The decision of whether a mud crab should be retained at harvest has traditionally been based on shell hardness. This is most commonly assessed by using thumb pressure applied to the carapace of the mud crab. The carapace of a recently moulted mud crab will flex considerably and is therefore returned to the water. This assessment has also been used to divide mud crabs into three meat fullness grades (A, B and C). The higher meat fullness grade A mud crabs fetch a greater price at market compared to the lower B and C grades. The subjective nature of this assessment will always result in disputes at the boundaries of the grades. By developing a more objective science-based method downgrades at the market will be reduced while consumer satisfaction and the overall industry profitability will increase.
A scoping study was conducted that evaluated innovative non-invasive technologies to assess mud crab meat fullness based on percentage yield recovery of cooked meat from the dominant individual mud crab claws. The non-invasive technologies assessed included near infrared spectroscopy (NIRS), candling using visible light, and acoustic velocity. NIRS showed the most potential and was reassessed in a second study with slight improvements to spectra capture methods and NIR light sources.
94 live mud crabs from the Moreton Bay area were used in the second study. Partial least squares regression (PLS-R) was performed to build a calibration model to predict the percentage yield recovery of cooked meat based on the spectral data. The PLS-R had R2=0.77 and RMSECV=4.8.
A principal components linear discriminant analysis (PC-LDA) was also conducted to discriminate between the standard three grades of mud crab meat fullness. This was compared to the industry standard shell hardness method. The NIRS PC-LDA achieved a minimum of 76% correct classification for each of the three grades, compared to 24% for the shell hardness method.
The non-invasive technologies trialled along with the results will be discussed in this talk.