2014 Volume 20 Issue 3 Pages 607-611
Small and large deformation rheological tests for mixed gels of gelatin, konjac glucomannan (KGM) and locust bean gum (LBG) were performed. For a total ingredient content of 1.1%, the hydrocolloid mixtures behaved as entangled polymer solutions. Higher relative weight fraction of KGM at this total hydrocolloid content resulted in G'-G" crossover at a lower frequency. Addition of KGM and LBG to 4% gelatin resulted in an increase in storage and loss moduli. Presence of LBG led to a stronger increase of the elastic modulus and a stronger decrease of the fracture force in puncture. The puncture peak force of binary gelatin/KGM and gelatin/LBG mixed gels was similar to that of ternary gelatin/LBG/KGM mixed gels at the same ratio of polysaccharide to protein, and was lower than the puncture peak force of pure gelatin gels at the same total hydrocolloid concentration. Addition of LBG and/or KGM to gelatin decreased the sensory hardness of the gels. Difficulty in chewing and swallowing was highest for hard pure gelatin gels and soft, but very sticky mixed gels with a substantial addition of LBG. The gel judged to be the easiest for mastication and swallowing was a gel obtained from 5% gelatin, 1% KGM and 0.5% LBG.
When solutions of different biopolymers are mixed, interactions between unlike chains are either more favourable or less favourable than interactions between chains of the same type, leading to association or segregation, respectively (Harrington and Morris, 2009a). Mixtures of proteins and polysaccharides form gel structures used in food technology, medicine and material engineering (Quirit and Liaguno, 2004). In food technology, there is an increasing demand for nursing-care foods for people with mastication and swallowing difficulties. Such products should be optimized from rheological, colloidal and tribological aspects so that foods can be easily chewed and swallowed (Chen, 2009; Ishihara et al., 2011). Gelatin is a natural polymer obtained from collagen. There are two main types of gelatin: type A, obtained from collagen using acid pretreatment exclusively; and Type B, which is the result of alkaline pretreatment. Gelatin is a rigid-chain high molecular weight compound, and exhibits typical rheological properties of polymeric substances, which is not the case with native collagen (Harrington and Morris, 2009b). In aqueous solutions gelatin molecules assume a statistical-coil conformation. Under different conditions of pH, ionic strength and temperature, gelatin molecules display large conformational variety. In aqueous solution, gelatin forms thermally reversible gels with a melting temperature a little below human body temperature. Such gels have been used in a large range of applications (Haug et al., 2004). Gelatin gels are characterized by unique organoleptic properties as well as their ability for flavor release. Galactomannans occur as storage polysaccharides in the seed endosperm of plants in the Leguminosae family. KGM is extracted from konjac flour, obtained from Amorphophallus konjac C. Koch, and consists of β-1, 4-linked D-mannose and D-glucose at a ratio of 1.6:1. These gels are sometimes presented as a vegan substitute for gelatin gels (Nishinari et al., 1992). Locust bean gum (LBG, from Ceratonia siliqua) is a galactomannan with roughly one galactose unit per 4 mannose units and is only partially soluble in cold water. It can be dissolved completely at high temperature, and can form thermally-irreversible weak gels under certain conditions (Alves et al., 2000). In the literature, there are no reports on mixed gelatin, KGM and LBG gels. The objective of this research was to investigate the rheological and sensory properties of such ternary gels in order to create a gel matrix texture suitable for people with mastication and swallowing difficulties.
Locust bean gum was purchased from Sigma Aldrich (St. Louis, MO, U.S.A.). Konjac glucomannan was purchased from Puritan’s Pride (Oakdale, NY, U.S.A.) and gelatin was obtained from Dr. Oetker (Gdansk, Poland). Mixtures of KGM and LBG were prepared in a 25% sucrose and 0.22% citric acid solution under mechanical stirring at 75°C for 30 min. Gelatin was mixed in the solution by manual and mechanical stirring depending on the concentration. Dispersions were stained by the addition of the fluorescent dye Rhodamine B (Fluka, Oakville, ON, Canada) at 0.03 wt%. Gels were set on glass slides and immediately placed in a refrigerator (5°C) for curing overnight (16 − 20 hrs). Gels were equilibrated to room temperature (22 ± 2°C ) for 60 − 120 min before measurement. The gel microstructure was observed by an optical polarizing microscope (Eclipse E600 Pol; Nikon, Tokyo, Japan) using a 430 − 490 nm filter. Puncture test was performed using an XT.T2 Texture Analyser (Stable Micro Systems, Godalming, UK). Gels were prepared in beakers (40 mm diameter, 60 mm height), kept at 7°C for 18 hrs, removed from a thermostat and kept at 25°C for 2 hours prior to the analysis. Puncture test was performed using a 7 mm diameter steel plunger at 2 mm/s for 10 seconds. Flat gel cylinders (35 mm diameter, 2 mm thick) were cut using a scalpel. Frequency sweeps were performed in a linear viscoelastic region using a Haake RS300 rheometer (Thermo Haake, Karlsruhe, Germany) equipped with serrated parallel plate geometry (PP35; diameter 35 mm, gap 2 mm). All measurements were conducted at 25°C. Sensory evaluation of the gels was carried out by 28 female and male subjects between 20 and 24 years of age. Fifty subjects were trained and 28 final panelists were selected based on their performance in screening tests. The samples were served at 22°C in random order to avoid the position effect. Sensory evaluation was conducted based on a 5-point scale for 4 different attributes. The following attributes were evaluated: A — Hardness (+ 2 very hard, + 1, 0, -1, -2 very soft); B — Difficulty of chewing (+ 2 very difficult, + 1, 0, -1, -2 very easy); C — Slipperiness or stickiness (+ 2 very slippery, + 1, 0, -1, -2 very sticky); D — Difficulty of swallowing (+ 2 very difficult, + 1, 0, 1, 2 very easy). Higher chewing difficulty meant that a higher chewing frequency and longer total mastication time was required by the panelists before swallowing. Each test sample was presented in duplicate.
Statistical analysis of results (standard deviation and analysis of variance) was performed using a statistical program (STATISTICA 5.0 PL; Stat Soft Polska, Warsaw, Poland). Significant differences between means were determined using Tukey’s test at a confidence level of p ≤ 0.05 based on the least significant difference.
The mechanical spectra of storage and loss moduli for mixtures of hydrocolloids with a total concentration of 1.1% are shown in Fig 1. The spectra show the typical form of a solution of densely entangled disordered chains. At low frequencies, where there is sufficient time for chains to disentangle, G" values are higher than G', and viscous liquid behavior is observed. At higher frequencies, with less time for disentanglement, G' values are higher than G" values above a characteristic crossover frequency. The product behaves as a viscoelastic liquid with an entangled network. The increase in relative weight fraction of the KGM shifted the crossover frequency to lower frequencies. This means that the addition of KGM resulted in a more elastic structure, which entangled even at lower frequencies. A previous report showed that viscosity of 0.5% KGM was about twice that of a 0.25% LBG + 0.25% KGM mixture (Yang et al., 2013). Thus, at low concentrations where the gelatin/polysaccharide mixture does not form a self-standing gel, the effect of polysaccharide addition on viscoelasticity is as expected from the solution properties of the respective polysaccharide.
The addition of KGM and LBG to 4% gelatin gels resulted in an increase in storage and loss moduli (Fig 2). A similar behavior was observed for 4.5% and 5% gelatin gels (data not shown). For all mixed gels, G' was about 4 times higher than G" at low frequency (0.1 Hz) and about 11 times higher at high frequency (10 Hz). This indicates viscoelastic relaxation that is present even at frequencies above 0.1 Hz. The elastic modulus values found for ternary gels show clearly that LBG has a greater effect on the small-deformation rheology of gelatin-based gels, and leads to a greater increase of G’. This behavior is probably caused by the LBG-induced concentration of gelatin during the process of phase separation, leading to an increase in the elastic modulus (Alves et al., 2000). The images obtained using an optical polarizing microscope in the UV region are presented in Fig. 3. Phase separation is observed at greater length scales for the gelatin/LBG gel in comparison to the gelatin/KGM gel. The effect of LBG and KGM addition to gelatin gels on the elastic modulus is inversely correlated to its effect on fracture stress in the puncture test, as seen from results. Mixed gels of gelatin with LBG or KGM show different force-distance curves in a puncture test (Fig. 4). For pure gelatin gels, a peak corresponding to the initial fracture is observed, followed by another peak caused by further fracture. Mixed gels have smoother force-distance curves, and following the fracture a monotonous increase in force is observed. This is probably caused by the “sticky” nature of the mixed gels, which was observed in sensory analysis (Fig. 6). The frictional force exerted on the side surface of the plunger contributed to the detected force, especially for these “sticky” gels at larger distances (>10 mm), shown in Fig. 4. Solowiej (2013) found during the investigation of processed cheese analogues that plungers made of different materials gave different TPA adhesiveness. Ternary gelatin/LBG/KGM mixed gels were characterized by lower puncture peak force than pure gelatin gels, with a similar peak force to binary gelatin/KGM and gelatin/LBG gels (Fig. 5). The opposite effects of plant polysaccharides on the large and small deformation rheology of gelatin have previously been reported. Alves et al. (2000) investigated gelatin/LBG mixed gels and found that an increase in LBG concentration caused an increase in G' values and a decrease in maximum penetration force. Aeration of whey protein gels yielded a structure with higher storage and loss moduli compared with the undisrupted gels, but with lower TPA hardness (Tomczyńska-Mleko, 2009). Whipping of the whey protein gels followed by healing at rest produced a more homogenous structure with higher density of crosslinks, leading to higher G' values and lower fracture stress, owing to the introduction of air bubbles, which can lower the fracture stress (Tomczyńska-Mleko, 2010).
Frequency dependence of storage and loss moduli of mixed gelatin, KGM and LBG gels. Total polymer concentration, 1.1 wt%. Measurement temperature, 25°C.
Frequency dependence of storage and loss moduli of 4% gelatin gel with and without KGM and LBG addition with different total polymer concentration. Measurement temperature, 25°C.
Optical polarizing microscope images showing phase separation for 5% gelatin gels with 1% KGM (left) and 1% LBG (right) addition.
Force-distance curves for puncture test of 5% gelatin gels with and without 1% KGM or 1% LBG. Plunger, 7 mm diameter; speed, 2 mm/s; temperature, 25°C.
Peak puncture force for 5% gelatin gels with and without the addition of KGM and LBG. Plunger diameter 7 mm diameter; speed, 2 mm/s; temperature, 25°C.
Sensory analysis data of 5% gelatin gels with and without added KGM and/or LBG.
Structural differences were also the origin of different yield stress values in Parmesan cheeses of similar storage moduli (Govindasamy-Lucey et al., 2004).
Our present results show that mixing different hydrocolloids leads to gels with different small and large strain rheological properties. During mastication and swallowing both small and large strain phenomena are involved. Pure gelatin gels can cause problems for older people with mastication and swallowing difficulties. Figure 6 presents sensory analysis data. The addition of LBG and/or KGM to gelatin decreased the sensory hardness of gels, which is in agreement with the puncture test data presented in Fig. 4. Gels judged as the most difficult to chew and swallow were gelatin gels and gels obtained with the addition of 1% LBG, 0.5% KGM + 1% LBG and 0.5% KGM + 2% LBG. These mixed gels were also the stickiest. Panelists had the greatest challenges chewing and swallowing the hard pure gelatin gels and the soft, but very sticky mixed gels with a substantial addition of LBG. Panelists judged sticky gels difficult to chew because they stick to the teeth and/or the hard palate, and as a result they cannot be located in the right position in the oral cavity to be fractured effectively during mastication. The gel perceived easiest to masticate and swallow was a mixed gel containing 5% gelatin, 1% KGM and 0.5% LBG. Pure gelatin gels are harder and more slippery, and can therefore cause problems for older people with mastication and/or swallowing difficulties.
