Pāua, the Māori name for an abalone species found in New Zealand, are highly valued by Māori and are recognised as a taonga
, or treasure. Their colourful shell was traditionally used to decorate wood carvings, and today they are used for making iconic jewellery. The flesh is considered a delicacy. There is a large commercial market for their flesh, primarily in Asia. It is a highly priced New Zealand export product that is sold live, fresh or in processed form. In the five years to 2015, on average, the total value of pāua caught was $
56.9 million [1
]. The most important commercial New Zealand species is Haliotis iris
or the black foot pāua, which is among the largest abalone species in the world.
Meat toughness and chewiness are common problems associated with abalone species in many parts of the world [2
]. There is a wide range of literature available on the use of different processing techniques for tenderising abalone, mostly for Asian abalone species. Several articles focus on the effect of thermal [3
] and high-pressure processing on abalone [9
]. However, limited studies have been done on Haliotis iris
, and these have been mainly focused on the effects of diets [11
], habitats and physiological and environmental conditions [13
] on the quality of the raw abalone meat.
Studies by Zhu et al. 2018 [14
] have shown the potential of the proteolytic enzyme from kiwifruit, actinidin, and its combination with sous-vide cooking in tenderising tough beef cuts. Positive results on red meat texture have also been reported for non-thermal processing methods, such as ultrasound and marinating [15
]. Changes in the muscle structure, induced by processing, have been reported to be determinants of product texture and digestibility [14
]. Cepero-Betancourt et al. [16
] examined the protein digestibility, in rats, of dried abalone, Haliotis rufescens
, pre-treated using high pressure processing. From the nutritional parameters, which included the amount of essential amino acids (EAA), protein efficiency ratio (PER), true digestibility (TD), net protein ratio (NPR) and protein digestibility corrected amino acid score (PDCAAS), the control untreated abalone sample exhibited high-quality nutritional protein [16
In the current study, it was hypothesised that the structural changes induced by ultrasound would not only affect texture, but would also affect the protein digestibility of canned pāua. Addition of actinidin enzyme during ultrasound treatment might allow penetration of the enzyme into the inner meat layers that could lead to further tenderisation of pāua meat. Therefore, this study was focused on investigating the effects of ultrasound alone or along with actinidin enzyme treatment on the tenderness, in vitro protein digestibility and microstructure of canned meat from Haliotis iris.
The objective of this study was to identify processing techniques that will lead to the tenderisation of canned pāua and to obtain information on the in vitro protein digestibility of the product. Some preliminary experimentation was performed using ultrasonication and actinidin enzyme solution, individually and in combination. The cooked pāua appearance was evaluated based on industry standards for canned pāua [41
]. The epipodium should retain its defined shape, and the adductor should be smooth without any damage or cracking. The treatments presented in this study were selected as they led to lower SSFVs than the control canned commercial sample but did not cause any undesirable effects on the appearance of the canned paua. Automated injection of enzyme solution and soaking pāua in higher enzyme concentration (5% w
) were eliminated from further studies, despite their tenderising potentials, due to their adverse effects on the appearance of the cooked pāua. These treatments led to cracks on the surface of the pāua and caused sliminess and loss of definition on the pāua lip, which were deemed unacceptable.
Ultrasound pre-treatment in water for 5 min followed by soaking in water for 24 h at 4 °C, yielded the lowest average SSFV (42.88 N) and the samples were on average 31% more tender than the canned control (61.99 N). Increased tenderness caused by low frequency, high-intensity ultrasound applied for a sufficient time, has previously been reported in several studies on beef muscles [42
]. The average SSFV exhibited by the above-mentioned sample was within the reported acceptable slice shear force reported by previous studies. Dong et al. [6
] reported highest sensory acceptability in terms of structure (meat integrity), hardness and elasticity for cooked abalone that yielded shear force values of 36.52 ± 5.19 N. Shear force (Warner-Bratzler) values have been strongly positively correlated with the sensory hardness and chewiness (product of hardness, elasticity and cohesiveness) of abalone [3
]. In a different study, Zhu et al. [5
] recommended a heating temperature for cooking abalone that yielded an average SSFV of 56.47 ± 9.79 N. There were differences in the methods for shear force measurements, such as sample thickness and type of blade, which could have affected the values.
The commercial canned samples had comparable SSFV (68.54 ± 6.58 N) with canned control (61.99 ± 15.40 N), which means that the treatments have shown potential in improving the tenderness of canned pāua. However, the commercial samples are usually cleaned in brine, bleached and were cooked in a medium that has salt and sugar, which could affect abalone meat tenderness. Ultrasound treatment may be applied during the cleaning steps prior to canning, but industrial practices use salt solution for this process [45
], which may have an influence on abalone tenderness. Further investigations on these interactions are needed.
Changes in the structure of the muscle fibres during cooking related well to the texture measurements. Significant disruption of muscle fibres and a widening of extracellular spaces was seen in ultrasound-treated samples and this could be responsible for their lower SSFVs compared to the control. Although the ultrastructure of the cooked sample pre-treated by ultrasound in enzyme showed much-staggered myofibrils, its mean SSFV was higher (but not significantly, p
> 0.05) than the mean SSFV of samples ultrasonicated in water. Cooking led to a narrowing of the extracellular spaces, with the structure becoming more compact than the raw uncooked pāua. This could be due to loss of water as cook loss. Similar results have been reported by Kaur et al. [46
] after cooking at 100 °C for beef muscles. Hatae and co-authors [8
] reported an increase in the total amount of free amino acids and oligopeptides in the meat drips after extended cooking, and this was attributed to the shrinking of the meat that squeezed out these components [8
]. Shrinkage of the myofibrillar proteins could be observed in the TEM images of cooked pāua (Figure 6
, Section 3.2.2
). New inter- and intra-protein interactions arising from exposure of hydrophobic areas, along with shrinkage of some proteins such as titin during cooking, can result in a compact protein structure [47
]. Although the pāua samples in the current study were cooked for only 30 min, the temperature used (116 °C) was substantially higher than that used in previous studies. Other than the cooking time, the increased temperature has also been implicated in the reduction of soluble protein in cooked meat [48
]. In agreement with the previous literature reports, cooking of pāua also led to a change in the protein breakdown pattern as observed through SDS-PAGE along with a significant reduction in soluble nitrogen during simulated gastric digestion. There was a relatively small amount of gastric hydrolysis as observed from free amino N despite the loss of some intact protein as seen in the gels. Pepsin preferentially cleaves adjacent to aromatic amino acids (e.g., phenylalanine, tyrosine and tryptophan) [49
], but it does not hydrolyse peptide bonds adjacent to valine, alanine or glycine [50
]. It converts proteins to smaller peptides and is reportedly responsible for less than 20% of protein hydrolysis during gastro-small intestinal digestion [51
]. Additionally, the low pH (3) of the simulated gastric fluid might have induced the formation of a matrix of coagulated proteins that were insoluble and resistant to further pepsin hydrolysis, thus explaining the lower free amino N values during the gastric digestion.
A decrease in protein digestibility has been reported after cooking by Santé-Lhoutellier et al. [52
] and Kaur et al. [46
] for pork and beef meat. In contrast, Shi et al. [53
] stated that heating did not affect the digestibility of proteins in abalone, Haliotis discus
. The paper did not indicate the temperature and time combination used in heating. A negative and strong correlation with pepsin activity has been reported for carbonyl group formation and aggregation that was induced by cooking [52
]. In the present study, pāua samples were cooked at 116 °C for 30 min, and protein aggregation was observed in the electron micrographs, however, the differences observed in free amino N among raw and cooked samples are consistent with the loss of soluble protein components during cooking. SDS-PAGE results showed a group of peptides (MW 77 and 75 kDa, Figure 8
J and Figure 9
A) that are seen in the raw pāua digest during gastric digestion, that were resistant to further digestion and could be observed at the end of small-intestinal digestion, were not seen in the cooked samples. Similar bands were observed during the gastric digestion of raw beef meat by Kaur et al. [46
] They reported these peptides to be the hydrolysis products of a myosin-heavy chain (MHC, 220 kDa). Meat is oxidised and denatured upon cooking, leading to MHC’s breakdown into even smaller MW peptides, which could be the reason for not observing these bands in the cooked meat digests. Bands corresponding to HMW protein aggregates could still be observed in all the digests at the end of digestion, with their higher intensities observed for the control-cooked digest. A recent study on the digestion of the myofibrillar protein fractions of raw abalone, Haliotis discus,
showed some 60–200 kDa protein bands that were seen after 120 min of pepsin-trypsin digestion [53
]. The combination of thermal treatment and ultrasonication of cowpea proteins has been reported to contribute to protein unfolding, which has been suggested to lead to increased hydrolysis [54
]. This could explain the higher protein digestibility of ultrasound-treated cooked pāua compared with cooked pāua muscle.