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Sweetness resulting from sucrose released from agar gels of varying concentrations of agar (0.5, 1.0, and 1.5% w/w) and sucrose (10%–50% w/w) was evaluated. The intensity of sweetness as perceived by subjects was investigated during consumption of the gels. A time–intensity method was applied: the maximum intensity and area under the time–intensity curve were higher in gels with lower concentrations of agar as expected from previous studies. The intensity of sweetness as estimated by the area under the time–intensity curve increased with sucrose concentration, but the maximum intensity tended to reach a plateau above a certain sucrose concentration. The manifestation of the maximum sweetness was delayed with increasing concentrations of sucrose and agar. The total duration of the perception of sweetness was longer in gels with higher sucrose concentrations but did not depend on the concentration of agar in the range examined. Instrumental characterization of these gels was also conducted. Young's modulus, fracture stress, fracture strain, and energy obtained from a conventional uniaxial compression test of sample gels increased with sucrose concentration and the modulus, stress, and energy were higher in gels with higher agar concentration. Although mechanical characteristics such as Young's modulus, fracture stress, strain, and energy, increased with increasing sucrose concentration, the sweetness intensity tended to a plateau; therefore, physiological and psychological factors experienced during oral processing should be taken into account to understand the perceived sweetness intensity.

The objectives of this study are to identify mechanical properties that dictate natural oral processing of soft solid foods and to quantify the effects of mouthful sizes. Two types of hydrocolloid gels with similar fracture loads but different elastic moduli and fracture strains were prepared using agar (A) or a mixture of konjac mannan, κ-carrageenan, and locust bean gum (K). Subsequently, 3, 6, 12, and 24 g mouthfuls of A and K gels were randomly served to subjects and the effects of mechanical properties and mouthful sizes were quantitatively analyzed within an individual. Electromyographic (EMG) activities of both masseter muscles and the suprahyoid muscle were recorded as jaw-closing and jaw-opening muscles, respectively. Total 33 EMG variables relating mastication and swallowing difficulties were classified into six groups by influences of gel types and mouthful sizes. Despite similar fracture loads, K gels with greater fracture strain and work required more mastication effort and less swallowing effort than A gels, which had higher elastic modulus. Doubling of mouthful sizes between 3 g and 24 g increased oral processing times and the numbers of chewing actions and swallows by 1.4 fold. These variables were approximately proportional to 0.7 of the power of the mouthful mass, and they can be predicted using this relation. Mastication effort exhibited lower dependence on sample mass than the three variables and durations and muscle activities per masseter muscle action did not vary significantly with mouthful sizes and gel types.

Eating difficulty of solid foods was analyzed using electromyography (EMG) of five hydrocolloid gels with varying textures. Eleven subjects ate gel samples under normal conditions and EMG activities of both sides of masseter (jaw-closing) and suprahyoid (jaw-opening) muscles were recorded during eating. Differences in EMG variables among samples were analyzed separately before and after the first swallow using repeated-measure statistics. The time before the first swallow was related to chewing effort and was estimated according to numbers of chewing actions, masseter muscle activities per chew and the sum of masseter muscle activities during chewing. EMG variables were also correlated with resistance to fracture, which was extracted as the first principal component from sensory evaluations of gel texture, but did not directly relate to any single parameter from mechanical tests or sensory evaluations. The time after the first swallow to the end of eating corresponded with the number of swallows, the EMG amplitude of suprahyoid muscles during chewing, the sum of masseter activity and suprahyoid muscle activity during the period after the first swallow. These EMG variables corresponded with sensory evaluations of adhesiveness but not with other single mechanical and sensory parameters or principal components. The present data suggest that at least two independent factors affect eating difficulty. Although differences among the five gels were predominantly reflected by chewing difficulty, these remained significant after the first swallow.

Eating difficulty was evaluated by sensory analyses of hydrocolloid gels as a food model. Twenty sample gels were created with a wide range of textures. Twelve panelists rated the eating difficulty for each gel on an unstructured line scale. Subsequently, they also evaluated six texture attributes including firmness, cutting effort, elasticity, extensibility, adhesiveness, and melting rate in the mouth. A principal component analysis characterized the texture attributes of the sample gels on axes of resistance to fracture (principal component 1) and stickiness and flexibility (principal component 2). The contour of the resulting eating difficulty scatter diagram revealed that resistance to fracture and stickiness and flexibility were critical determinants of eating difficulty.

Some in vivo measurements have been carried out using polysaccharide gels of different physical properties (i.e., elastic and plastic) and degrees of hardness. In vivo measurements tested included electromyography (EMG) and acoustic analysis of the swallowing sound to investigate the dynamic changes of food texture during oral processing. As a model of foods for dysphagia patients, the gels were soft enough to be eaten by compression between the tongue and the hard palate without biting by the teeth. From EMG, no significant differences were found between elastic gels and plastic gels in the duration of oral processing and the EMG activity of the suprahyoid musculature when compared at equivalent hardness. The EMG activity of the suprahyoid musculature correlated well with the compression load of gels at 95% strain. From the acoustic analysis, the plastic gels required shorter time to transfer through the pharynx and were scored higher in sensory cohesiveness than the elastic gels. Results indicate that oral processing of soft gels requires equivalent EMG activity of the suprahyoid musculature when the gel hardness is the same. Also, the plastic gels flow through the pharynx as one coherent bolus with smaller variation of the flow speed. Texture of foods for dysphagia patients should be optimized in terms of viscoelasticity so that they can easily transform to swallowable bolus during oral processing.

The applicability of a powerful but still easy to use technique, based on a phenomenological theory of viscoelasticity, for processing and analyzing dynamic mechanical data of some rice gels was investigated. Based on this theory a continuous relaxation spectra was generated by application of Tikhonov regularization procedure on continuous Maxwell model. Interpretation of relaxation spectra in terms of number of peaks, its peak intensity H ( λ ) and appearance of its main distribution peak and magnitude of equilibrium elasticity modulus ( G e ) of continuous Maxwell model was found to appropriately reflect main peculiarities of the viscoelastic behavior of rice starch gels. An increase in number of peaks in the relaxation spectra was observed for starch gels having higher amylose content indicating the creation of more heterogeneous structure. H ( λ ) and G e values also increased with increase in amylose content demonstrating a transition of the system to more stable state like a gel.

The effect of acid–methanol treatment using 0.36% HCl in methanol at 25 °C for 4–216 h on molecular structure and physicochemical properties of lentil starch was investigated. The recoveries for modified starch granules were quite high (91.9–95.6%). Upon modification, the starch granules showed formation of cavities, surface roughness and development of protuberances. The solubility of lentil starch increased, while swelling power, pasting viscosity and gel strength decreased with increase in treatment duration. These changes were found to be due to preferential degradation of amylose and amylopectin long chains in acid–alcohol treatment, which was confirmed by studies conducted on high-performance size-exclusion chromatography (HPSEC). Data obtained from HPSEC was also used to develop an exponential model to illustrate the degradation kinetics of acid–methanol-treated lentil starch.

Rheological and thermal properties for 5:5 mixed gels of konjac glucomannan (KGM) with five different molecular weights and κ-carrageenan (CAR) were studied. Molecular weight distribution was analysed by gel permeation chromatography (GPC). Dynamic and static viscoelastic moduli for the mixed gels were much smaller than those for gels of CAR alone and became larger with increasing molecular weight of KGM. Gel—sol transition for mixed systems with total gum content 1.5% w/w occurred at the temperatures between the transition temperature for 0.75% w/w CAR and that for 1.5% w/w CAR. Gel-to-sol and sol-to-gel transition temperatures for the mixed systems were slightly increased with increasing molecular weight of KGM in the mixture observed in differential scanning calorimetry (DSC). One exothermic peak was observed in cooling DSC curves, while two endothermic peaks appeared in heating DSC curves for 5:5 mixed systems. The intensity of the lower peak increased with increasing KGM proportion in the mixture and with increasing molecular weight of KGM at a given mixing ratio. This fact confirms that the mixed gels involve two crystalline regions: some are made by CAR molecules in the helical structure and others are formed by the interaction between helical CAR and KGM. On heating, the latter is broken at a lower temperature and then the former melts at a higher temperature. The junction zone formed by CAR and KGM is easily made in a gel containing KGM with a higher molecular weight or in a higher ratio to CAR, or a system of lower total gum content.

Rheological properties of mixed gels of konjac glucomannan (KGM) of four different molecular weights with kappa-carrageenan (CAR) were studied by dynamic viscoelastic measurements and tensile testing. Both storage and loss moduli for sol and gel state mixtures containing higher molecular weight KGM were highest. However variation in molecular weight of KGM produced little effect on the gel-to-sol and sol-to-gel transition temperatures of the mixtures. Two systems containing KGM of lower molecular weights showed almost the same stress—strain curves in tensile testing. The breaking stress and the breaking strain for mixed gels became larger with increasing molecular weight of KGM, and the difference was larger than that observed in the dynamic viscoelasticity measurements. It was suggested that weak junction zones are created by the addition of KGM and that the number of weak junction zones and the contour length of flexible chains connecting these weak junction zones increase with increasing molecular weight of KGM.

Gelation kinetics of soybean 11S protein by glucono-δ-lactone (GDL) was investigated by dynamic viscoelastic measurements. The gelation time decreased with increasing GDL concentration at a constant 11S concentration (4%). The mechanical loss tangent at the final stage of the gelation decreased with increasing protein concentration, but was independent of the GDL concentration. The gelation time of systems which consist of 11S and GDL at a fixed ratio decreased with increasing 11S concentration. However, the time at which the gelation curve reached saturation did not decrease so much with increasing GDL concentration at higher concentrations of the protein. It was found that the decrease of pH by addition of GDL greatly contributes to the gelation time. The relationship between the gelation time and 11S concentration at low concentrations suggested that the minimum 11S concentration for gelation is 1.03% and the junction zones of the systems are formed by 2.6 single polymer chains.