Novel methods to characterise texture changes during food breakdown : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosphy in Food Technology at Massey University, New Zealand
The purpose of the mastication process is to break down food for bolus formation so that it can be swallowed safely. Although light has been shed on the criterion for a swallow safe bolus, quantifying these in terms of the bolus properties is not fully understood. There is a lack of suitable measurement techniques to quantify these identified bolus properties. Thus, the purpose of this work was to develop novel techniques that would be useful in in-vitro studies of food breakdown for the characterisation of bolus properties.
A mastication robot (MR) had been previously developed to enable the reproducible mastication of food so that masticatory efficiency and food breakdown dynamics can be assessed quantitatively. To evaluate if the MR could be a controllable and reproducible alternative to subjects for food break down studies, a series of experiments involving the mastication of peanuts using a range of machine parameters was conducted. The bolus particle size distributions were used to characterise the breakdown of the peanuts. There were significant differences in the average particle size of the particles chewed by the different chewing trajectories during the initial chews. The performance of the mastication robot was validated against human subjects (n=5) by comparing the particle size distribution (PSD) of peanut boluses collected from subjects and the MR. Although the MR was unable to achieve similar breakdown capability as that for the human subjects, the MR proved to have good reproducibility in bolus preparation.
Two novel techniques were developed to characterise bolus properties. The slip extrusion test was developed to characterise two determinant properties for safe swallowing, the bolus deformation and slippage properties. The test measures the force needed to extrude a bolus through a test bag imitating the swallowing action of a bolus. The multiple pin penetrometer was previously developed to measure the spatial distribution of texture in foods exhibiting heterogenous structures. The forces experienced by each pin is measured independently as they pushed through the food, providing a pressure distribution for each food. This allowed the characterisation of fibrous (non-fracturable) foods in a similar way to PSD analysis, offering a method to characterise boluses that do not form discrete particles. The variability in the structure of the boluses was also characterised using the grey level co-occurrence matrix through the image textural features: contrast, energy and homogeneity.
Finally, these developed novel techniques were applied to five real foods with varying textures to show how the MR and these techniques may be used to characterise the changes in bolus properties across the mastication stages. Subjects (n=5) were asked to masticate the foods to determine their chewing behaviour and the bolus properties (deformation and slip properties) at swallow point. The chewing parameters from the median subject (subject A) was used to establish the parameters for the mastication robot’s set up for the factorial design of experiments. The developed models from the factorial study were used to optimize the conditions needed for the MR to achieve boluses with similar DR and SR properties as subject A. The five foods were then broken down using the MR configured in this way, and bolus properties were evaluated at various stages of the mastication process through the application of the slip extrusion test, textural mapping using the multiple pin penetrometer, and the back-extrusion test. Factor analysis was applied to the various data collected, which showed that the properties related to the hardness, swallowability and homogeneity attributes were best at describing the changes in the boluses as they were masticated to swallow point.
In conclusion, the mastication robot could be used to replicate human chewing trajectories to consistently produce boluses in a controlled trajectory with controlled “simulated saliva” rates throughout the various stages of mastication. Thus, it is relevant as a tool to produce boluses for comparative analysis especially for studies investigating the properties of boluses collected from various stages of the mastication process. In addition, the developed characterisation techniques could be used to track the dynamic changes in the bolus properties for most of the mastication stages from initial chews to the swallow point and beyond that.
Figures 2-6 (=Ogawa et al., 2001 Fig 2) & 2-12 (=Hutchings & Lillford, 1998 Fig 1) were removed for copyright reasons. Some possibly copyrighted Figures remain for the sake of clarity, while other Figures are in the public domain.