Methods of Quality Assessment of Fish
a. Biochemical methods
A variety of chemical compounds or groups of compounds accumulate post-mortem fish muscles. These chemicals are either intermediate or end products of biochemical changes occurring in the muscles of fish after death.
Proximate composition: Because of influence of chemical composition on keeping quality, proximate chemical composition like moisture, lipid, protein and ash contents of fish samples from the time or day of harvest to different storage periods or conditions are often investigated. Proximate composition may vary with species, sex, season, place of harvest, feeding condition, etc. So, conclusive results are very difficult to obtain.
Hypoxanthin value:
As a consequence of post-mortem changes, breakdown of ATP to ADP, AMP, IMP and finally to hypoxanthin takes place. Hypoxanthin content of muscle increases on storage ofmfish. Estimation of hypoxanthin is an objective test of freshness of fish. However, the estimation of hypoxanthin is too cumbersome and it is seldom employed in practice. Fish with a hypoxanthin value of 7-8 micro moles/g is considered spoiled (Gopakumar, 2002a).
Histamine content:
Histamine develops in freshly caught fish after 40-50 hours of death, if the fish is not properly iced. To avoid histamine formation in tuna, skipjack and mackerel, care is taken to ice or freeze fish as quickly as possible. Histamin is a major problem in warm water pelagic species that causes a form of food poisoning known as scombroid poisoning, as the name derived from the family name of tuna and mackerel, scombroidae. The term is also applied to other family members too. The symptoms of scombroid poisoning are facial flushing, rashes, headache and gastro-intestinal disorders.
Histamin is produced from histidine which is one of the constituents of muscle protein of all fishes. Generally, pelagic fishes contain sizable amount of histidine in Free State as well as bound state within protein. Upon death, histidine in fish muscle is converted to histamine by bacterial enzymes. Histamin content over20mg/100g in canned fish is prohibited by the US-FDA standard.
pH:
Change in pH of the fish muscle is an usual good index for freshness assessment.
Trimethylamine:
Marine fish contains sizable amount of trimethylamine oxide (TMAO) which is reduced to trimethylamine (TMA) during the spoilage of fish. TMA determination is a useful index to measure the quality of stored fish. Fish with a level of 1.5 mg TMA nitrogen/100 g fish is considered acceptable, while 10-15 mg/100 g limit is set for moderately spoilt fish and beyond this range is set for highly spoilt fish (Gopakumar, 2002a).
Total volatile base nitrogen (TVBN): A number of volatile bases like ammonia are released in fish during decomposition t) bactaria.
A limit of 35-40 mg TVBN per 100 2 of muscle is considered acceptable for Good quality fish, while a value of 50-70 mg/100g of muscle can be taken as upper limit beyond which fish is considered inedible. For salted and dried fish the range is 100-200 mg/100 g beyond which the products are marked unacceptable.
Peroxide value:
Oxidative rancidity developed in fish tissue is determined by the estimation of peroxide value. Good quality fish should have a peroxide value quite less than 10. Peroxide value above 20 for any fish is considered rancid.
Thiobarbituric acid value (TBA):
It is also determined as an index of oxidative rancidity in fish. For a good quality moderate lipid fish. TBA value of less than 2 is usually accepted.
K value:
High level of adenosine related compounds or inosine mono phosphate in fish muscle imparts sweet, meaty flavor and is regarded as a reliable index of freshness. Post-mortem accumulation of inosine or hypoxanthin generally reflects poor quality. The conversion of ATP to LMP is very fast and is usually complete within a day. Subsequent accumulation of inosine or hypoxanthin is related to both autolytic and microbial action. Based on hypoxanthin values and their correlation with freshness of fish, Saito et al (1959) first proposed the K value as an index of the freshness of fish. K value is calculated from the values of hypoxanthin, inosine and total nucleotide levels in fish at the point of measurement as shown in the equation (i). However in practical situation adenosine compounds are deleted because of their conversion into IMP, HxR and Hx, shown in equation (ii). In freshly caught fish K value would be as low as zero, in moderate quality fish the value could be 10 to 20, but in spoilt fish it can go upto 90 (Gopakumar, 2002a). K vales are found to have excellent agreement with sensory data of fish. K values also rise with the ice storage period of fish.
HxR + Hx
K Value = x 100% (Theoretical)…(1)
ATP+ADP-i-AMP+IMP+HxR+Hx
HxR + Hx
K Value = x 100% (Theoretical)…(2)
IMP+HxR+Hx
b. Biological method
Total plate count (TPC):
Total number of microbial flora is changed with the time in fish or fish products. The numbers per gram of fish or fish products or per square centimeter of the surface area indicate the quality of fish from the microbiological view point. Total plate count or viable bacterial count is determined by the culture of bacteria present in fish sample using a suitable bacteriological media that could recover maximum number of bacteria in fish tissue. A known weight of fish sample is minced aseptically and serial decimal dilutions are pour-plated with the media. For marine fish aaar agar and for processed fish products tryptone glucose beef extract agar media are commonly used. Inoculatd plates are incubated at 37'C for 24 hours and the bacterial colonies are counted. From the colony Counts, TPC is calculated by multiplying with appropriate dilution factor. TPC does not strictly indicate the edibility of the fish. Fish with low TPC may bear pathogen that would have more dangerous if consumed. Therefore, qualitative analysis is done to determine. the presence of any pathogenic or health hazard bacteria. Different types of selective or ordinary media are used for the identification of pathogenic organisms in fish. For example, SS-agar or XLD agar medium are Used to detect coliform bacteria like Salmonella. Shigella, E. coils etc.
C. Organoleptic method
Sensory methods are used to assess the degree of freshness based on organoleptic characteristics such as general consistency of flesh, odour, color. eye and gill condition etc. These characteristics are judged by panel members. i.e. subjective judgment are made by individuals. Various numerical scoring or ranking systems have been developed to evaluate the judgments or results. Sensory methods have advantages that it can be adapted by the human being easily and the quality can be assessed by odour or visual inspection for quality defects. Human senses indeed are more efficient in some complex tasks than the instruments. The method is described more in chapter 16. There is, however, no single satisfactory test developed for the quality assessment of fish. A combination of chemical, biological and organoleptic tests would be the best procedure to assess such freshness.
a. Biochemical methods
A variety of chemical compounds or groups of compounds accumulate post-mortem fish muscles. These chemicals are either intermediate or end products of biochemical changes occurring in the muscles of fish after death.
Proximate composition: Because of influence of chemical composition on keeping quality, proximate chemical composition like moisture, lipid, protein and ash contents of fish samples from the time or day of harvest to different storage periods or conditions are often investigated. Proximate composition may vary with species, sex, season, place of harvest, feeding condition, etc. So, conclusive results are very difficult to obtain.
Hypoxanthin value:
As a consequence of post-mortem changes, breakdown of ATP to ADP, AMP, IMP and finally to hypoxanthin takes place. Hypoxanthin content of muscle increases on storage ofmfish. Estimation of hypoxanthin is an objective test of freshness of fish. However, the estimation of hypoxanthin is too cumbersome and it is seldom employed in practice. Fish with a hypoxanthin value of 7-8 micro moles/g is considered spoiled (Gopakumar, 2002a).
Histamine content:
Histamine develops in freshly caught fish after 40-50 hours of death, if the fish is not properly iced. To avoid histamine formation in tuna, skipjack and mackerel, care is taken to ice or freeze fish as quickly as possible. Histamin is a major problem in warm water pelagic species that causes a form of food poisoning known as scombroid poisoning, as the name derived from the family name of tuna and mackerel, scombroidae. The term is also applied to other family members too. The symptoms of scombroid poisoning are facial flushing, rashes, headache and gastro-intestinal disorders.
Histamin is produced from histidine which is one of the constituents of muscle protein of all fishes. Generally, pelagic fishes contain sizable amount of histidine in Free State as well as bound state within protein. Upon death, histidine in fish muscle is converted to histamine by bacterial enzymes. Histamin content over20mg/100g in canned fish is prohibited by the US-FDA standard.
pH:
Change in pH of the fish muscle is an usual good index for freshness assessment.
Trimethylamine:
Marine fish contains sizable amount of trimethylamine oxide (TMAO) which is reduced to trimethylamine (TMA) during the spoilage of fish. TMA determination is a useful index to measure the quality of stored fish. Fish with a level of 1.5 mg TMA nitrogen/100 g fish is considered acceptable, while 10-15 mg/100 g limit is set for moderately spoilt fish and beyond this range is set for highly spoilt fish (Gopakumar, 2002a).
Total volatile base nitrogen (TVBN): A number of volatile bases like ammonia are released in fish during decomposition t) bactaria.
A limit of 35-40 mg TVBN per 100 2 of muscle is considered acceptable for Good quality fish, while a value of 50-70 mg/100g of muscle can be taken as upper limit beyond which fish is considered inedible. For salted and dried fish the range is 100-200 mg/100 g beyond which the products are marked unacceptable.
Peroxide value:
Oxidative rancidity developed in fish tissue is determined by the estimation of peroxide value. Good quality fish should have a peroxide value quite less than 10. Peroxide value above 20 for any fish is considered rancid.
Thiobarbituric acid value (TBA):
It is also determined as an index of oxidative rancidity in fish. For a good quality moderate lipid fish. TBA value of less than 2 is usually accepted.
K value:
High level of adenosine related compounds or inosine mono phosphate in fish muscle imparts sweet, meaty flavor and is regarded as a reliable index of freshness. Post-mortem accumulation of inosine or hypoxanthin generally reflects poor quality. The conversion of ATP to LMP is very fast and is usually complete within a day. Subsequent accumulation of inosine or hypoxanthin is related to both autolytic and microbial action. Based on hypoxanthin values and their correlation with freshness of fish, Saito et al (1959) first proposed the K value as an index of the freshness of fish. K value is calculated from the values of hypoxanthin, inosine and total nucleotide levels in fish at the point of measurement as shown in the equation (i). However in practical situation adenosine compounds are deleted because of their conversion into IMP, HxR and Hx, shown in equation (ii). In freshly caught fish K value would be as low as zero, in moderate quality fish the value could be 10 to 20, but in spoilt fish it can go upto 90 (Gopakumar, 2002a). K vales are found to have excellent agreement with sensory data of fish. K values also rise with the ice storage period of fish.
HxR + Hx
K Value = x 100% (Theoretical)…(1)
ATP+ADP-i-AMP+IMP+HxR+Hx
HxR + Hx
K Value = x 100% (Theoretical)…(2)
IMP+HxR+Hx
b. Biological method
Total plate count (TPC):
Total number of microbial flora is changed with the time in fish or fish products. The numbers per gram of fish or fish products or per square centimeter of the surface area indicate the quality of fish from the microbiological view point. Total plate count or viable bacterial count is determined by the culture of bacteria present in fish sample using a suitable bacteriological media that could recover maximum number of bacteria in fish tissue. A known weight of fish sample is minced aseptically and serial decimal dilutions are pour-plated with the media. For marine fish aaar agar and for processed fish products tryptone glucose beef extract agar media are commonly used. Inoculatd plates are incubated at 37'C for 24 hours and the bacterial colonies are counted. From the colony Counts, TPC is calculated by multiplying with appropriate dilution factor. TPC does not strictly indicate the edibility of the fish. Fish with low TPC may bear pathogen that would have more dangerous if consumed. Therefore, qualitative analysis is done to determine. the presence of any pathogenic or health hazard bacteria. Different types of selective or ordinary media are used for the identification of pathogenic organisms in fish. For example, SS-agar or XLD agar medium are Used to detect coliform bacteria like Salmonella. Shigella, E. coils etc.
C. Organoleptic method
Sensory methods are used to assess the degree of freshness based on organoleptic characteristics such as general consistency of flesh, odour, color. eye and gill condition etc. These characteristics are judged by panel members. i.e. subjective judgment are made by individuals. Various numerical scoring or ranking systems have been developed to evaluate the judgments or results. Sensory methods have advantages that it can be adapted by the human being easily and the quality can be assessed by odour or visual inspection for quality defects. Human senses indeed are more efficient in some complex tasks than the instruments. The method is described more in chapter 16. There is, however, no single satisfactory test developed for the quality assessment of fish. A combination of chemical, biological and organoleptic tests would be the best procedure to assess such freshness.