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).
(Place for figure 1)
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.