Ultrasound and actinidin enzyme treatments : effects on tenderness and in vitro protein digestibility of New Zealand abalone (Haliotis iris) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Manawatu, New Zealand

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Canned pāua, Haliotis iris, is a premium New Zealand product that is exported to Asia. However, meat toughness is a common problem of thermally processed abalone, worldwide. The objective of this research was to investigate the effects of different technologies such as ultrasound and actinidin enzyme treatments on pāua texture, microstructure and in vitro protein digestibility. Ultrasound treatment was chosen as it has been reported to cause tenderisation by physical disruption of muscle cells or release of endogenous enzymes and/or calcium that hastens proteolysis in meat. Actinidin enzyme from kiwifruit has been reported to tenderise tough beef cuts by hydrolysing the myofibrillar proteins and connective tissues. Whole pāua meat was pre-treated in ultrasound (20 kHz, 464 ± 9 W) for 5 min in water (with or without subsequent soaking in water at 4 °C for 24 h), or in 1% actinidin solution. Post-treatment cooking of canned pāua was done in a water retort at 116 °C for 30 min. All ultrasound pre-treated cooked pāua yielded lower slice shear force values (SSFV) than untreated canned and cooked samples; the lowest SSFV was attained when ultrasound pre-treatment in water was followed by soaking at 4 °C for 24 h. The increased tenderness in ultrasound pre-treated cooked pāua could be linked to the observed changes in the microstructure. Histological analysis of raw samples revealed disintegration of myofibers and formation of gaps between myofibers. Transmission electron micrographs (TEM) of raw pāua showed collagenous fragmentations in ultrasound pre-treated samples, and this was more pronounced in pāua ultrasonicated in enzyme solution, which also exhibited myofibril fragmentations. Cooked control pāua muscle fibres appeared very compact while ultrasound pretreated samples had wider spaces between myofibers. In vitro protein digestibility was determined for raw and cooked control; and cooked ultrasound pre-treated pāua. Raw pāua exhibited significantly higher (p < 0.05) free amino N values during in vitro digestion and also protein breakdown as observed through SDS-PAGE than cooked control and ultrasound pre-treated samples However, cooked ultrasound pre-treated pāua was more digestible than control cooked sample. In conclusion, ultrasound treatment in water for 5 min, followed by soaking in water for 24 h caused changes in the microstructure of pāua muscle tissues, which led to increased tenderness of cooked pāua. The SSFV for this sample was 31% lower than the control cooked and a commercial canned sample.