The problems in this study

In this study, actually, a total of 20 fish were tested. However, because of the unstable excitation power for the xenon lamp and because there were other spectra areas that we tried to test, the study was limited to eight fish subjects. In future work, we might use a laser as a power source for its stability and its concentration of energy. In addition, the delay for several minutes between slaughtering the fish and performing the initial measurements could have resulted in missed information related to the fish tissue.

Also, in this study, a Y type fiber was adopted to receive the fluorescence. Although fixing the fiber directly to the tissue surface might have prevented errors related to hand movements, the fluorescence received was only from a small point of fish tissue. This was done to reduce the non-homogeneous effect of fish tissue, but separating and measuring more parts of fish tissue could be beneficial in future research.

4.7 To develop a portable detector for fish freshness

4.7.1. The potential of nitrogen laser as excitation power

The 337 nm wavelength of a nitrogen laser would be a proper power source for a fish freshness detector. Nitrogen lasers have been proven to cause little destruction to tissue [10].

With the less invasive character of a nitrogen laser, it could be commercialized for the development of a portable instrument.

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4.7.2. The usage and problem of G index for freshness

From my study, the sliced fishes were measured to analyze the spectrum. However, for now, it only can be applied as the freshness detector of sashimi or other fish fillets as a quick-detection method. The challenge is to investigate the change of spectra of fish muscles beyond spot measurements. By confirming the spectra change of muscle in the whole fish and developing a standard, the method can be applied as a faster and less invasive detection method.

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V. Conclusion 5.1 The optical index

The fluorescence spectrum can present the concentration of type I collagen, type V collagen, and NADH. NADH is the energy source of the live body. When fish die, the fish muscles start glycolysis to maintain the energy of the fish muscles. After the fish muscle is acidified by the accumulation of many lactic acids from glycolysis, it stops generating NADH or ATP. At this point, the NADH may degrade. Also, it has been proven that collagen is related to the resolution of muscle. Collagen is a component of fish muscle structure and has been proven to participate in the tenderization of muscle. When collagen is transformed, the texture of fish may soften, which indicates the breakdown of muscle structure. NADH and collagen are the materials that maintain the normal metabolism and structure of muscle, and they may degrade soon after a fish dies. Thus, they play important parts in the early process of fish putrefaction. From this study, two species of fish and a total of 16 fish samples were analyzed. The G index value that we developed might descend from approximately 1.5 to 0.5 with 24 hours of refrigeration with an ascending K value. Therefore, based on the G index we developed from the fish, the condition of the fish can be confirmed. If the G index of the fish is higher, the fish could be assumed as a good quality or fresh fish which has longer expiry date. On the contrary, if the G index of the fish is lower, the fish can be assumed as a bad quality or “less fresh” fish which has shorter expiry date. In other words, we can use the G index to validate freshness in the early stages of fish putrefaction, and it can act as a measure of taste and freshness of sashimi or fish fillets.

5.2 The application of the index

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For the light source for the optical index, the nitrogen laser has been commercialized for many years and is appropriate for our optical system. Also, because of the laser’s less destructive wavelength to tissue [10], the index has great potential for freshness detection [33].

The index can be used to monitor the quality of sashimi in restaurants or for quality control in the aquaculture market. Restaurants and the aquaculture market have strong operation performance standards and are capable of applying these indexes. With the G indexes, they can detect the freshness of fish faster.

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VI. Future Work

Up to now, we have only used 1D signals to analyze the EEM signal. However, 2D matricess of EEM may contain more information that we have not found. A 2D algorithm has faster computation and is more appropriate for 2D signal processing. It is a method to analyze the EEM spectrum for other fluorophores. Also, with a 2D algorithm, maybe more target fluorophores can be discovered. Second, the amount of fish data is not enough to prove how the index works with other fish species. Accumulating more data of other fish species is necessary. Finally, if the optical index is to be applied in the food or restaurant industries, we need to find more useful indexes and combine them to build a valuable portable instrument.

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Appendix A

Abbreviation Full name

ADP Adenosine diphosphate

AMP Adenosine monophosphate

ATP Adenosine triphosphate

EEM Excitation emission matrix

HPLC High performance liquid chromatography

Hx Hypoxanthine

HxR Inosine

IMP Inosine monophosphate

NADH Nicotinamide adenine dinucleotide PCA Principle component analysis

PMT Photomultiplier tube

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Appendix B

Function Description G(emi.,exci)

where F(emi, exci) denotes the fluorescence amplitude of certain excitation wavelength (exci nm) and emission wavelength (emi nm) and F(exci+50, exci) denotes the fluorescence amplitude of certain excitation wavelength (exci nm) and emission wavelength (exci+50 nm). The G index is developed as a optical index for freshness.

where [ATP], [ADP], [AMP], [IMP], [HxR], [Hx] mean the concentration of themselves. In most of fish, K values increase linearly during the first days of refrigeration storage and it is an index of freshness detection. When K value is higher, it means the less fresh of fish meat.

r value value is between -1 and 1 which can compare the relationship between x and y variables.

The value of recovery is used as the efficiency for our experiment process.

Based on the value, it can confirm the quality of our experiment method.

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where x , S, and n denote as mean, standard deviation, and the number of {x_i}, respectively. RSD is used to confirm the precision of HPLC results.