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Food Chemistry > (2) Water in Foods > Flashcards

(2) Water in Foods Flashcards

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1
Q

mention some role of water in food

A
  • affect texture
  • affect freshness & preservation
  • universal solvent
  • ionizable
  • affect anzyme activities
  • affect microbial growth
  • heat transfer medium
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2
Q

food categories according to preservation

A
  • easily rotten (fresh meat, beef, etc)
  • slightly easily rotten (vegetables, fruits, bread)
  • preservable (sugars, dried spices, seeds)
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3
Q

example of water as a solvent

A

salt dissolution

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4
Q

example of water involvement in chemical reaction

A
  • dehydration of a polymer
  • hydrolysis of a polymer
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5
Q

example of water involvement in amylose and peptide synthesis

A
  • water in starch-glucose polymerization
  • water in peptide synthesis
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6
Q

water involvement in lipid synthesis

A

as a bond linkage in triglyceride

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7
Q

what’s water ph, mr, melting point, boiling point, and density?

A

ph= 7.0
mr= 18.0153 gr
melting point= 0.0 celsius
boiling point 1 atm= 100 celsius
density= 1 g/ml

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8
Q

what type of bond linked the H-O vs. H-H atom in water

A

covalent bond and hydrogen bond respectively

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9
Q

whats the amount of hydrogen bond energy?

A

10 kcal/mol

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10
Q

why does water have a high boiling point?

A

because of hydrogen bond

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11
Q

definition of moisture content (MC)

A

measure the amount of water contained in food (in percent %)

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12
Q

definition of relative humidity (RH)

A

the water vapor content in the air (in percent %)

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13
Q

definition of water activity (aw)

A

the degree of water activity in food that will react with microorganism

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14
Q

what’s the range value of water activity (aw)

A

0-1 (without units)

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15
Q

what are the 3 degree of water binding

A

physically bound water, chemically bound water, free water

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16
Q

3 sub-categories of physically bound water

A

capillary water= water bound in cavities in fine capillary in food
dissolved water= water seems to dissolve in soild materials (sugar water, salt water)
absorbed water= bound physically to the surface of food solid

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17
Q

the 3 sub-categories of chemically bound water

A

consitution water= binds to other compounds
crystal water= water is bound as a molecule in form of H2O (CaSO4.5H2O)

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18
Q

what are the 3 types of water in food

A
  1. actual bound water
  2. water molecules form hydrogen bond with other water
  3. water that is physically bound in food matrix (free water)
  4. unbounded water
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19
Q

why is water activity used as an index of water content for microbial growth?

A

because microorganism use free water for growth

20
Q

what does value aw=1 mean?

A

pure water

21
Q

what happens when pure water is stored in a closed room?

A

equilibrium will be formed with the surrounding air

22
Q

formula of water activity (aw)?

A

aw= P/Po = ERH/100

*P= water vapor pressure of food
Po= water vapor pressure of pure water

23
Q

definition of equilibrium relative humidity (ERH)

A

relative humidity of the air surround good that is in equilibrium with its environment.

also described as the percentage ratio of P to Po.

*P= water vapor pressure of food
Po= water vapor pressure of pure water

24
Q

what happens when aw of food > ERH

A

water will be released into air (decrease of moisture content)

25
Q

relation between relative humidity and water activity?

A

if the food is left in a specific relative humidity, the moisture content will balance overtime.

*if the air is very humid and product has low water activity, the product will absorb moisture from the air. vice versa

26
Q

definition of equilibrium moisture content (emc)

A

when the vapor pressure of the material is in a condition in balance with its environment (no change in weight)

27
Q

what’s the curve of the relation between water activity and moisture content?

A

motion sorption isotherm curve (msi)

28
Q

identify the different zones in msi curve

A

zone 1: tightly bound water. water is adsorbed on the surface of food. forces include hydrogen bond and van der waals
zone 2: intermediately bound water. monolayer is adsorbed and multilayers are added
zone 3: free water. water fills the intergranular space. water is mobile and retains the properties of water.

29
Q

what is adsorption

A

the attachment of one compound to the surface of another compound

30
Q

what is desorption

A

the remove of one compound from the surface of another

31
Q

explain msi curve adsorption

A

the curve starts from dry to wet conditions

32
Q

what’s the x and y-axis in msi curve

A

x-axis= water activity (0-1)
y-axis= moisture content (%)

33
Q

at what aw value does microbial growth is inhibited?

A

aw <0.6

34
Q

at what aw value does mold, yeast and bacteria commonly starts to grow?

A

mold (0.80), yeast (0.88) and bacteria (0.91)

35
Q

how many risk region is in food group and risk diagram?

A

3 risk region (low risk, medium risk and high risk)

36
Q

what’s the x and y-axis of food risk diagram

A

x-axis= pH
y-axis= aw

37
Q

ph and aw value for food with high risk

A

ph>4.5 and aw>0.85

38
Q

ph and aw value for food with low risk

A

ph<4.5 and aw<0.85

39
Q

ph and aw value for food with medium risk

A
  1. ph>4.5 and aw<0.85
  2. ph<4.5 and aw>0.85
40
Q

mention the importance of water activity

A
  • affecting texture
  • affect speed of agglomeration (easily absorb water)
  • affect chemical reaction (fat oxidation, maillard reaction, enzymatic reaction)
41
Q

definition of maillard reaction

A

a non enzymatic browning reaction between a reducing sugar and amino acids

42
Q

at what aw does maillard reaction (non-enzymatic browning) occurs

A

in practice, it occurs rapidly at aw= 0.5-0.8

43
Q

why does enzymatic reaction was surpressed in lower regions of msi curve

A

at low aw<0.2 free water is not available and so reaction wasn’t carried out

44
Q

at what aw value does free radicals reaction occurs

A

aw<0.2

45
Q

why does at higher aw value, the free radical reaction is inhibited

A

because at higher aw, the ability of water to dislodge oxygen from dried tissues

46
Q

but why does at aw>0.4 oxidation increases

A

the hydration of food cause swelling of protein and carbo -> exposure of oxidizable sites -> increased rate of oxidation

Food Chemistry (4 decks)

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