It might be, firstly, necessary to draw attention to the misconception of cohesiveness in the food science literature. Bourne27 was impressed by how many texture terms (more than 450, see ref. 28) are used in Japan. However, as mentioned in his textbook, “cohesiveness” is not included in the most frequently used ten texture terms, neither in USA, nor in Japan or Austria, although conceptually, it is thought to be an important term to describe food texture. A possible reason could be that the significance of the term is difficult to be grasped or represented.

In order to define cohesiveness of a fluid bolus, an overview of the historical usage of this term for solid particles (such as powders or granules) is necessary. The term “cohesiveness” was first defined in food science as the strength of the internal bonds making up the body of the product29 and it was determined experimentally as the ratio of the area under the curve of the second bite to that of the first bite in texture profile analysis (TPA).30 However, the physical meaning of this concept was not clear, because the x-axis in TPA was the time, and Peleg31 changed it to the deformation, so that the area under the curve would represent the work or energy.10,32 Back in 1977, Peleg used the term “cohesiveness” to represent the state in which particles stick to each other and agglomerate to form a lumped mass.26,33,34 Therefore, an increase in cohesiveness could induce a decrease in flow-ability.

Flow-ability of solid particles is characterized quantitatively by the flow index, the ratio ff c of unconfined yield strength to the consolidation stress. The larger value of ff c indicates the higher flow-ability, where the consolidation stress and the unconfined yield strength are determined in the uniaxial compression test of solid particles filled in a hollow cylinder and those without a hollow cylinder.35 The flow behaviour of solid particles is classified by the value of the flow index ff c , as: ff c < 1 not flowing, 1 < ff c < 2 very cohesive, 2 < ff c < 4 cohesive, 4 < ff c < 10 easy-flowing, 10 < ff c free-flowing.35

The flow-ability of solid particles has been evaluated by modified Jenike’s shear cell, which is used for the analysis of flow of the powders and granules in bins and hoppers.36 It was found that the flowing behaviour of solid particles depends on particle size and distribution, particle shape, particle surface roughness and friction, surface electric charges, voidage, moisture and temperature.33

Fluid cohesiveness may be quantified also by the degree of fragmentation of a fluid which flows out from the space such as a tube or a nozzle where it has been confined, as is commonly done in spray drying process. Although cohesiveness of powder has been studied extensively in spray drying technology using a simple index called Hausner ratio defined as the ratio of the bulk density and tap density,37,38 the cohesiveness of a fluid droplet before drying into a powder state has not been experimentally determined as far as the authors are aware.

Prinz and Lucas25 pointed out the importance of cohesiveness in the oral processing. They proposed an optimization model: food is crushed into small particles, by mastication, and bound together by salivation when viscous cohesion is promoted, leading to bolus formation. By assuming that the particles are spherical, the cohesive force, F V − F A , was defined as the difference between the viscous force and the adhesive force. The term cohesive force represents the force that makes cohere or stick the like particles each other while adhesive force signifies the force which makes particles cohere or stick to the organs of oral cavity. Prinz and Lucas assume the viscous force as an attractive force between two disc-like surfaces, the gap of which is filled with saliva, where the disc-like surfaces are on the imagined section in the centre of this spherical particle. If swallowing is delayed, particles are separated by excessive saliva and the cohesion is reduced. Prinz and Lucas25 examined the validity of the model for humans using Brazil nuts and raw carrots. They found that both foods, in spite of the difference in mechanical characteristics, are swallowed at approximately the same number of chews, when the cohesive force becomes maximum. According to their model, the time at which the cohesive force becomes maximum is the optimum moment for swallowing. Although the concept of viscous force and adhesive force introduced by Prinz and Lucas25 is interesting and may be a starting point for the further clarification, it is still difficult to understand and to apply these concepts for real boli at molecular level.

Examining the particle size distribution of the bolus produced from 3 g petal wheat-flake cereals collected at different stages of mastication sequences, Peyron et al. (2011)39 found that the cohesiveness increased from the middle stage to the final stage just before swallowing while the particle size and the hardness of the bolus decreased with progressive mastication. They also found that the dryness sensation increased just like the cohesiveness, which was attributed to the absorption of saliva into the bolus. This behaviour might be observed for boli obtained with wheat-flake cereals, but may not be generalized to other solid foods containing more water. Peyron39 recognized a narrow variability in particle size of the food bolus just before swallowing, in contrast to a broader variability of the physiological parameters.40,41,42 However, since Fontijn-Tekamp et al. (2000)43 reported a greater variability of the particle sizes in the bolus, Peyron et al.39 stated that other determinants (e.g. rheological/saliva content) are probably involved in the swallowing threshold concept. Jalabert-Malbos et al.40 found that the mean particle size of food bolus should be smaller than a certain critical value, which supports Hutchings and Lillford’s model, suggesting that this critical size should be small for hard brittle foods (e.g. peanuts) and much larger for softer foods. The reason for the larger critical size for softer foods was attributed to the lower liability of softer foods to injure the upper digestive mucosae, which is consistent with findings for grittiness sensation. Harder particles are felt gritty at a smaller size than softer particles.44,45,46

However, it is still difficult to identify only one critical factor triggering the swallowing considering that swallowing threshold is probably an integrative process combining the perception of various bolus properties39.

Chen and Lolivret24 questioned the statement of swallowing at the maximum cohesive force introduced by Prinz and Lucas25 because swallowing muscles will have to work much harder to create a high enough oral pressure to push bolus through the oropharyngeal system and, therefore, it is not the logical choice. They have found that the oral residence time for 28 different fluid foods increased with increasing apparent shear viscosity which is in good agreement of Taniguchi et al.47 who observed that the total swallowing time and oral ejection time increased in the order of liquid < syrup < thin paste < thick paste. The swallowing evaluation was shown to be correlated well with the apparent shear viscosity, but even higher correlation was found between swallowing and stretch-ability. Chen and Lolivret24 concluded that the bolus extensional behaviour could be the key determining factor in triggering the swallowing and that the incorporation of a sufficient amount of saliva improves the flow-ability and stretch-ability of the bolus for a safe and comfortable swallowing. This was recently supported by Morell et al.48 This assumption clearly denies the statement of Prinz and Lucas25 in which excessive saliva will flood the bolus and may induce the risk of aspiration. It should be noted that the swallowing easiness was evaluated by Chen and Lolivret24 in healthy young subjects, who assessed water as the easiest fluid to be swallowed. This appears contradictory to many papers reporting the effectiveness of thickening agents in preventing aspiration.

Devezeaux de Lavergne, van Delft et al.49 supported the hypothesis of Chen and Lorivret24 that flow-ability of bolus is more important than high cohesiveness, since they observed that Young et al.,50–52 experiments on biscuits failed in confirming the hypothesis that cohesiveness is a deciding factor for swallowing, although Young et al. found the validity of this hypothesis for cereal flakes. Based on their observations of dynamic change of boli prepared from emulsion-filled semi-solid food model, gels were perceived either as creamy or grainy in the last stage of oral processing. Only gels perceived as creamy revealed a high bolus flow-ability. Devezeaux de Lavergne, Derks et al.53 compared the oral processing of sausages and found that both, long-duration and short-duration eaters, perceived the same sausage similarly in the early stages of oral processing, but started to perceive its texture differently from the middle of oral processing towards the end. They found no compensation for a shorter eating time of the short duration eaters by applying a higher chewing frequency or an increased muscle effort. They found that the incorporation of saliva into bolus was more significant for longer duration eaters which is in agreement with Tarrega et al.54 They also found that although roundness of fragments was similar at the moment of swallowing in both groups, short-duration eaters generate less broken-down hard sausage boli containing fewer and larger fragments at all eating times than long-duration eaters. Thus, between the two groups, the bolus properties were found different at the end of mastication.

As already mentioned, food cohesiveness was first determined experimentally in the texture profile analysis (TPA).30 Since then, some papers have been published and erroneous applications of TPA to fluids have been practiced, unfortunately. The cohesiveness, defined as the ratio of the energy required for the second compression to that of the first compression in TPA, has some meaning only for solid foods.10,32,55 When a rubber-like material is masticated, the cohesiveness is found very close to one,56 indicating that such food model recovers the initial shape and size after biting. Although such a too rubbery-like food is not disintegrated at all and it may not be liked, the significance of the cohesiveness was clearly shown in this experiment. The ideal elasticity in mechanics has two aspects: (1) the deformation is instantaneous (no time delay), (2) total recovery of the original shape and size after removing the force. Since plastic solids do not recover the initial size and shape, the cohesiveness determined by TPA procedure is zero because the area in the second bite is zero if the solid is purely plastic and not elastic. In solid particles, such as powders and granules, cohesiveness is caused by agglomeration of particles and higher cohesiveness indicates the poor flow-ability as mentioned above.

Rosenthal57 showed that the cohesiveness of a starch gel model decreased from about one to one-third with increasing compression from 25 to 90%, and he interpreted this reasonably consistent with the original significance of the cohesiveness proposed by the originators of TPA: “a direct function of the work needed to overcome the internal bonds of the material”.58 But, he warned about the misappropriation of TPA data since TPA is an easy experiment and the number of papers is rising. He recommends performing TPA at higher compression speed than 2 mm/s where he observed leveling off of the hardness for his model starch gels but below that speed the hardness obtained is much smaller. In most cases TPA is performed to find the correlation with the sensory evaluation, therefore TPA parameters obtained at lower compression speed should not be used.10

Young et al.50 tried to find the swallowing trigger for biscuits with three different sugar/fat ratios, B1 the lowest sugar and B3 the highest sugar content. They preferred to use back extrusion (BE) method proposed by Osorio and Steffe.59 They observed that the consistency index, η, by BE and the peak force by TPA decreased significantly, while the cohesiveness by TPA increased significantly from the middle stage to final stage of processing. They also found that flow behaviour index, n, determined by BE increased slightly from the middle stage to final stage of processing. Since both, the cohesiveness and the consistency index of the boli at the swallowing point decreased with increasing sugar content, they concluded that “cohesiveness” is not a swallowing trigger for the biscuits as it was significantly different for the three biscuit recipes, thus questioned the optimum swallow model of Prinz and Lucas.25 Since the study mentioned above was done based on only one single subject (a 25-year- old healthy woman),51 examined the same problem employing five subjects using BE and modified TPA for a biscuit with the same sugar/fat ratio as B1. They determined the bolus yield stress by BE at each stage, and found that the yield stress decreased from early stage to the point of swallow. This yield stress was found to be comparable to the static yield stresses of cereal bolus at the swallowing point (1.3–4.3 kPa), which was determined by vane geometry.60 Authors found many inter-individual differences in perceived texture and breakdown paths, mastication period, number of chews, and chewing frequency, bolus mass, yield stress, peak force, “adhesiveness”, “cohesiveness”, and description of bolus properties. The authors concluded that one single parameter determining the point of swallow could not be found for their biscuit recipe. Young52 raised the advantage of BE because it could test boluses which still contain larger particles and be carried out using the same simple testing apparatus as TPA. The present authors would not like to discourage him and rather wish the further development of the method, but the consistency index and the flow index that they obtained may have meaning only when the bolus consists of smaller particles. Depending on the mastication ability, some boli contain larger particles and in such cases the diameter of the cylinder and the gap should be enlarged, and then these parameters may not be obtained. This is similar to the usage of falling sphere or needle viscometer where the diameter of sphere or the needle should be increased to get an average rheological value for inhomogeneous bolus containing larger particles.3

If this TPA method is used for fluids it leads to a contradictory conclusion: the “cohesiveness of water” is exactly one (water conforms to the shape of the container, as has been known from ancient times), and much larger than that for xanthan solutions, while the “cohesiveness of xanthan solution” decreases with increasing concentration of xanthan. This is obviously illogical.61 It can be predicted without measurement, but since such a strange situation prevailed in Japan, these authors were obliged to write such a paper.

Even though starch-based thickening agents are still widely used in dysphagia products, in the last years gum-based thickeners gained more and more attention, namely due to their ability to form thicker fluids at considerably lower concentration than starches and to provide higher thickness with lower oropharyngeal residues.62 Another benefit of gums in dysphagia management is the lower degradation by the salivary enzymes during the oral processing, which makes them safer to swallow. Therefore, in the present paper, three polysaccharide thickening agents, xanthan, locust bean gum and guar gum (GG) were selected as model boli and the significance of cohesiveness is discussed based on their fluid extension, acoustic and video-fluorographic analysis of swallowing. The decision to use gums instead of starches for such illustration is that starch is as well, prone to show thixotropic behaviour and viscosity changes during storage, while xanthan gum and galactomannan gums are free from retrogradation.

The main focus of this work is not only to discuss the role of cohesiveness as a triggering factor of swallowing, but also its effect on the swallowing speed of the bolus through the pharynx and therefore, its ability in preventing aspiration. Consequently, this study can be considered as a basis to understand the role of cohesiveness using an ideal homogenous food bolus model which does not require chewing and it is not strongly affected by the salivary enzymes, since chewing and enzymatic degradation would complicate the picture of the swallowing mechanism, as already discussed above. To develop further understanding, it is surely necessary to study more in depth non-homogeneous boli since they represent a vast majority of the real fluids.