Seafood

Question 1

Part 1

What are the 2 main ways in which we obtain fish?

Answer 1

1. Wild caught
2. Commercially farmed

Question 2

Part 1

What are the main problems with catching fish in the wild? [2]

Answer 2

1. Non-sustainable → supply will eventually run out and fish may take very long to reproduce etc
2. Negative environmental and by-catch impacts
   - when fishermen catch some seafood that they don’t want, these organisms are left on the trunk on the ship while sorting their catch → organisms may die → dead organisms thrown back into the sea

Question 3

Part 1

What are the main types of fish based on their habitats? [3]

Answer 3

1. Marine fish – spend their lives in salt water (sea water)
2. Diadromous fish – partially spend live in seawater, partially in freshwater
3. Freshwater fish – spend their lives in freshwater only

Question 4

Part 1

Marine fish is the most commonly consumed type of fish. What are the 2 different kinds of marine fish?

Answer 4

1. pelagic (fatty, store lipids in body tissue) → have higher % of dark muscle → dark meat
2. demersal (lean, store lipids in liver only) → very low % of dark muscle, white muscle (surimi)

Question 5

Part 1

What are crustaceans, and how are they characterized? Give some examples.

Answer 5

Crustaceans are arthropods characterized by a hard, close-fitting shell that is shed periodically. Examples include shrimp, lobsters, crabs, and crayfish.

Question 6

Part 1

What are molluscan shellfish?

What are the different types of common molluscs? (not v impt)

Answer 6

Molluscan shellfish are aquatic invertebrates characterized by a shell that wholly or partly encloses the soft unsegmented body.

1. Bivalves (2 shells) – oyster, clam
2. gastropods (one shell) – abalone
3. Cephalopods (shell inside body) – squid

Question 7

Part 1

Why is fish such an important source of food?

Answer 7

Fish muscle is a valuable source of:

1. proteins
   2. micronutrients
2. polyunsaturated fatty acids.

Question 8

Part 1

What are the differences between white and dark muscle in fish? [3]

Answer 8

White muscle is used for quick bursts of energy and has lower myoglobin levels. Gets energy from anaerobic respiration, using glycogen as a fuel.

Dark muscle has more myoglobin, capillaries for a more sustained source of energy and employs aerobic metabolism using* fat *as the main fuel.

Question 9

Part 1

What are the 3 main types of proteins in fish?

Answer 9

1. Structural proteins
2. Sarcoplasmic proteins
3. connective proteins

Question 10

Part 1

What are the types of structural proteins found in fish muscle and what are their functions?

Answer 10

Structural proteins = myofibrillar proteins

• actin, myosin, tropomyosin, and actomyosin
• Function : mainly involved in muscle contraction

Question 11

Part 1

Structural proteins make up ____% of total proteins in fish muscle.

Answer 11

70-80

Question 12

Part 1

What are the types of sarcoplasmic proteins found in fish muscle and what are their functions?

Answer 12

Small moelcular weight proteins like myoalbumin, globulin, and consists of mostly enzymes

• sarcoplasm surrounds the myofibrils, it is the cytoplasm of fish muscle containing organelles (mitochondria, lysosome) and these organs contain enzymes
• Function: responsible for the metabolism of animal cells and are made up of oxidative enzymes, heme pigments, mitochondrial oxidative enzymes, lysosomal enzymes, and nucleoproteins.

Question 13

Part 1

Sarcoplasmic proteins make up ____% of total proteins in fish muscle.

Answer 13

25-30

Question 14

Part 1

What 3 processes mainly leads to spoilage of fish meat?

Answer 14

1. Autolytic processes
2. Chemical processes
3. Bacteriological changes

Question 15

Part 1

At different stages of storage, how does spoilage of fish change?

Answer 15

At first, when fish is freshly caught, spoilage is due to autolytic process (enzymes breaking down compounds). As storage time increases, spoilage is due to bacteria spoilage

Question 16

Part 1

What are 2 autolytic processes that occur that causes fish spoilage?

Answer 16

1. Enzymatic breakdown of ATP
2. Enzymatic reduction of TMAO

Question 17

Part 1

Normally when a fish is caught, post-mortem ATP metabolism is one of the intrinsic factors pertinent to the quality of fish.

Describe the steps of post-mortem ATP metabolism from “fish caught” to “changes in nucleotide metabolism”.

Answer 17

• After fish is caught, ATP is still being synthesised from creatine phosphate.
• During post-mortem, endogenous enzymes in the fish muscle tissue become active and contribute to the autolytic breakdown of ATP and formation of other compounds.
  This autolytic process includes the activation of enzymes like ATPases and proteases.

[MUST KNOW]
- ATP → ADP → AMP → IMP (inosine monophosphate) → Ino (inosine) → Hx (Hypoxanthine) → Xa (xanthine) → Uric acid

• Breakdown products, such as inosine and hypoxanthine, can contribute to bitter off-flavours in fish

Question 18

Part 1

In the metabolism of ATP, what compound is responsible for the fresh flavour of fish?

Answer 18

IMP, inosine monophosphate

Question 19

Part 1

In the metabolism of ATP, inosine (Ino) and Hypoxanthine (Hx) contribute to ____ off-flavours

Answer 19

Bitter

Question 20

Part 1

What is k value and its equation?

Answer 20

K-value : gives a relative freshness rating based primarily on autolytic changes that take place in muscle during fish storage

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Equation = [([Ino] + [Hx]) / everything] x 100%

• everything = [ATP] + [ADP] + [AMP] + [IMP] + [Ino] + [Hx] +

Question 21

Part 1

The higher the k value, the fresher the fish. True or False?

Answer 21

False, since numerator is [ino] and [hx], which contribute to off-flavours

Question 22

Part 1

Enzymatic reduction of Trimethylamine N-oxide is carried out by which enzyme and what volatile compounds are produced?

Answer 22

TMAO demethylase breaks down TMAO to give products like TMA (trimethyl amine) + DMA (dimethylamine) + .. + formaldehyde

Question 23

Part 1

One of the products formed by emzymatic reduction of TMAO is formaldehyde. What does formalehyde do to the quality of fish (texture)?

Answer 23

induces cross-linking of fish muscle proteins, reducing water holding capacity → tougher texture + shrinkage

• cross-linking also causes protein aggregation and denaturation, lowering gel strength of surimi

Question 24

Part 1

What can be done to reduce the effects of Trimethylamine demethylase in fish products (minced/whole fish)?

Answer 24

1. Ensure minced fish is free of organ tissue so that there is no TMAO demethylase (enzyme mostly found in organs like kidney, gallbladder, spleen)
2. minimise temperature fluctuations in cold storage. → TMAO demethylase becomes active when tissue membranes are disrupted by freezing (ice crystals pierce through tissue, causing enzyme leakage)
3. Avoid rough handling / application of physical pressure on fish prior to freezing → prevent leakage of enzyme and contact of enzyme w substrate

Question 25

# **Part 1** What are the 2 main chemical changes that occur in fish during spoilage?

Answer 25

Marine fish, high in lipids, are usually consumed. 1. Lipid oxidation → oxidative rancidity 2. Lipid hydrolysis →hydrolytic rancidity

Question 26

# **Part 1 ** In live fish, the flesh is sterile. Why is the flesh not sterile once the fish dies?

Answer 26

immune system collapses, bacteria allowed to proliferate freely.

Question 27

# **Part 1** During storage, most bacteria proliferate inside the flesh of the fish. True or False?

Answer 27

False, Only a limited number of microbes invade flesh. Microbial growth mostly occurs on surface (found in between scales, gills)

Question 28

# **Part 1** Bacterial spoilage of fish is mainly due to ? | Hint : bacterial enzymes and nutrients

Answer 28

bacterial enzymes diffusing into the flesh + nutrients diffusing outside

Question 29

# **Part 1** When spoilage bacteria diffuses into the flesh of fish, what compound does it break down to cause spoilage? How does bacteria utiise this compound?

Answer 29

Spoilage bacteria use TMAO as an **electron acceptor** (gets reduced) in anaerobic respiration Action of spoilage bacteria: - TMAO → **TMA (fishy odour)**

Question 30

# **Part 1** What is an estimate of fish spoilage caused by bacteria, and the equation?

Answer 30

total volatile bases (TVB) - [TVB] = [ammonia] + [TMA] + [DMA]

Question 31

Total volatile bases (TVB) will come to a plateau and stops increasing after all TMAO has been broken down into TMA by bacteria. True or False?

Answer 31

False, TVB will continue to rise even after TMA reaches it maximum → due to **proteolysis**, where bacteria acts upon other **nitrogenous** compounds **(proteins)** in fish → gives rise to other compounds like ammonia

Question 32

# **Part 1** What is melanosis?

Answer 32

Melanosis is a discolouration that occurs along the swimmerets (legs), head, tail and nearby shell areas**in crustaceans** before spreading further along shell edges and through the body.

Question 34

# **Part 1** What factors contribute to melanosis in crustaceans, and how can it be prevented?

Answer 34

Melanosis is caused by the oxidation of polyphenols, leading to the production of quinones and melanin. It can be prevented by : 1) usage of acidulants to denature PPO, thus reducing [PPO] 2) Careful handling of commodity and ensuring it doesn’t get crushed or get subjected to physical force / pressure -- lower incidence of PPO coming into contact with the substrate 3) Use reducing agents (e.g. sulfiting agents, antioxidants) to shift the equilibrium, reduce quinones back into polyphenols 4) Reduce O2 by using MAP / vacuum package 5) **Lower rate of reaction by storing at lower temperature – MAINTAIN COLD CHAIN**

Question 35

# **Part 1** What are the common forms of water/ice in chilling seafood? [4]

Answer 35

- chilled water - refrigerated sea water - ice (blocks/cubes) - ice slurries (water + small ice crystals)

Question 36

# **Part 1** What are the reasons that makes ice a great medium to chill seafood? [5]

Answer 36

1) Low temperature: Consistent cold temperature around 0°C is sufficient to slow microbial growth and chemical reactions, slowing down the rate of spoilage
2) Heat capacity: The relatively high specific heat capacity of ice allows it to absorb a significant amount of heat before melting. Provides a longer cooling effect over time.
3) Phase change: When changing from solid phase to liquid phase, additional heat energy is absorbed from the surrounding, providing cooling without any external energy sources. Additionally, melting of ice can also help to wash away bacteria from the fish.
4) Moisture: The ice provides moisture for the fish tissues, maintaining the texture and prevent dehydration, preserving the quality of the fish.
5) Uniform cooling: Ice can be crushed into slurry which can fit in the gaps between the fishes, allowing a even distribution of ice across the fishes, preventing the formation of hot spots that support microbial growth.
6) Chemical reactions: Ice is unlikely to undergo chemical reactions with the compounds within the fish. Ice from clean sources would thus be effective as a safe cooling medium that does not affect the properties of the fish. Availability: Ice can be easily made in various forms and transported at a low cost. Allows it to be accessible in remote areas and at sea

Question 37

# **Part 1** What are the advantages of using ice slurries over ice cubes / blocks to chill seafood?

Answer 37

***1. Able to minimise temperature fluctuations + more efficient cooling*** - Heat transferred from fish → ice in slurry → water
- If in slurry, heat gained from ice is transferred to water, temperature of ice can be maintained at 0℃ and reducing fluctuations in temperature
- But if use ice block, there is no water surrounding it → solid ice absorbs all the heat → leading to more temperature fluctuation → more membrane disintegration of fish, leakage of enzymes and spoilage
2. Semi-liquid nature of slurry → it can **conform to the shape** of the fish, hence **maximising the surface area of the fish in contact** with the slurry → efficient cooling

Question 38

# **Part 2** What is surimi?
Which country invented surimi and its purpose?

Answer 38

paste that is derived from fish (mostly) or other meat.**It is a wet protein concentrate made of fish myofibrillar proteins that is obtained from mechanically deboned fish flesh that is processed through mincing, washing, mixing with GRAS cryoprotectant and freezing**
Invented in Japan to convert cheaper fish into higher priced products

Question 40

# **Part 2** What is the official definition of surimi?

Answer 40

wet protein concentration made of fish **myofibrillar proteins** obtained from mechanically deboned fish that is processed through : - mincing - washing - mixing with GRAS (generally recognised as safe) cyroprotectants - freezing

Question 41

# **Part 2** A surimi gel consists of inter-connected ____ chains that holds ____ and low molecular weight particles

Answer 41

protein ; water

Question 42

# **Part 2** What are 5 desirable properties of fish used in making surimi [5]

Answer 42

1) Strong gel-forming ability - marine fish > freshwater fish - White-fleshed fish > dark-fleshed fish
2) Good organoleptic quality - Good flavour, free from odour - White flesh fish : visually nicer, while dark flesh may turn grey aft cooking
3) Abundant supply 4) All year round availability 5) reasonable price

Question 43

# **Part 2** Why do proteins from freshwater fish have lower form gel forming ability than proteins from marine fish?

Answer 43

Proteins in freshwater fish are more prone to denaturation

Question 44

# **Part 2** How do proteins in fish muscle denature, leading to lower gel forming ablity?

Answer 44

Protein chains aggregating together through forming various types of bonds, making them unavailable for subsequent gel formation

Question 45

# **Part 2** What are 3 factors that cause proteins in dark-fleshed fish to denature faster than white-fleshed fish?

Answer 45

1. Higher amounts of polyunsaturated lipids and iron - more PU lipids undergo lipid peroxidation, forming free radicals which cause protein denaturation - iron in myoglobin acts as a pro-oxidant in lipid peroxidation
2) Higher proteolytic activity - High content of sarcoplasmic proteins with enzymes to break down myofibrillar proteins, interfering with gelling process
3) Lower pH - dark muscles have lower pH leading to faster protein denaturation (recall : dark muscles are for swimming, higher glycogen levels in them, and glycogen broken down into lactic acid via anaerobic glycoysis during periods of intense activity)

Question 46

# **Part 2** In surimi production alone, there are 3 washing steps with **fresh water**. What are the purposes of having so many washing steps? [3]

Answer 46

1. Removal of protein denaturing factors to enhance gel-forming characteristics of surimi during frozen storage
2. Concentrate the raw material, whcich is the water-insoluble myofibrillar protein - wash away sarcoplasmic proteins which contain enzymes which may denature proteins
3. Remove undesirable compounds in fish muscle

Question 47

One of the main purposes of washing steps in surimi production is to remove undersirable compounds? List all 5 compounds and how they may lead to undesired qualities in the end product.

Answer 47

1) lipids : undergo peroxidation / hydrolysis which leads to off-flavours
2) Heme proteins (myoglobin): undesirable pigmentation
3) TMAO demethylase : brings about production of formaldehyde which can cause undesirable cross-linking of myofibrillar proteins, affecting gel strength
4) Nitrogenous compounds : break down of TMAO into TMA and DMA which leads to fishy odour
5) Proteolytic enzymes : destroy protein network during gelation, resulting in mushy texture

Question 48

# **Part 2** How do cryoprotectants prevent protein denaturation?

Answer 48

- When frozen, water forms ice crystals and dissociates from hydrophilic sites of myofibrillar proteins
- Cyroprotectants interact with hydrophilic sites of proteins, resulting in increased hydration
- Therefore, this prevents protein chains from aggregating and forming cross-linkages, reducing denaturation so that gelling ability is not compromised

Question 49

# **Part 2** What are some examples of cryoprotectants?

Answer 49

Molecules with many hydrophilic --OH groups, like sorbitol (sugar alcohol), sucrose (sugar) etc

Question 50

# **Part 2** How are surimi products frozen?

Answer 50

Surimi with added cyroprotectants is usually packed in a freezing pan to form a block and frozen quickly to below -25°C using contact freezer

Question 51

# **Part 2** What are the 2 main steps in making surimi gel and the rationale behind it

Answer 51

**Step 1** - high concentrations of salt (up to 3%) added to induce dissociation of actomyosin complex - Protein network changes from solution to gel at temperatures up to 40°C --> known as suwari phenomemnon - protein molecules unfold, exposing reactive surfaces which interact with nearby protein molecules to form intermolecular bonds to form a 3D gel-like network
**Step 2** - after setting, suwari gel cooked at 80-90°C to yield a rigid and irreversible gel to form final product

Question 52

# **Part 2** What is the quality of surimi products affected by?

Answer 52

Texture, which is affected by gel strength

Question 53

# **Part 2** What is the Modori phenomenon in surimi products?

Answer 53

Gel softeningm where there is structural disintegration that occurs between 50-70°C, which lowers gel strength
- induced by remaining endogenous heat-stable proeinases that are difficult to remove via extensive washing - Degrade myofibrillar proteins, esp myosin

Question 54

# **Part 2** What is the negative environmental impact in surimi production? how can it be tackled? [2]

Answer 54

Large amounts of waste water containing many water-soluble sarcoplasmic proteins and lipids.
Solutions: 1. recover some of the functional sarcoplasmic proteins and lipids from wastewater
2. Waste water can be recycled, or treated to allow safe discharge back into environment