The Mechanical Properties of Starchy Foods in Relation to Texture and Digestibility

This dissertation, "The Mechanical Properties of Starchy Foods in Relation to Texture and Digestibility" by 隋中泉, Zhongquan, Sui, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled THE MECHANICAL PROPERTIES OF STARCHY FOODS IN RELATION TO TEXTURE AND DIGESTIBILITY Submitted by ZHONGQUAN SUI for the Degree of Doctor of Philosophy at The University of Hong Kong in December 2007 Starchy foods are some of the most important in the human diet. Some researchers have even attributed the dominance of humans as terrestrial organisms today to the invention of cooking, in that heat treatment facilitates a great increase in starch digestibility, so liberating a huge energy resource. This study was aimed at pinpointing exactly what cooking does to starchy foods and how this affects its treatment by the mouth, in particular, and by the gut, in general. The model product chosen was a starchy food - Chinese noodles. The mechanical properties of two noodle types were examined as a function of cooking time. Noodles were boiled for 2-14 minutes and tested at regular intervals from a raw to an overcooked state. The optimally-cooked state was defined as approximately 7 minutes by sensory (taste panelist) assessment. Stress-strain and energy dissipation characteristics of noodles, both pre-fracture and post-notching, at all cooking times were examined in detail. The true stress-true strain curves started out as r-shaped convex curves that became linear around the sensorily-defined 'cooked' state and then subsequently become concave J-shaped curves in overcooked noodles. Viscoplastic energy dissipation was >50% per cycle in uncooked noodles, but decreased by an order of magnitude with cooking, turning the material by 7 minutes of boiling into what was effectively an elastic band. Fractional dissipation values remained approximately constant at longer cooking times. Toughness, judged by energy dissipation and by a nonlinear critical stress intensity factor, dropped by about half with cooking, with plateau values after about 7 minutes. Overall, a greater effect of cooking was seen in viscoplastic dissipation characteristics than in fracture resistance, but a linear stress-strain relationship was found to be probably the best correlate of taste panel judgments on cooking. Digestibility studies using an in vitro technique, showing that cooking increases amylase digestibility. However, all improvements were seen after brief cooking times, showing that panel judgments on the optimally-cooked state did not relate to chemical aspects of digestion. To study the basic effects of mastication, six human subjects were asked to chew noodles after the various cooking times and count chewing time taken before swallowing. The chewing time dropped when noodles were cooked. The thin form of noodles proved unsuitable for such studies, so starch gels were made into cubes measuring 12.8 mm 4 mm and loaded by dental models of upper and lower incisors. The results suggested that friction is likely to play a role in textural judgments. In summary, this study, which plots the evolution of a starchy food from a viscoplastic mixture into an essentially elastic material in the space of 7 minutes, has broad implications for understanding what cooking does to food materials and suggests what an exacting 'materials science' approach can achieve in characterizing foods. In particular, the results show that textural assessments by consumers of the optimally-cooked state of foods may have a physical rather than chemical basis. DOI: 10.5353/th_b3955775 Subjects: