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Y1S1_FST1101B > CHAP 11 : FOOD CHEMISTRY > Flashcards

CHAP 11 : FOOD CHEMISTRY Flashcards

1
Q

All foods are composed of chemicals. True or False?

A

True.
All food is made up of chemical substances. Chemicals in food are largely harmless and often desirable – for example, nutrients such as carbohydrates , protein , fat and fibre are composed of chemical compounds. Many of these occur naturally and contribute both to a rounded diet and to our eating experience.

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2
Q

What is function of water? [2]

A
  1. Acts as a solvent for biochemical reactions, promotes growth of microorganisms.
  2. Acts as a solvent for chemical reactions to take place
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3
Q

What is free-flowing water and bound water in foods?

A
  • Free flowing water can be extracted with ease from foods, through cutting, pressing or squeezing.
  • Bound water cannot be extracted easily, it is chemically bound to food

NOTE : Fruit juice is not considered as free flowing water in food. While fruit juice is primarily composed of water, it also contains other components such as sugars, acids, and nutrients, which are not present in free-flowing water. Free flowing water refers to plain, unsweetened water

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4
Q

What is meant by the water activity of a food?

What direction does water move in?

A

It estimation of how much water is available for microbial growth and chemical reactions.

Water moves from area of high water activity to area of low water activity.

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5
Q

What is the equation for water activity?

A

vapour pressure in food / vapour pressure of water

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6
Q

What is the maximum level of water activity such that a food is considered low moisture?

A

0.85

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7
Q

What are 3 ways to reduce the water activity of a food?

A
  1. Evaporate the water
  2. increase [solutes]
  3. freeze water (water is frozen and locked in ice so they are not available to act as a solvent)
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8
Q

What are the 4 types of catrohydrates?

What are carbohydrates made of and list chemical formula?

A
  1. Monosaccharides
  2. Disaccharides
  3. Oligosaccharides – short chain of monosaccharides (~3-10)
  4. Polysaccharides

Formula : (CH2O)n

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9
Q

What bonds are monosaccharides linked by in disaccharides / oligosaccharides /polysaccharides ?

A

Glycosidic bonds

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10
Q

What is the name of the carbohydrates that

  1. our bodies can hydrolyse
  2. our bodies cannot hydrolyse?

give examples

A
  1. Digestible carbohydrates
    - Disaccharides such as sucrose, maltose
    - Polysaccharides such as starch
  2. Dietary fibre
    - polysaccharides such as cellulose (found in cell walls of plants)
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11
Q
  • How is dietary fibre categorised?
  • Dietary fibre that can be _____ and have a potential health benefit are called _____
A
  • They are categorised according to whether they can be fermented by micrroorganisms in our gut.
  • feremented, prebiotics
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12
Q

Why are monosaccharides soluble in water? [jc explanation]

A

There are several -OH (hydroxyl groups) in monosaccharides. The -OH groups form strong hydrogen bonds with water. [check again]

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13
Q

What are reducing sugars? What reaction can they undergo?

A

Carbohydrates that are aldehydes.

  • They can react with amino groups from protein and amino acids in a reaction called the millard reaction.
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14
Q

What is meant by
1. Primary structure
2. Secondary structure
3. Tetiary structure
4. Quaternary structure

of a protein?

A
  • Primary structure is the sequence of amino acids in the protein
  • Secondary structure is when the chain of amino acids fold to form a alpha-helix / beta-pleated sheets DUE TO INTRAMOLECULAR interactions between carboxyl and amino groups of an amino acid.
  • Tertiary structure is formed when secondary structure FURTHER INTERACT AND FOLD to form 3d structures that consist of alpha helix and beta pleated sheets
  • Quaternary structures are proteins consisting of more than 1 amino acid chain.
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15
Q

During denaturation, which of the following structures of protein are destroyed?

  1. Primary structure
  2. Secondary structure
  3. Tetiary structure
  4. Quaternary structure
A

2,3,4
The SHAPE of the protein destroyed, but the STRONG PEPTIDE BONDS in the primary structure (amino acid sequence) is not destroyed.

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16
Q

What are enzymes, and what are some of its characteristics?

A
  • An enzyme is a protein that catalyses chemical reactions
  • Enzymes shape and strucuture is determined by its pri/sec/tet/quat structures and thus is very specific in nature ;; only substrates with complementary shape to enzymes can fit.
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17
Q
  • What temperature and duration is blanching usually carried out?
  • How does blanching affect enzymes?
A
  • 80-100°C, for 30s-1 min

Blanching denatures enzymes

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18
Q

Why are lipids soluble in organic solvents but not in water? (find if jc notes got explanation)

A

Lipids are non-polar and thus can dissolve in organic solvents as they are non-polar too, can form id-id interactions which require lots of energy to overcome.

However, water is polar and thus lipids are unable to interact with water molecules

(“like dissolves like”)

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19
Q
  • What are the different categories of lipids, and name the 4 kinds of lipids.
  • What is the most common type of lipids seen?
A
  • Fatty acid chain lipids
    1. Triglycerides
    2. Phospholipids
    3. Waxes
  • Non fatty acid chain
    4. Steroids (e.g. cholestrol which has 4 c rings)
  • Triglycerides where there is a glycerol backbone + 3 long chains of fatty acid
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20
Q

What are the common elements that all lipids are made of? What is the characteristic of lipids (structure)?

A

Carbon,Hydrogen, oxygen (same as lipids)

Lipids have long hydrocarbon chains.

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21
Q

Lipids that are solid at room temp are called _____ while lipids that are liquid in room temperature are called _____.

A

fats;; oils

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22
Q

Why are most lipids solids at room temperature? [check jc notes for expln]

A

Lipids have a huge Mr (relative molecular mass). This gives rise to stronger id-id interactions between lipid molecules which require a large amount of energy to overcome, thus they are mostly solids at room temperature.

[ Lipids have a simple molecular structure. they have a large Mr and thus a large polarisable cloud of electrons, leading to strong id-id requiring a large amount of energy to overcome. Thus, they are solids at rm temp]

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23
Q

Fatty acids with one C=C are called _____ fat while fatty acids with two or more C=C are called _____fat.

A

monoUNsaturated ; polyUNsaturated

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24
Q

Why are trans fatty acids usually solids at room temperature while cis fatty acids are liquid at room temp? [check jc notes]

A
  • Trans fatty acids do not form kinks, molecules can pack closer together, causing stronger id-id interactions which require more energy to overcome due to more efficient stacking.
  • Cis fatty acids form a kink in fatty acids, stacking is not so effiecient and id-id interactions between molecules are weaker –> liquid
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25
Q

What is the structure of phospholipids?
Where can they be found?

A

2 fatty acids and a polar head group bonded to glycerol.

They are builing blocks of cell membrane (plasma membrane)

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26
Q

-food spoilage-
What is meant by
1. Hydrolytic rancidity
2. Oxidative rancidity

A
  1. Hydrolytic rancidity is when LIPIDS (not just fats) to hydrolyse to produce free fatty acids and glycerol (also produces off flavors and aromas)
  2. Oxidative rancididty is when C=C in lipids are oxidised, generating off flavours and contributing to the odour of food.
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27
Q

What are the other 6 chemical components of food other than water, carbs, proteins, lipids?

A
  1. Vitamins
  2. Minerals
  3. Colour compounds
  4. Flavour compounds
  5. Organic acids
  6. Secondary metabolites
28
Q

What are secondary metabolities?

Compounds that are not required for the _____ or _____ of an organism; but are produced to confer a _____ _____(2 words) of the organism.

A

growth; reproduction; selective advantage of the organism.

29
Q

Name 2 secondary metabolites and describe what they are.

A
  1. Alkaloids – naturally occuring alkaline organic compounds whick contain N atoms in their structures. N atoms are usually found in a ring structure (e.g. caffeine)
  2. Polyphenols – naturally occuring organic compounds with aromatic ring with more than one hydroxyl groups attached to the ring
30
Q

What are the 3 sources of toxic compounds? Give examples

A
  1. Naturally occuring (cyanide in cassava, the root from which tapioca is extracted from ;; Wild mushrooms may contain several toxins, such as muscimol and muscarine)
  2. Contaminants (pesticides)
  3. Micrroorgansims (mould pproducing mycotoxins)
31
Q

What are the 5 main functions of chemicals in food?

A
  1. Structural (provide structure to food)
  2. Colour
  3. Taste
  4. Aroma
  5. Prevention of spoilage
32
Q

The primary function of food additives is to prevent growth of micrroganisms. True or False?

A

False. Their primary function is to improve taste, odour, appearance or lengthen the shelf of foods.

Preservatives are the ones that prevent growth of microorganisms.

33
Q

The most common structure of food is the colloidal system. What is a colloid?

A

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended through another substance.

34
Q

In colloids, the dispersed phase is the phase that exists as ____ _____ (2 words) and _____in the surrounding liquid.

The continous phase is the _____ ______

A

small droplets ;; suspended;; surrounding liquid

35
Q

What are the 4 diff kinds of colloids? State the dispersed phase, continous phase, and the name of the colloid w examples.

A
  1. Colloidal suspension (aka sols) – dyes, paint,gravy stirred custard and other thick sauces

Dispersed phase (D) : solid ;;
Continuous phase (C) : liquid

  1. Gels – Jam, jelly, cheese
    D : Solid
    C: liquid
  2. Emulsion – milk, mayonnaise
    D: liquid ;;
    C: liquid

milk : milk fat is dispersed in water. Milk fat is usually liquid

  1. Foam – whipped cream, whisked egg whitee
    D: air
    C: liquid
36
Q

What is the difference between sols and gels?

A

Sol is a liquid state of colloidal solution whereas gel is a solid or semisolid state of colloidal solution. No definite structure is present for sols whereas generally a honeycomb like structure is present for gel.

36
Q

Liquid milk can be considered a colloidal suspension of proteins dispersed in water. True or False?

A

True. Proteins such as caesin and whey are suspended in water in liquid milk.

Liquid milk can be considered an emulsion as there are fat globules dispersed in water too.

But milk are mostly emulsions (e.g. powdered milk etc)

37
Q

What kinds of biological molecules can contribute to the colloidal and gel-like properties of foods?

A
  1. Protein
    - Casein and whey can coagulate
  2. Carbohydrates
    - starch – upon heating, interacts w water through H bonding and can form viscous paste/gel when cooled
38
Q

What is modified starch?

A

When starch is chemically modified

  1. to become resistant to hydrolysis and maintain a high viscocity over shelf life
  2. modified such that they thicken or form gels even with cold water
39
Q

Lipids form important strucutral components in _____.

A

emulsions

40
Q

How does caesin act as an emulsifier to prevent separation of milk fats from water?

A
  1. During homogenisation, particle size of fat is reduced and caesin is partially ufolded
  2. Hydrophobic parts of caesin interacts with fat droplets and hydrophilic parts interact with the aqueous phase (aka water) and
41
Q

The red and yellow pigments in many fruits and vegetables belong to the _____ family. An example is _____ which is found in carrots and oranges, and it is also the precursor for _____.

A

carotenoid ;; β-carotene ;; vitamin A

**β-carotene can be converted into vitamin A in the body thus it is said to be a precursor of vitamin A

** However, not all carenoids can be converted into vitamin A

42
Q

The orange colour in shrimps comes from _____, a type of carotenoid.

Can this carotenoid be converted in vit A in the body?

A

Astaxanthin

No

43
Q

Another source of colour in food is the Millard Reaction. During the rxn, _____ compounds can be formed. The flavour profile of pyrazine compounds differ depending on the _____ of the _____.

Pyrazine compounds can further react to form brown coloured compounds which contribute to the (golden to brown) colours of food.

A

pyrazine ;; substituent ;; R group

44
Q

List 2 sources of undersirable, unwanted browning in food products.

A
  1. millard rxn
    - e.g. evaporated milk looks more brown than fresh milk due to millard rxn, and this browning can be more severe over shelf life and can create the impression to consumers that the product is spoilt
  2. Enzymatic browning
45
Q

The enzyme _____ _____ , catalyses the oxidation and polymerisation of _____ present in fruits and vegetables in a process called enzymatic browning.

A

Polyphenol oxidase ;; polyphenols

46
Q

What is the key limitation of naturally occuring pigments as compared to synthetic pigments?

A

Naturally occuring pigments are less stable, meaning they are more prone to fading or discolouration.
- insufficient to create bright and vibrant colours, may require high concs

47
Q

How do we perceive taste?

A

When specific compounds bind with receptors on the tongue, as well as on the hard and soft palette

48
Q

What does the taste of umami refer to?

A

The taste imparted by glutamic acid (an amino acid), savoury, mouth-watering sensation associated with foods like broth / meat

  • Often added in the form of MSG to foods (monosodium glutamate)
49
Q

What compounds contribute to the aroma of foods?
How do we perceive aroma of foods?

A

Volatile compounds ;;
Aroma is perceived through olfactory receptors in the nose that detects these molecules

50
Q

_____ is a compound that when is smelled on its own is highly characteristic of the odour of the vanilla bean. _____ is also a volatile molecule found in vanilla beans, but smells _____ on its own.

A

Vanallin ;; Guaiacol ;; smoky

51
Q

What creates the sense of aroma/odour of foods?

A

The combined sensation from all the different volatile compounds present in a food

52
Q

During fermentation, yeast produce _____ that have _____ odours.

A

The combined sensation of odour and taste DURING chewing and swallowing of food

53
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

During food spoilage, volatile molecules w unpleasant odours may be produced. The hydrolysis of _____ in butter and milk can generate volatile short-chain _____ _____ which smell like _____

A

triglycerides ;; fatty acids ;; vomit

**hydrolytic rancidity, catalysed by lipases in contaminating bacteria

**NOTE : When there are free fatty acids at low levels, it contributes to the characteristic odour of dairy products, which are desirable.

54
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

What kinds of food are antioxidants added to?
What is the function of antioxidants?

A

Antioxidants are added to oils to prevent them from oxidising (rancid)

55
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

Name examples of 2 natural oxidants and 2 synthetic oxidants.

A

Natural oxidants
- α-tocopherol : Vitamin A
- Ascorbic acid : Vitamin C

Synthetic oxidants
- Butylated Hydroxyanisole (BHA)
- Butylated Hydroxytoluene (BHT)

56
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

Why are synthetic antioxidants preferred over natural antioxidants sometimes?

A

Synthetic antioxidants are much more cost effective in preventing oxidation.

57
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

Other than antioxidants, what other kinds of substances can prevent oxidants?

A

Sequestrants, such as EthyleneDiamineTetraacetic Acid (EDTA), to bind to transition metals since transition metals often catalyse oxidation.

** A sequestrant is a food additive which improves the quality and stability of foods. A sequestrant forms chelate complexes with polyvalent metal ions, especially copper, iron and nickel, which can prevent the oxidation of the fats in the food. Sequestrants are a type of preservative

58
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

What 2 functions does sodium nitrite serve in cured meat?

A
  1. Prevents growth of pathogens such as the bacteria clostridium botullinium
  2. Gives pinkish ccolour of cured meat
59
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

What are some common preservatives to :

  1. Lower water activity of food
  2. Lower pH of food
A

1, sucrose (sugar), NaCl (salt)
2. citric acid, acetic acid

60
Q

FUNCTIONS OF CHEMICALS IN FOODS : PREVENTING SPOILAGE

What is the function of other organic acids, such as benzoic acid and sorbic acid?

A

They have a direct inhibitory effect on micrrorganisms.

61
Q

CONTROVERSIES IN FOOD CHEMISTRY

What is a clean label?

A

A food label that contains only easy to pronounce ingredients without any chemical sounding names.

62
Q

CONTROVERSIES IN FOOD CHEMISTRY

“A product with a longer ingredients list (esp w long complex chemical names, unclean label) means it is more harmful to consume”. Give 3 reasons why this may not be the case.

A

Products with longer ingredients list…

  1. may be fortified, contain more vitamins and minerals which are beneficial to health.
  2. though there are lab chemicals present like EDTA, it is usually in trace amounts and is not harmful to health. Safety aspects of ALL additives are carefully monitored by the government
  3. may be safer to consume. In natural foods with cleaner labels, additives extracted from natural foods may be unsafe, as there may be trace amounts of contaminants present in natural foods.
63
Q

CONTROVERSIES IN FOOD CHEMISTRY

What is the naturalistic fallacy?

A

a type of logical fallacy which argues if something is natural, it must be good.

64
Q

CONTROVERSIES IN FOOD CHEMISTRY

Additives may be synthetic but this does not mean that they are harmful, and a lot of harmful chemicals occur naturally too. True or False?

A

True

Y1S1_FST1101B (12 decks)

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