4th lecture 17.3.22 BFTC-601.pptx
- 1. MEAT, FISH, AND POULTRY TECHNOLOGY BFTC-601 UNIT-4 BY SHIVANI SINGH PhD RESEARCH SCHOLAR JAMIA HAMDARD
- 2. CONTENT Unit: 4 ◦ Fish: structure, rigor mortis (continued) , autolytic changes, bacteriological changes, rancidity and physical changes
- 3. Fish: structure ◦ Preprocessing of fish prepares the raw material for final processing. It is often performed on shipboard or in a shore-based plant and includes such operations as inspection, washing, sorting, grading, and butchering of the harvested fish. ◦ The butchering of fish involves the removal of non-edible portions such as the viscera, head, tail, and fins. Depending on the butchering process, as much as 30 to 70 percent of the fish may be discarded as waste or reduced to cheap animal feed.
- 6. Rigor mortis ◦ Rigor in fish usually starts at the tail, and the muscles harden gradually along the body towards the head until the whole fish is quite stiff. ◦ The fish remains rigid for a period which can vary from an hour or so to three days, depending on a number of factors, and then the muscles soften again. How long does a fish stay in rigor? The time a fish takes to go into, and pass through, rigor depends on the following factors: 1. The species - some species take longer than others to go into rigor, because of differences in their chemical composition 2. Its physical condition - the poorer the physical condition of a fish, that is the less well nourished it is before capture, the shorter will be the time it takes to go into rigor; this is because there is very little reserve of energy in the muscle to keep it pliable. Fish that are spent after spawning are an example
- 7. 3. The degree of exhaustion before death - in the same way, fish that have struggled in the net for a long time before they are hauled aboard and gutted will have much less reserve of energy than those that entered the net just before hauling, and thus will go into rigor more quickly. 4. Its size - small fish usually go into rigor faster than large fish of the same species. 5. The amount of handling during rigor- manipulation of pre-rigor fish does not appear to affect the time of onset of rigor, but manipulation, or flexing, of the fish while in rigor can shorten the time they remain stiff. 6. the temperature at which it is kept-This is perhaps the most important factor governing the time a fish takes to go into, and pass through rigor because the temperature at which the fish is kept can be controlled. The warmer the fish, the sooner it will go into rigor and pass through rigor. For example, gutted cod kept at 32-35°F may take about 60 hours to pass through rigor, whereas the same fish kept at 87°F may take less than 2 hours Small fish with low reserves of energy, that is exhausted and in poor condition, and kept at a high temperature will enter and pass through rigor very quickly.
- 8. Rigor changes occurring in fish before it is frozen may affect the quality in three main ways: • Toughness and high drip loss in frozen whole fish or fillets; • Gaping in fillets taken from frozen whole fish • Shrinkage of frozen fillets These undesirable effects can be reduced or prevented by: • keeping the fish cool, particularly before it goes into rigor; • handling it carefully when in rigor; • freezing fillets taken from pre-rigor fish as soon as they are cut. • Careful treatment of the fish before and during rigor will result in a higher quality frozen product with a correspondingly better market value.
- 9. Autolytic change in Fish The spoilage of fish can be defined as “irreversible changes occurring in postmortem fish muscle making it unacceptable to consumers”. Such changes are brought about as a result of careless handling and faulty pre- processing or storage. Spoilage can be broadly classified into two types; bacterial spoilage and autolytic spoilage. Bacterial spoilage results from growth and multiplication of microorganisms at the expense of muscle constituents. Even though bacterial growth is the major cause of spoilage of fish, it can be effectively controlled by proper processing methods. The rate and extent of autolytic spoilage in fish are considerably less than bacterial spoilage, but at first, autolysis plays an important role in flavour development and the onset of bacterial spoilage. Absolutely fresh and healthy fish is impermeable to bacteria due to the intact skin. Further, the absence of simple and easily available nutrients in absolutely fresh fish makes it difficult for bacteria to grow and multiply. However, after the death of the fish, autolysis sets in, making the fish skin permeable to bacteria and at the same time releasing simple sugars, free amino acids, free fatty acids, etc. These nutrients provide a nutrient rich medium for bacteria to grow and multiply.
- 10. ◦ In simple terms, autolysis is defined as the degradation of muscle and skin constituents by endogenous enzymes. Since the enzymes causing autolysis arise from within the fish muscle (endogenous), the prevention and control of autolysis is very difficult unless drastic treatments are used. However, a clear understanding of autolysis would be useful in devising suitable methods to effectively reduce spoilage, thereby preserving the delicate flavour. Role of Enzymes in Autolysis Live fish contain numerous enzyme systems required for the complex metabolic reactions taking place. These enzymes are distributed both in the intracellular (within the cell) and extracellular (outside the cell) compartments throughout the fish muscle. Their individual concentrations vary with the nature and function of the tissue. In live fish, all these enzyme systems are used in metabolic processes. Consequently, most of the enzymes occur as some sort of inactive precursors. In certain other cases, the enzymes are kept isolated from their substrates. Once the fish is dead, the ability of its body to regulate the enzymes is lost. The absence of blood circulation, depletion (reduction) of oxygen, depletion of energy sources such as CTP (creatine triphosphate) and ATP (adenosine triphosphate), and the breakdown of the body’s scavenging mechanism bring an end to all anabolic or biosynthetic processes. In effect, in post-mortem fish muscle, only the catabolic and degrading reactions are active. These changes lead to the accumulation of catabolic products..
- 11. Autolsysis and nucleotide catabolism • The degradation of nucleotides starts during the rigor mortis stage and continues throughout the period of autolysis. In the autolytic period the ATP sequentially degrades to adenosine diphosphate (ADP), adenosine monophosphate (AMP). Inosine monophosphste (IMP) and hypoxanthine (Hx). The rate of breakdown of ATP is mainly dependent on the type of fish and condition of storage and the degradation of ATP has been associated with the quality of fish. It has therefore, been observed that it is possible to assess the freshness of fish from the relative accumulation of different degraded products of ATP.
- 13. Bacteriological changes ◦ Even before the autolytic process is over the spoilage bacteria become active. Though the inside of the live fish muscle is sterile the skin, viscera and gill harbor a high load of bacteria. About 10² -10⁷ colony forming units (c.f.u.)/square inch of bacteria are found in the skin and between 10³ -10⁹ c.f.u./g are found in gill and intestine of a tropical fish. The high variation in the number, however, is due to the type of water the fish live in. The gill and skin of fish caught from polluted water show higher load of bacteria in comparison to fish caught from clean waters. Based on the growth temperature requirements, bacteria can be grouped as psychrotrophic (cold-tolerant), psychophilic (cold-loving), mesophylic and thermophylic. Table 1: Optimum and maximum growth temperature of different categories of bacteria
- 14. ◦ Based on temperature tolerance, bacteria are divided into three classes. i) Mesophilic bacteria- Mesophilic bacteria live in room temperature, 20-45ºC. Majority of bacteria fall in this group. They grow within the temperature range of 20-45º C with an optimum of 30-37º C. Most pathogenic bacteria fall in this group e.g. Salmonella, Escherichia, Staphylococcus. ii)Psychrophilic bacteria- They are cold loving bacteria and grow in temperatures between 0-20º C. Their optimum growth temperature is 15ºC. This group is the principal bacteria that cause spoilage of foods kept in refrigerated temperatures. In actual practice, these types of organisms are not really encountered. Those bacteria seen growing in cold temperatures are often adapted to grow in elevated temperatures up to 35ºC. Such bacteria are called Psychrotropic bacteria. Ex. Pseudomonas, Alteromonas, Acinetobacter. iii) Thermophilic bacteria- Tolerate high temperatures, normally, 45-70º C. Their Optimum growth temperature is 55º C. Bacteria belonging to this group are rare. e.g. Bacillus stearothermophilus.
- 15. Rancidity This is caused by the oxidation of fat, which is present in the fishes. Rancidity is more pronounced in oil rich fishes like mackerel, sardine etc. The unsaturated fat in the fish reacts with the oxygen in the atmosphere forming peroxides, which are further broken down into simple and odoriferous compounds like aldehydes, ketones and hydroxy acids, which impart the characteristic odors. At this stage the colour of the fish changes from yellowish to brown this is known as rust. This change results in an unpleasant flavour and odor to the product, thus leading to consumer rejection. Though a certain degree of rancidity can be accepted, it is seen that the nutritional value of these fishes are much lower than non-oxidized ones. These fatty fishes continue to become rancid during storage. Certain impurities in salt and traces of copper accelerate this. The two distinct reactions in fish lipids of importance for quality deterioration are: 1. oxidation 2. hydrolysis
- 16. 1. Lipid hydrolysis Lipases bring about hydrolysis of lipids producing free fatty acids and resulting in hydrolytic rancidity. Free fatty acids trigger protein insolubilization and texture degradation in frozen stored fish. 2. Lipid oxidation It is a very series problem in fish, in view of the highly unsaturated nature of fish lipids. The double bonds of unsaturated fatty acids are highly susceptible to oxidation and this leads to the production of various carbonyls and other secondary oxidation products, which impart the characteristic rancid off flavour to the product. These products, besides producing off flavour reduce the shelf life and nutritional value of the product also. Some of them are toxic in nature. These reactions are initiated by free radicals generated from unsaturated bonds, which start chain reactions resulting in the production of various undesirable compounds like peroxides, hydroperoxides, aldehydes, ketones etc. Finally, the free radicals form non-radical polymers, which terminate the chain reaction.
- 17. Oxidised lipids interact with proteins reducing the nutritive value of the proteins considerably. Melonaldehyde is one of the major oxidation products and estimation of this compound by forming the thiobarbituric acid (TBA) complex is the accepted method for monitoring the extent of lipid oxidation. In lipid oxidation, the first step leads to formation of hydroperoxides, which are tasteless but can cause brown and yellow discoloration of the fish tissue. The degradation of hydroperoxides gives rise to formation of aldehydes and ketones. These compounds have a strong rancid flavour. Lipid oxidation primarily non enzymatic in nature, recently the involvement of microsomal enzymes and lipoxygenase has been reported. This lipid oxidation takes place in fishes having more than 2% of the lipids e.g. fatty fishes. Factors affecting the oxidation 1. Content and composition of unsaturated fatty acid 2. Oxygen availability 3. Light radiation 4. pH 5. Temperature 6. Moisture content 7. Content of pro and anti oxidant.
- 18. Physical changes Apart from the chemical effects, growth and multiplication of bacteria also bring about certain changes which are perceptible by human sense organs. These changes can also be evaluated to determine the extent of spoilage. Such physical changes are called ‘organoleptic indices’. The important organoleptic indices of spoilage are as follows: 1) Texture: In case of fresh fish, the texture of fish meat on pressing with finger will be firm and elastic. In other words, the distortion created by finger pressing will be removed immediately and the pressed surface will come back to its original shape. On spoilage, with extend of spoilage, the texture will gradually change to soft and flabby with retention of finger impression or distortion of finger pressing. 2) Eyes: In case of fresh fish, the eye balls will be protruding and the eye lens will be transparent and pupil will be jet black. On spoilage, the eye balls will sink (Sunken eyes), the eye lens will become opaque and cloudy. 3) Gills: The gills of fresh fish will be bright red and free from mucous deposit. With spoilage the bright red colour turns brown and then gets bleached. The gills also get covered with thick mucous. This mucous covering also changes its thin transparent nature to thick and yellow in colour on spoilage.
- 19. Physical changes 4. Fish surface: The colour and surface of fish body also undergo changes with spoilage. In fresh fish, the body surface will show a characteristic colour with metallic sheen. The surface also will be covered with a thin and transparent layer of slime. On spoilage, the characteristic colour and metallic sheen will be lost and the surface will get covered with thick cloudy or yellow slime. 5. Cross-section: A critical observation of the cross-section of the fish is also found to give a clear indication about the extent of spoilage. In case of fresh fish, the tissue around backbone at the cross-section of fish will be bluish and transparent without reddish brown colour. On spoilage, the muscle will turn waxy and opaque with or without reddish brown discoloration. During spoilage, the odor changes as follows Very good : Fresh sea weedy (Fish) or shell fish smell (Shrimp) Good : Loss of sea weediness or shell fish smell Fair : No odor
Editor's Notes
- #10: Gapping A fillet is said to be “gapped” when the individual flakes of muscles come apart, giving the fillet a broken and ragged appearance. This happens when the material that binds the flakes together, known as connective tissue, breaks down.