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FST2106 -- Post harvest food processing > Seafood > Flashcards

Seafood Flashcards

1
Q

Part 1

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

A
  1. Wild caught
  2. Commercially farmed
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2
Q

Part 1

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

A
  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
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3
Q

Part 1

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

A
  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
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4
Q

Part 1

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

A
  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)
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5
Q

Part 1

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

A

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

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6
Q

Part 1

What are molluscan shellfish?

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

A

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
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7
Q

Part 1

Why is fish such an important source of food?

A

Fish muscle is a valuable source of:

  1. proteins
    2. micronutrients
  2. polyunsaturated fatty acids.
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8
Q

Part 1

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

A

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.

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9
Q

Part 1

What are the 3 main types of proteins in fish?

A
  1. Structural proteins
  2. Sarcoplasmic proteins
  3. connective proteins
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10
Q

Part 1

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

A

Structural proteins = myofibrillar proteins

  • actin, myosin, tropomyosin, and actomyosin
  • Function : mainly involved in muscle contraction
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11
Q

Part 1

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

A

70-80

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12
Q

Part 1

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

A

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.
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13
Q

Part 1

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

A

25-30

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14
Q

Part 1

What 3 processes mainly leads to spoilage of fish meat?

A
  1. Autolytic processes
  2. Chemical processes
  3. Bacteriological changes
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15
Q

Part 1

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

A

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

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16
Q

Part 1

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

A
  1. Enzymatic breakdown of ATP
  2. Enzymatic reduction of TMAO
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17
Q

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”.

A
  • 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
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18
Q

Part 1

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

A

IMP, inosine monophosphate

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19
Q

Part 1

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

A

Bitter

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20
Q

Part 1

What is k value and its equation?

A

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

</br>

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

  • everything = [ATP] + [ADP] + [AMP] + [IMP] + [Ino] + [Hx] +
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21
Q

Part 1

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

A

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

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22
Q

Part 1

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

A

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

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23
Q

Part 1

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

A

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
24
Q

Part 1

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

A
  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
25
Q

Part 1

What are the 2 main chemical changes that occur in fish during spoilage?

A

Marine fish, high in lipids, are usually consumed.

  1. Lipid oxidation → oxidative rancidity
  2. Lipid hydrolysis →hydrolytic rancidity
26
Q

**Part 1 **

In live fish, the flesh is sterile. Why is the flesh not sterile once the fish dies?

A

immune system collapses, bacteria allowed to proliferate freely.

27
Q

Part 1

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

A

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

28
Q

Part 1

Bacterial spoilage of fish is mainly due to ?

Hint : bacterial enzymes and nutrients

A

bacterial enzymes diffusing into the flesh + nutrients diffusing outside

29
Q

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?

A

Spoilage bacteria use TMAO as an electron acceptor (gets reduced) in anaerobic respiration

Action of spoilage bacteria:
- TMAO → TMA (fishy odour)

30
Q

Part 1

What is an estimate of fish spoilage caused by bacteria, and the equation?

A

total volatile bases (TVB)

  • [TVB] = [ammonia] + [TMA] + [DMA]
31
Q

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?

A

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

32
Q

Part 1

What is melanosis?

A

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

33
Q
A
34
Q

Part 1

What factors contribute to melanosis in crustaceans, and how can it be prevented?

A

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

35
Q

Part 1

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

A
  • chilled water
  • refrigerated sea water
  • ice (blocks/cubes)
  • ice slurries (water + small ice crystals)
36
Q

Part 1

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

A

1) Low temperature: Consistent cold temperature around 0°C is sufficient to slow microbial growth and chemical reactions, slowing down the rate of spoilage

<br></br>

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.
<br></br>
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.
<br></br>
4) Moisture: The ice provides moisture for the fish tissues, maintaining the texture and prevent dehydration, preserving the quality of the fish.
<br></br>
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.
<br></br>
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

37
Q

Part 1

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

A

1. Able to minimise temperature fluctuations + more efficient cooling

  • Heat transferred from fish → ice in slurry → water
    <br></br>
  • If in slurry, heat gained from ice is transferred to water, temperature of ice can be maintained at 0℃ and reducing fluctuations in temperature
    <br></br>
  • 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

<br></br>
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

38
Q

Part 2

What is surimi?
<br></br>
Which country invented surimi and its purpose?

A

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

<br></br>
Invented in Japan to convert cheaper fish into higher priced products

39
Q
A
40
Q

Part 2

What is the official definition of surimi?

A

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

41
Q

Part 2

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

A

protein ; water

42
Q

Part 2

What are 5 desirable properties of fish used in making surimi [5]

A

1) Strong gel-forming ability
- marine fish > freshwater fish
- White-fleshed fish > dark-fleshed fish

<br></br>
2) Good organoleptic quality
- Good flavour, free from odour
- White flesh fish : visually nicer, while dark flesh may turn grey aft cooking

<br></br>
3) Abundant supply
4) All year round availability
5) reasonable price

43
Q

Part 2

Why do proteins from freshwater fish have lower form gel forming ability than proteins from marine fish?

A

Proteins in freshwater fish are more prone to denaturation

44
Q

Part 2

How do proteins in fish muscle denature, leading to lower gel forming ablity?

A

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

45
Q

Part 2

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

A
  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

<br></br>
2) Higher proteolytic activity
- High content of sarcoplasmic proteins with enzymes to break down myofibrillar proteins, interfering with gelling process

<br></br>
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)

46
Q

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]

A
  1. Removal of protein denaturing factors to enhance gel-forming characteristics of surimi during frozen storage

<br></br>
2. Concentrate the raw material, whcich is the water-insoluble myofibrillar protein
- wash away sarcoplasmic proteins which contain enzymes which may denature proteins

<br></br>
3. Remove undesirable compounds in fish muscle

47
Q

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.

A

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

48
Q

Part 2

How do cryoprotectants prevent protein denaturation?

A
  • When frozen, water forms ice crystals and dissociates from hydrophilic sites of myofibrillar proteins
    <br></br>
  • Cyroprotectants interact with hydrophilic sites of proteins, resulting in increased hydration
    <br></br>
  • Therefore, this prevents protein chains from aggregating and forming cross-linkages, reducing denaturation so that gelling ability is not compromised
49
Q

Part 2

What are some examples of cryoprotectants?

A

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

50
Q

Part 2

How are surimi products frozen?

A

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

51
Q

Part 2

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

A

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

<br></br>
Step 2
- after setting, suwari gel cooked at 80-90°C to yield a rigid and irreversible gel to form final product

52
Q

Part 2

What is the quality of surimi products affected by?

A

Texture, which is affected by gel strength

53
Q

Part 2

What is the Modori phenomenon in surimi products?

A

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

54
Q

Part 2

What is the negative environmental impact in surimi production?

how can it be tackled? [2]

A

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

55
Q
A

FST2106 -- Post harvest food processing (4 decks)

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