased from Sigma-Aldrich (UK). Deionised water was used for all sample preparation. All materials were used as received with no further modification or purification.

Michael-Alex Kamlow, Julian Marks, Joshua E.S.J. Reid, Vlad Dinu, Ian D. Fisk, Gleb E. Yakubov, Ascorbic acid mediated hydrolysis of galactomannans, Food Hydrocolloids, 2025, 111996, ISSN 0268-005X, https://doi.org/10.1016/j.foodhyd.2025.111996.