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Food Science & Nutrition > Water - week 2 > Flashcards

Water - week 2 Flashcards

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1
Q

What’s the equation of Density

A

Mass/volume

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2
Q

At What temperature is water at its heaviest density

A

4 degrees (this is when there is ice on the surface of the water and underneath it is liquid) due to its expanding when freezing

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3
Q

What properties does water have?

A

-large surface tension
-large dielectric constant(relative permittivity)
-large heat capacity
-Large latent heat of vaporisation

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4
Q

Define what surface tension is

A

Property of a fluid to resist an external force which is due to HYDROGEN BONDING

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5
Q

Define Dielectric constant

A

The relative amount of electrical energy stored in a material vs vacuum, its a very polar solvent and so it allows ionic salts to dissolve

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6
Q

Define heat capacity

A

Amount of heat energy required to raise the temperature by 1degree WITHOUT changing phase (e.g water vaporising)

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7
Q

Define latent heat of vaporisation

A

Amount of heat energy required to convert liquid to vapour without changing temperature (e.g 100 degree water into 100 degree steam)

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8
Q

Explain the bonding in water

A

Then paired electrons on Oxygen each pair up with a single electron from hydrogen to form COVALENT BONDS
The unbounded pairs of electrons on the oxygen are available to form temporary HYDROGEN BONDS with other molecules - (forms cohesion)

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9
Q

How do you determine the water content of a certain food

A

1) Weigh a dry container and record weight
2)Grind or mince food (to give it enough surface area)
3)Weigh sample into container and record weight
4)Heat at specified temperature for a specified time (or freeze dry)
5)Weigh sample and container and record weight
6)repeat heating/weighing cycle until constant MASS
7)calculate mass loss

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10
Q

What are the 2 reasons why mass loss varies with heating conditions when determining the water content of food

A

1)Loss of substances other than water (volatiles)
2)Incomplete removal of “bound” water

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11
Q

What is the formula to calculate the percentage of water on a FRESH weight basis

A

% water= (mass loss/original food mass) x 100

% water is always less than 100- as no fresh food is made up of 100% of water

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12
Q

What is the formal to calculate the percentage of water on a DRY weight basis

A

% water dry basis= (mass loss/food mass after dying) x 100

Sometimes this will give you an answer greater than 100% (yoghurt,jelly, etc)

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13
Q

What can you do to inhibit deterioration

A

By making water unavailable for chemical, enzyme can and microbial reactions
- remove water e.g drying or concentrating (mil powder,condensed milk ,cheese)
-immobilise water e.g freezing or adding sugar (jams), or slat
This shows that water activity is more important than water content itself

This is because foods with high water content seem to spoil more rapidly than foods with lower water content

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14
Q

Define/express the availability of water (equations and definitions of the terms)

A

aw = p/p0

p, vapour pressure of water in equilibrium with sample
p0, vapour pressure of pure water at the same temperature
(Vapour pressure relates to the tendency of molecules to escape from a liquid)

aw x 100 = Equilibrium Relative Humidity (ERH %)

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15
Q

Whats the simplified version of what water activity is ?

A

aw is an average value for water in the food- some molecules are bound more strongly than others.

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16
Q

What are the subdivisions that water can be categorised based on the strength of its binding

A
  • free/bulk water
    -entrapped water
    -Bound/multilayer water
    -vicinal water (closest to food surface)
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17
Q

What are the properties of vicinal water (also known as mono layer or Langmuir water)

A

-mostly strongly bound
-present at 0.5 ± 0.4 % of total water (of a typical high moisture food)
‒ Largest binding energy
‒ Removal by heat is difficult and not reversible (binding surface damaged)
‒ Does not freeze (cannot move to crystal lattice)
‒ Not a solvent
‒ Associated with proteins, polysaccharides, salts, especially the insoluble structures
‒ The layer of water is complete at aw approx 0.25

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18
Q

What are the properties of bound or multilayer water

A

‒ 3 ± 2% of water of a typical high moisture food
‒ Successive layers bound by hydrogen bonding
‒ Strength of binding declines progressively, negligible when n>3 ‒ Water held in smaller capillaries <1 μm diameter
‒ Layers complete at aw about 0.8
‒ Freezing point decreased substantially • Does not freeze until <-40C

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19
Q

What are the properties of entrapped water

A

‒ Water physically held/entrapped in the food matrix
• behindmembranes
• in capillaries > 1 μm diameter
• prevented from flowing freely by gel structure
‒ Up to 96% of total water of a typical high moisture food
» Solvent capacity, freezing point, water mobility all reduced slightly vs free water

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20
Q

What are the properties of free or bulk water

A

‒ Behaves like pure water
‒ Up to 96% of total water of a typical high moisture food
‒ Held within structure, but readily squeezed out by pressure ‒ Solvent capacity and freezing point are normal

21
Q

What are the reasons why foods can change their water content

A

» If the atmosphere and a food have the same aw - no overall transfer of water
» If aw(food) > aw(atmosphere) then the food loses water
- cakes and cheeses go dry, fruits shrink and wrinkle
» If aw(food) < aw(atmosphere) then the food gains water
- sugar, instant coffee, milk powder etc become sticky and cake together
» Also applies to foods in contact with each other e.g. cucumber sandwiches

This happens because water moves from regions of higher aw(water activity) to regions of lower aw

22
Q

How do you estimate the water activity by using Raoults law- derivation

A

aw = nw/(ns + nw)

ns=moles of solute
nw=moles of solvent (water)

23
Q

what is the difference between Temporary hardness and permanent hardness

A

Temporary hardness- bicarbonates- removed by boiling
Permanent hardness- sulphites and chlorides- can’t be removed by boiling

24
Q

What are the 2 methods of softening water

A

Lime softening
Use of ion exchange cartridges

25
Q

Explain lime softening

A
  • addition of slaked lime Ca(OH)2 and soda ash, Na2CO3- the soda-lime process
    -calcium precipitate as CaCO3
    -Magnesium precipitate as Mg(OH)2
    -softened water useful in heat transfer
    -process also kills bacteria
26
Q

explain how the use of ion exchange cartridges soften water

A

-resins in cartridges have sodium ions temporarily associated
-Hard water replaces sodium ions with Ca+ and Mg2+ inside the exchange
-Soft water, with Na+, leaves exchanger
-sodium ions restore by adding NaCl to exchanger

27
Q

Give 3 examples of waterborne pathogens

A

-Faecal bacteria
-Enteric viruses
-parasites

28
Q

Give examples of faecal bacteria

A

-VTEC (verocytotoxin E. coli)
-salmonella
-shigella
-Vibrio cholerae (Cholera)
-Campylobacter

29
Q

Give examples of Enteric viruses

A

-Hepatitis A
-Norovirus

30
Q

Give examples of parasites (2 examples)

A

-Cryptosporidium
-Cyclospora

31
Q

What are the steps of water treatment

A

(Screen it to remove large particles)
1) flash mixer- mixed with aluminium sulphate
2)flocculation tank. Floc settles out (sludge)
3)slow or rapid sand filtration (then can add activated carbon)
4)chlorination kills microorganisms

32
Q

What is the reason for water treatment

A

To reduce the bacterial load

33
Q

what is the requirement of chlorine and what PH do you need to have in the water to wash your fruit and veg & state the contact time

A

50-200 ppm chlorine
PH 6.0
contact time: 1-2 min

34
Q

what are the common contaminants (4)

A

-Norovirus
-Salmonella
-Cyclospora
-Listeria

35
Q

what are the sources of contamination

A
  • Farm worker hygiene (might be sick and still go to work)
    -manure from animals
    -Unsanitary conditions for on farm processing
    -contaminated irrigation water/flooding- e.g sewage leaks
36
Q

what are the steps of treatment for sewage/Effluent Treatment?

A

(Screening to remove large particles)
1)Settlement tanks to sediment organic solid matter (sludge)
2)Aeration (‘good bacteria consume bad bacteria)
3)Final settlement
4)sand filtration & then return to river
DONT USE CHLORINE

37
Q

what are the 2 tests to test how polluted your water are

A

1) Biochemical oxygen demand-BOD
2) Chemical oxygen demand-COD

38
Q

what are the steps of BOD (Biochemical oxygen demand)

A

test:
-Add waste sample to aerated water
-incubate at 20 degrees for 5 days
-measure oxygen used
-expressed as mg/L or ppm

39
Q

explain what the BOD test is and include what the treated effluent number should be

A

it’s the amount of dissolved oxygen for the microbial oxidation of biodegradable matter on an aquatic environment
properly treated effluent should be <20mg/L

40
Q

explain what COD test is

A

its the amount of oxygen consumed in a chemical oxidation of organic compounds in an aquatic environment

41
Q

what are the steps for the COD test (include what the treated level should be)

A

1) Add waste sample to boiling dichromate solution
2) Measure oxygen used
(Treated: 200-1000 mg/L)
it deals with the chemicals within the water

42
Q

why is COD>BOD always

A

there are somethings that can be oxidised by dichromate that isn’t oxidised when u conduct BOD, but u can’t do COD without BOD as there are some reactions take place in BOD that doesn’t in COD

43
Q

what is the aerobic oxidation of the effluent to reduce the BOD value

A

(Organic compound +oxygen +minerals +biomass) —-> (C02 +H2O +more biomass + heat)

44
Q

what is the anaerobic digestion of effluent in the reduction in the BOD value

A

(Organic compound + minerals + biomass) —> (CO2 + CH4 ± more biomass + heat)

45
Q

when is anaerobic digestion of effluent usually used

A

when there is heavy contamination

46
Q

In terms of water content, what happens to a food with a high water activity when it is placed in an environment of lower water activity?

A

water content decreases as water is lost into the air

47
Q

Give the name of the most strongly bound water present in foods.

A

vicinal water

48
Q

Define Chemical Oxygen Demand (COD) as it is applied to water quality. (2 marks)

A

Amount of oxygen consumed in a chemical oxidation of organic compounds in an aquatic environment

Food Science & Nutrition (6 decks)

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