Physical methods for assessment of

fish quality deterioration

MOKSHA CHIB13FET1003

PARAMETERS FOR ASSESSING FISH QUALITY

Refractive index of eye

fluid

Texture changes

Electrical Conductance

Optical tests

Viscosity

Drip

Refractive Index of Eye FluidsPRINCIPLERefractive Index (RI) of the fluid, the vitreous humour, recovered from the interior of the eye is measured and related to the storage time of the fish.

REASON FOR CHANGE IN REFRACTIVE INDEX Eye fluid is bright and transparent but its brightness is lost with time due to drying. Materials transferred from tissues around the eye circle into the eye fluid increase the fluid concentration. Thus, the light refraction property of eye fluid changes.

• Change in RI likely due to the leaching of the potassium salts from the cells lining the eye to the extracellular vitreous humour, as the ion pump system across the cell membrane degrades on the death of fish

• RI change is not affected by any bacterial growth in the eye • Studies showed a linear relationship between the refractive index of eye fluid from

haddock & organoleptic score

Removal of eye from the head by

making a slit in the posterior portion of

the eyes

The fluids from each eye are

allowed to drain into a beaker

Fluids are then centrifuged &

filtered by passing through glass wool

RI is determined by Abbé

Refractometer using two or three drops of the eye

fluid

PROCEDURE

REFRACTIVE INDEX QUALITY OF FISH1.3347-1.3366 Very good1.3367-1.3380 Fair to good1.3381-1.3393 Poor> 1.3394 Not marketable

Gogoklu and Yerlikaya 2004

Yapar and Yetim 1998

Texture changes• Textural changes can take place in fish due to biochemical, structural and microbiological

changes. Cooking and freezing conditions also affect the texture• Textural assessment would include quantifying hardness, springiness, cohesiveness and

resilience by either instrumental or sensory means• The shape of the fish, the complex, non-uniform structure of the fish muscle, and the slippage

of the myotomes upon cooking make many instrumental methods difficult to apply in fish. The texture properties also varies from location of the fish, thus adding to the difficulty to the measurement

• Traditional Method: Finger Test- Evaluation of firmness by pressing on the skin or fillet of the fish by finger, and this method depends largely on subjective assessments of expert panel

Kramer Shear Cell• Standard cell contains an upper part with blades

that penetrate a box with slots• Upon application of force, the food materials

undergo shearing, compression and extrusion• SAMPLING: Fillets can be flaked by hand once

cooked, mixed thoroughly, and placed as a uniform layer in the cell before performing the test or evenly to spread a fixed amount of diced and cooked fish muscle in random fashion

• Parameters usually measured include maximum force at a given sample weight, slope, energy of the force-deformation curve

Warner- Bratzler Shear Cell• The Warner-Bratzler shear cell consists of a blade with two cutting

edges forming an angle of 60°, which penetrates another device with a slot.

• The blade cuts the sample like a guillotine and is thus subjected to a complex combination of tension, compression and shearing

• It is very to take into account how the fibers are oriented with respect to the blade, and therefore the Warner-Bratzler method is difficult to perform, especially with small pieces of fish muscle. The blade needs to be sharpened regularly

• The shearing takes place in a very localised area of the muscle, causing distortion of the muscle fibers

• Can be used for studying the influence of factors such as different cooking temperature & ice storage on the texture

Puncture Test• Consists of measuring the force required to push a plunger (conical,

cylindrical, wedge-shaped) into a food sample, which is thus subjected to a combination of compression and shearing in proportion to the area of the cross-section of the plunger

• The plunger penetrates either to a constant depth or to the point of rupture. The applied force can increase linearly or at a constant rate

• There is initial rapid increase in the force as the probe moves inside the food. After the punch begins to penetrate, this leads to a sudden change in slope called Yield Point, which marks the instant where the punch begins to break the food

• This method has been used to assess the changes occurring in texture during storage in ice. It can also be used to analyze the texture of fish gels

TENSION ANALYSIS• Measures the force required to

break a sample in two when it is held by two parallel clamps, one of which moves away from the other at a constant rate

• The sample is cut into dumb-bell shapes or strips and the parameters normally measured are maximum force or tensile strength and energy

• Sampling must be proper otherwise can lead to slipping or premature breaking in the clamps

COMPRESSION TEST• Performed on application of a uniaxial compression

force, between two parallel flat surfaces, one fixed and the other moveable

• There is a change in shape during compression but no change in the volume

• Probe should be larger than the sample• The deformation rate, the friction at the contact

surfaces, and the physical dimensions of the samples can have a high influence on the force deformation curves

• Slippage of myotomes in cooked fish samples can occur, thus it is not considered to be suitable for analysis of cooked products

• A compression force that exerts 50% deformation is used for a product of minced and cooked carp however, for fish mince or fillets 30% and 10% deformation is used

Cylinder Probe Texturometer • Mechanical devices for measuring the texture of fish usually involve some

kind of cylindrical probe that presses into the fish with the force, F, increasing to a preset value. The corresponding depth, h, of the concavity produced can be measured and related to the elasticity of the flesh

Open Cylinder Probe Texturometer • It involves a small open-ended cylinder (outer diameter dO , inner diameter dI ) that would

cause a "meniscus" of flesh of height h to swell up across the diameter of the probe. The advantage of this approach would be that a sensor to measure h could be built inside the probe

Specifications:

dO = 40mmdI = 30mmSpeed of the cylinder = 30mm/min

Hand-held device as initially envisaged, shown testing a piece of foam rubber

Close-up of the cylindrical tube and the meniscus height sensor 

Compression Test

Texture Profile Analysis (TPA)• Consists of compressing a sample twice in back-and-forth movement, mimicking the

action of the jaw• The force- deformation curve is analyzed to determine a number of texture parameters

like maximum force at 1st & 2nd compression cycles considered as hardness, and ratio of the force areas under the 1st & 2nd compression considered as cohesiveness

• Other parameters measured are load at maximum displacement, deformability modulus and relaxation time

• If the test is done in conditions where the sample size is much smaller than the probe, the forces derive mostly from uniaxial compression. In the opposite case, the forces derive mostly from puncture, that is, compression and shearing

PARAMETER

DESCRIPTION

Springiness Range within which a deformed material recovers its non-deformed shape after the force is released

Cohesivity Strength of the internal borders which form the body of the product

Masticability

Required energy to masticate a solid amount of food till it is ready to be swallowed

Gumminess Required energy to disintegrate a semi-solid food product till it is ready to be swallowed

Adhesiveness

Necessary amount of work to counteract the attraction forces between the surface of the food and the surface of other materials

Hardness Necessary effort to obtain a given deformation

Elasticity Property of a material allowing it to recover its non-deformed shaped after being compressed

Durometer• Measures hardness of the snow crab• Has a broad 3.175mm diameter hemispherical

indenter which doesn’t penetrate the shell• A 2.5 cm stainless steel or aluminum extension

rod separates the indenter from the body of the instrument

• Crab to be classified as hard shelled must have a reading of 68

Durometer

Electrical Conductance • Disruption of cell structures (containing electrically conducting fluids) with tissue decay

causes large changes in the overall electrical conductivity and in the capacitance due to autolysis

• During storage, several other post mortem changes in the fish muscle affect quality through mechanisms that could also alter the dielectric properties

• Most changes affect interactions between water and proteins, and degradation processes leading to the formation of polar compounds

Torrymeter• Measures the effects of spoilage on the

electrical properties of fish tissues by the use of four-electrode system

• The applied current is through two electrodes at which polarization develops. Positioned between these current electrodes, a further pair of electrodes connected to a very high input impedance, draw no current and just measure the electric potential

• The potential electrodes are made up of stainless steel & the concentric current electrodes are of graphite

• The output is read in the form of a digital number from a scale of 0-18 0-4 : Fish unfit for

consumption14-18 : Very fresh fish

Relationship between GR Torrymeter readings of various species of fish and freshness

Intellectron Fischtester VI• Measures the electrical properties ( conductance, impedance & capacitance) of the fish

flesh• Fischtester can measure the resistance of the cell tissue by dividing it into different

components: resistance of the interstitial liquid, resistance of the cell content, resistance of the cell wall & the capacity of the cell wall

• It measures transversally through the entire fish; the electrodes are applied on the lateral line of the body close to the anal opening

• The readings can range from 0-100 Ω• Suitable for fresh fish grading, for determination of days in ice & prediction of days in ice

left for lean fish species

Optical Tests• Ultraviolet fluorescence and luminescence of dispersions are carried out which report

changes in the color of fluorescence occurring during storage• Bacterial decomposition on the surface of the fish loaf shows increased turbidity of the surface

washings which cane be measured by turbid metric method• Optical test for establishing the presence of spoilage in crabmeat is based on turbidity of alcohol

extracts that developed upon the addition of saturated aqueous picric acid solution• Computer screen photo assisted technique (CSPT) is used to display millions of colors,

combining wavelengths in optical range & also evaluates the absorbance and fluorescence of the samples

Detection of spoilage by Colorimetry Triphenyl Tetrazolium dye ( Colorless, ionized, water soluble & capable of passing through the cell wall of bacterial cell)

Triphenyl Tetrazolium formazan ( Red-colored, non ionic, water-insoluble, deposits within the bacterial cell)

The intensity of the formed color is proportional to the concentration of bacteria present which is necessary to provide reduction reaction

Incase of dark-flesh finfish, such as tuna fish, additional oxidizing or bleaching agents like H2O2 are added to bleach the muscle pigment & lighten the color of the solution so that the only color seen is due to the assay

Colorless solution - Excellent QualityLight reddish color – Good QualityDarker red color - Borderline QualityIntense red color – Unacceptable Quality

Viscosity• Measurement of the apparent viscosity of a fish muscle homogenate has been proposed as

an indicator of quality• The conditions are optimized for measurement of apparent viscosity of a muscle homogenate

in a solution of 5% NaCl (1:4) and pH 6.5-7.0 as a quality control measure • Viscosity of an actomyosin dispersion can determine the apparent viscosity• Viscosity is expected to decrease on frozen storage and it depends upon the species of the

fish• It has also been found to depend upon the storage temperature & the season

Drip Measurement• Drip loss measurement includes subjecting the fish samples to High Pressure Thawing

(HPT) which will lead to reduction in the drip loss. Slower the freezing, larger are the ice crystals resulting in more tissue damage & thawing loss

PROCEDURE

Batches of the packed samples are subjected to

freezing

Samples are thawed by HPT with the temp of

the pressure medium to be

20°C

Fish sample is removed from

the pouch, leaving behind the

drip

Drip loss is computed from

the weight of the drip & the sample

Freezing processes: Conventional air freezing (CAF), Plate freezing (PF), Liquid Nitrogen Freezing (LNF)