1. INTRODUCTION
Dysphagia is characterized as difficulty swallowing (an oral disorder), which may range from total lack of swallowing ability to safely ingest food, fluids, or saliva (Sasegbon & Hamdy, 2017). In Canada, dysphagia affects about 35% of the elder population. Among hospitalized elders, approximately 50% have dysphagia which not only impacts their nutrition and hydration status but also medication intake and overall quality-of-life (Werstuck & Steel, 2021). Dysphagia can be caused by weak tongue, cheek, or throat muscles, hindering food movement in the mouth for chewing and transferring meals to the stomach (Sasegbon & Hamdy, 2017). Aspiration, aspiration pneumonia, dehydration, malnutrition, morbidity, and death are among risks associated with dysphagia (Tagliaferri et al., 2019).
Dysphagia is clinically managed through the provision of thickened liquids and texture-modified foods. These modified foods and liquids reduce the risk of aspiration and increase hydration and nutrition. (Seshadri et al., 2018). Texture modified foods include pureed foods that are naturally or mechanically altered so that the original food becomes moist, smooth and homogeneous requiring minimal oral preparation (Keller et al., 2012). In the preparation of pureed foods, thickeners are an integral ingredient which improve consistency and cohesiveness and decrease syneresis (e.g., released liquid) of food products (Nishinari et al., 2019; Alvarez et al., 2012). Thickened fluids delay flow than liquids (e.g., water), providing an appropriate reflex reaction time when swallowing (Hadde et al., 2021).
Food ingredients used as thickening agents in purees are typically hydrocolloids. Hydrocolloids are a heterogeneous group of long chain polymers (such as polysaccharides and proteins) that form viscous dispersions and/or gels in water (Saha & Bhattacharya, 2010). Starch is the most commonly used thickener in texture modified foods because it is relatively cheap and abundant, and does not impart any foreign taste (Saha & Bhattacharya, 2010). Therefore, starch-based thickeners are traditionally used in the management of dysphagia (Newman et al., 2016). However, starch-thickened liquid decreases viscosity by 90% in 10 seconds of oral processing (Hanson et al., 2011). This sudden viscosity reduction in starch-thickened food during oral processing is due to the action of α-amylase, an enzyme present in saliva. Amylase catalyzes the hydrolysis of α-1,4 glycosidic linkages between glucose units in starch, resulting in amylose and amylopectin breakdown. (Sukkar et al., 2018; Souza, 2010). Hence, the stability of thickener to carbohydrate-hydrolyzing enzymes should be taken into consideration in food preparation for dysphagia patients (Sukkar et al., 2018).
The main carbohydrate-hydrolyzing enzymes are α-amylase and α-glucosidases. Alpha-amylases are hydrolytic enzymes that act upon the α-(1,4)- and/or α-(1,6)-linkages of starch polymers (Goesaert et al., 2009). When food is ingested, α-amylase in saliva randomly hydrolyses α-(1–4) glycosidic bonds of starch components to produce oligosaccharides of various lengths and a different α-limit dextrins with α-(1–6) bonds. As a result, the digestion process also breaks down the mechanical food structure, which is created by starch granules. It can also lower the viscosity of fluids that are thickened by starch-based thickeners (Hanson et al., 2011). The oligosaccharides and dextrins released from amylase action on starch can be further hydrolyzed into monosaccharides by α-glucosidases which catalyze the hydrolysis of α-1,4 and α-(1,6) bonds (Tomasik & Horton, 2012).
Plantago ovata (Psyllium) husk polysaccharides are obtained after milling psyllium seeds and is well-known for its strong hydrophilic and gelling properties, enabling its use as stabilizers and thickeners in food industry (Zhou et al., 2022; Franco et al., 2020). Psyllium husk is an excellent source of both soluble and insoluble fiber, and about 80% of the fiber in the husk is soluble and classified as a mucilaginous fiber due to its powerful ability to form a gel in water (Raymundo et al., 2014). In addition, psyllium husk can bind with glucose, and thereby inhibit amylase activity (Ahmed & Urooj, 2010). Moreover, psyllium husk can absorb water more than 50 times its initial weight, expanding and creating a smooth bulky gel (Masood & Miraftab, 2010). However, due to its extremely strong gel-forming ability and water-absorption property, psyllium husk rapidly forms a solid gel when it binds with water, impeding its incorporation into food homogeneously and limiting its food product application (Yu et. al., 2003).
These limitations can be overcome by developing psyllium husk emulsion gel for use in modified-texture foods to enhance consistency and cohesiveness, and to reduce syneresis. Emulsion gels are formed by gelling the continuous phase of emulsions or by aggregating the emulsion droplets through the addition of hydrophilic polymers (Fig. 1). With the gelling of the continuous phase, the resulting medium is a soft solid that can entrap emulsified lipid droplets inside the gel matrix. As a result, functional compounds incorporated into emulsion gels often have better storage stability compared to those incorporated into emulsion (Cofrades et al., 2017).
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Fig. 1. The formation of an emulsion gel (O/W), modified from Lu et.al. (2019).
The formation of emulsions requires energy to agitate the two immiscible phases together. Ultrasonic homogenization is a high-energy, cost-effective, energy-efficient, easy, and environmentally friendly emulsification process to split aggregates and generate tiny droplets with a narrow size distribution (Leong et al., 2017). Smaller droplet size can improve emulsion stability by avoiding gravity separation; hence, ultrasound-treated emulsion has higher stability than untreated emulsion. However, the effect of ultrasound on emulsion gels needs to be further explored and assessed.
In this study, we first developed and optimized food-grade psyllium husk emulsion gel. Then, the stability of ultrasonic-treated and untreated emulsion gels was investigated by polarized light microscopic, cryo-scanning electron microscopic observations and particle size analysis. We also studied the total expressible fluid and the inhibitory effects of psyllium husk emulsion gel on alpha-amylase and alpha-glucosidase activities in puree samples.