Instrumental methods

Instrumental methods for fish quality evaluation involves correlating the data generated by these systems with the sensory data. Dielectric properties of fish muscle have been relied upon as indices of quality during the past 40 years. The Intelectron Fishtester VI (Intelectron International Electronics, Hamburg, Ger-many), the Torrymeter (Distell Industries Ltd., Fauldhouse, West Lothian, UK), and the RT-Freshtester (RT Rafagnataekni, Reykjavik, Iceland) use instruments with increasing degrees of sophistication. Readings from all the instruments reflect dielectric properties of fish, which almost linearly change with storage time. The main advantages of these instruments are their immediate response and poten-tial in field use. Based on these rapid and nondestructive measurements, the RT-Freshtester allows automatic grading of 60 to 70 fish per minute. The elec-trical measurements can also be used to determine whether the fish has been previously frozen.⁶³Nevertheless, electrical properties of fish may not be always directly related to sensory spoilage. Further, these instruments need calibration depending on the season and fish handling practices, and they are not suitable for grading frozen or thawed, superchilled fish, and fish fillets. These disadvantages coupled with the high cost of the instruments limit their practical use in the seafood industry.

Spectroscopic methods have gained acceptance in detecting whether a fish has been frozen and also to estimate the storage time of fish in ice. Near infrared (NIR)

reflectance is another technology where measurements are rapid and have the potential for on-line quality grading.⁶³ This technology is useful for the indir-ect measurement of oil, water, and WHC in different types of fish. L^∗a^∗b^∗is an international standard for color measurements, adopted by the Commission Inter-nationale d’Eclairage (CIE) in 1976. L^∗is the luminance or lightness component, which ranges from 0 to 100, and a^∗ (from green to red) and, b^∗ (from blue to yellow) are the two chromatic components which range from−120 to +120. The principles of food color measurement based on this system have been discussed.⁷⁹ Table 2.3 summarizes some of the conventional methods for quality evaluation of fishery products.

TABLE2.3

Conventional Methods for Quality Evaluation of Fishery Products

Method Remark

Sensory evaluation Depends upon sight, smell, taste, touch, and hearing as judged by experienced panelists

Total volatile nitrogen Good correlation with bacterial spoilage

Trimethylamine Not generally useful for freshwater fishery products.

Not very good correlation with total bacterial counts

Ammonia Indicates advanced spoilage of finfish and shellfish Volatile acids Good correlation with bacterial spoilage Nucleotide catabolites (K-value), Hx Degradation products of ATP. Reliable quality

indices for several fish/shellfish based on K-value Indole in shrimp Rapid test for shrimp quality

Biogenic amines The amines are thermostable and hence cooked fish can also be analyzed. HPLC method is used H2S, CH3SH, (CH3)2S Indicates advanced degree of spoilage

Ethanol Good quality index for several fish such as salmon, raw tuna, redfish, pollock, flounder, and cod Rancidity TBA value is a good index of oxidative rancidity.

Reasonable correlation with sensory properties.

Other methods include peroxide and carbonyl values

Instrumental methods Electrical properties, pH, and vision properties of fish muscle. Presence of parasites and blood clots in fish fillets are determined by computer-aided vision techniques

Total microbial counts, SSOs Indicative of microbial spoilage of fresh and processed fish

Pathogenic microorganisms and toxins

Indicative of hazards. Above threshold levels indicate unsafe for consumption

Source: Adapted from Venugopal, V., Biosens. Bioelectr., 17, 147–157, 2002. With permission from Elsevier.

2.4 DETERMINATION OFS^POILAGER^{ATES AND}S^HELFL^IFE

The ultimate aim of freshness evaluation techniques is to combine different stand-ard methods that use rapid measurement techniques with a mathematical model to predict freshness, as well as the postharvest or remaining shelf life of an unknown fish sample. Spoilage of fish is linearly related to storage temperature, since auto-lytic reactions by muscle as well as microbial enzymes are directly related to temperature. In most cases, the spoilage rate can be determined by the slopes of the plots of the sensory, bacteriological, or chemical quality indices of fish against storage time at each temperature, since these plots are essentially linear. The QIM has also proved useful for obtaining a straight line relationship between quality scores and storage time.

If the shelf life of a fish at 0^◦C and another temperature, t^◦C are known, their ratio gives relative rate of spoilage (RRS) at t^◦C.⁸

Relative rate of spoilage at t^◦C= shelf life at “0^◦C”

shelf life at t^◦C (2.1) The relationship between shelf life and temperature has been studied in detail.⁸⁰ It was found that spoilage rate at varying temperatures followed the Spencer and Baines equation,⁸¹given below:

k= k0(1 + Ct) (2.2)

where k is the spoilage rate at temperature t^◦C, k0is the spoilage rate at 0^◦C, and C is the linear temperature response. The relative rate concept has made it possible to quantify and mathematically describe the effect of temperature on the rate of spoilage of various types of fish products. In the temperature range 0 to 8^◦C, the RRS of fish may be computed as:

k/k0= 0.24 × t (2.3)

where k and k0 represent the rate of spoilage in spoilage units per day at temperature t and 0^◦C, respectively.

The spoilage rate of fish can also be determined by knowing the influence of temperature on growth of contaminant microorganisms. A two-parameter square root model for the effect of sub-optimal temperature on microbial growth is given as:

√µmax= b(T − Tmin) (2.4)

where T is the absolute temperature (Kelvin) and Tminis a parameter expressing the theoretical minimum temperature of growth. The square roots of the micro-bial growth rates plotted against the temperature give a straight line from which Tminis determined. Based on the Tmin, a spoilage model has been developed on

the assumption that the relative growth rate of microorganisms would be similar to RRS.

Relative spoilage rate= 0.1 × t^◦C+ 1 (2.5)

From this, the shelf life at any temperature, T^◦C, is given as:

Shelf life at T(^◦C) = Shelf life at 0^◦C

[1 + 0.1 × T(^◦C)]² (2.6) While broad differences are observed in the shelf lives of various seafood products, the effect of temperature on RRS is almost similar for different fresh fish in general. At temperature of 5^◦C, the RRS of salmon and cod have been found to be 1.5 and 2.3, respectively, where as at 10^◦C, the corresponding values are 3.9 and 4.7.⁷The square root model (Equation 2.5) is the most popular to predict the effect of temperatures on the shelf life of different fresh seafoods from cold and temperate waters. This model has been successfully validated for products stored between−3 and +15^◦C.

Fresh tropical fish typically have a longer shelf life at 0^◦C than fresh fish from cold and temperate waters. The relative spoilage rates for tropical fish are more than twice as estimated for temperate fish species. For these fish, the logarithm of the relative spoilage rate has been found to be 0.12× T^◦C.⁷ An exponential tropical spoilage model has been developed for shelf life prediction,⁶⁹ based on Equation 2.4.

Shelf life at T(^◦C) = Shelf life at 0^◦C

exp[0.12 × T(^◦C)] (2.7) Compared to fresh seafood, the effect of temperature on the shelf life of lightly preserved seafood is more variable and less well described. Temperature influences the RRS of cooked and brined MAP shrimp more than the fresh shrimp. However, in the case of hot smoked cod and mackerel, the influence of temperature on RRS is less than that of fresh fish. The reference temperature of 0^◦C that is applied for the calculation of RRS in fresh fish may be inappropriate for lightly preserved products, and a different reference temperature (Tref), namely, 5^◦C can be used in the equation,⁶⁹given below:

Shelf life at T(^◦C) = Shelf life at Tref(^◦C)

exp[0.15 × (T − Tref(^◦C))] (2.8) The cumulative effect of time and temperature on storage and product quality allows spoilage models to be used for prediction of the effect of variable temper-atures on product shelf life. Electronic time-temperature integrators (TTIs) have been developed based on these results. The shelf life of different types of fresh and

lightly preserved seafood varies substantially with temperature and processing treatments. Despite this, it is possible to develop simple and entirely empirical relative-rate-of-spoilage-models to predict the shelf life of different seafoods at various storage temperatures.⁶⁹

2.5HAZARDSASSOCIATED WITHFISHERYPRODUCTS

Food-borne hazards are on the rise throughout the world. A hazard is defined as a biological, chemical, or physical agent in food, or a condition of food with the potential to cause harm. An estimate of the probability and severity of the hazard to populations caused by the consumption of foods is called risk. Diseases caused by consumption of food contaminated with pathogenic microorganisms are food infections, while those resulting from their toxins are referred to as food intoxica-tions. The severity of the diseases depends upon the nature of contamination, and may range from mild diarrhea to death. The major reasons for such outbreaks are:

the increase in population, tourism, industrialization, and the rise in international trade of processed food associated with lesser care during processing.^82–86 The various types of hazards associated with fishery products have been summarized recently.⁸⁴

The World Health Organization (WHO) has observed that in Asia-Pacific region alone, more than 700,000 people die every year from consuming contamin-ated food.⁸⁵An estimated 76 million cases of foodborne illness occur each year in the United States costing between $6.5 and $34.9 billion in medical care and loss of productivity.⁸³ In the late 1980s seafood attracted significant media attention as a carrier of environmental pollutants and other health hazards. Much processed seafood have been recognized as carriers of pathogenic bacteria, viruses, and para-sites responsible for food-borne hazards worldwide. Contamination of wild as well as farmed fish can occur during harvesting, distribution, and storage, through bio-logical and chemical hazards from both freshwater and coastal ecosystems.^84–88 The tendency of some fish to absorb and concentrate heavy metals such as mer-cury and other industrial pollutants and also hazards associated with aquacultured fish, such as presence of antibiotics at nonpermissible levels attracted the attention of regulatory authorities and consumers. The public health issues associated with seafood have been grouped as environment, process, distribution, or consumer-induced.⁸⁹ Some of the important hazards associated with fishery products are discussed below.