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In the bakery industry, glucose oxidase is usually used in combination with xylanase. Although many theories exist on the mechanism of action of each enzyme, the positive effect of combining the two is as yet unexplained. In this paper we studied a possible basis for this synergy by focusing on the involvement of arabinoxylans. Water-extractable arabinoxylans and water-unextractable solids were studied in dough with or without glucose oxidase. Addition of glucose oxidase led to a stiffer and less extensible dough. Addition of arabinoxylans caused a further decrease in dough extensibility. Addition of xylanase could correct for this decrease. The role of arabinoxylans was further investigated by studying modified water-extractable arabinoxylans and the effect of agents affecting arabinoxylan crosslinking (ferulic acid). Water-extractable arabinoxylan was modified with xylanase to generate a low-molecularweight fraction (WEAX XYL ). Alternatively, it was modified with NaOH to prepare a fraction with a decreased ferulic acid content (WEAX OH ). Addition of modified arabinoxylans diminished the effect of glucose oxidase. Glutenin macropolymer and water-extractable arabinoxylan viscosity experiments were performed to obtain further detail on the role of arabinoxylans (AX). These results give rise to a new theory explaining the contribution of xylanase in its synergy with glucose oxidase. In this theory glucose oxidase not only catalyses the formation of protein disulfide bonds, but also of AX-AX crosslinks. The latter negatively affect bread quality. Xylanase corrects for this latter effect by cleaving arabinoxylan complexes and generating small ferulic acid-containing arabinoxylan fragments interfering with the crosslinking of high-molecular weight arabinoxylans.

A detailed study was performed to simultaneously measure the mechanical and acoustic properties of crispy cellular solid foods. Different critical aspects are discussed in order to assess optimal test conditions. These are primarily data sampling rate, microphone positioning, frequency spectrum of interest, sound/noise ratio and selection of measuring probe. A data sampling rate of more than 50 kHz was shown to be sufficient to register fracture event and acoustic event, and the frequencies audible by human ear (at least 40 kHz needed). The optimum positioning of the microphone with respect to the test piece should be a compromise between a distance that the microphone registers a good sound over the whole human audible frequency spectrum and a good sound/noise ratio. It is shown that test method selection has to depend on whether the goal is determining material fracture behavior or correlation of data to consumer perception. The best method from a fracture mechanics point of view does not have to be the best choice for a combined fracture and acoustic measurement. PRACTICAL APPLICATIONS The method described will especially be useful for the study of materials that fracture in a brittle way whereby during the fracture process, an audible sound is emitted (as is the case for crispy and crunchy food products). Although the data reported are for dry products, the method will be relevant for the study of all kinds of crispy/crunchy products. This work also shows that for a study directed on elucidating the mechanisms determining crispy/crunchy behavior of foods, a much higher data sampling rate is required than used in most studies published in literature. Moreover, guidelines are given for the positioning of the microphone and the selection of the measuring probe for measuring simultaneously the mechanical and acoustic fracture properties of crispy/crunchy cellular solid foods.

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