(2) Water in Foods Flashcards
mention some role of water in food
- affect texture
- affect freshness & preservation
- universal solvent
- ionizable
- affect anzyme activities
- affect microbial growth
- heat transfer medium
food categories according to preservation
- easily rotten (fresh meat, beef, etc)
- slightly easily rotten (vegetables, fruits, bread)
- preservable (sugars, dried spices, seeds)
example of water as a solvent
salt dissolution
example of water involvement in chemical reaction
- dehydration of a polymer
- hydrolysis of a polymer
example of water involvement in amylose and peptide synthesis
- water in starch-glucose polymerization
- water in peptide synthesis
water involvement in lipid synthesis
as a bond linkage in triglyceride
what’s water ph, mr, melting point, boiling point, and density?
ph= 7.0
mr= 18.0153 gr
melting point= 0.0 celsius
boiling point 1 atm= 100 celsius
density= 1 g/ml
what type of bond linked the H-O vs. H-H atom in water
covalent bond and hydrogen bond respectively
whats the amount of hydrogen bond energy?
10 kcal/mol
why does water have a high boiling point?
because of hydrogen bond
definition of moisture content (MC)
measure the amount of water contained in food (in percent %)
definition of relative humidity (RH)
the water vapor content in the air (in percent %)
definition of water activity (aw)
the degree of water activity in food that will react with microorganism
what’s the range value of water activity (aw)
0-1 (without units)
what are the 3 degree of water binding
physically bound water, chemically bound water, free water
3 sub-categories of physically bound water
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
the 3 sub-categories of chemically bound water
consitution water= binds to other compounds
crystal water= water is bound as a molecule in form of H2O (CaSO4.5H2O)
what are the 3 types of water in food
- actual bound water
- water molecules form hydrogen bond with other water
- water that is physically bound in food matrix (free water)
- unbounded water
why is water activity used as an index of water content for microbial growth?
because microorganism use free water for growth
what does value aw=1 mean?
pure water
what happens when pure water is stored in a closed room?
equilibrium will be formed with the surrounding air
formula of water activity (aw)?
aw= P/Po = ERH/100
*P= water vapor pressure of food
Po= water vapor pressure of pure water
definition of equilibrium relative humidity (ERH)
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
what happens when aw of food > ERH
water will be released into air (decrease of moisture content)
relation between relative humidity and water activity?
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
definition of equilibrium moisture content (emc)
when the vapor pressure of the material is in a condition in balance with its environment (no change in weight)
what’s the curve of the relation between water activity and moisture content?
motion sorption isotherm curve (msi)
identify the different zones in msi curve
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.
what is adsorption
the attachment of one compound to the surface of another compound
what is desorption
the remove of one compound from the surface of another
explain msi curve adsorption
the curve starts from dry to wet conditions
what’s the x and y-axis in msi curve
x-axis= water activity (0-1)
y-axis= moisture content (%)
at what aw value does microbial growth is inhibited?
aw <0.6
at what aw value does mold, yeast and bacteria commonly starts to grow?
mold (0.80), yeast (0.88) and bacteria (0.91)
how many risk region is in food group and risk diagram?
3 risk region (low risk, medium risk and high risk)
what’s the x and y-axis of food risk diagram
x-axis= pH
y-axis= aw
ph and aw value for food with high risk
ph>4.5 and aw>0.85
ph and aw value for food with low risk
ph<4.5 and aw<0.85
ph and aw value for food with medium risk
- ph>4.5 and aw<0.85
- ph<4.5 and aw>0.85
mention the importance of water activity
- affecting texture
- affect speed of agglomeration (easily absorb water)
- affect chemical reaction (fat oxidation, maillard reaction, enzymatic reaction)
definition of maillard reaction
a non enzymatic browning reaction between a reducing sugar and amino acids
at what aw does maillard reaction (non-enzymatic browning) occurs
in practice, it occurs rapidly at aw= 0.5-0.8
why does enzymatic reaction was surpressed in lower regions of msi curve
at low aw<0.2 free water is not available and so reaction wasn’t carried out
at what aw value does free radicals reaction occurs
aw<0.2
why does at higher aw value, the free radical reaction is inhibited
because at higher aw, the ability of water to dislodge oxygen from dried tissues
but why does at aw>0.4 oxidation increases
the hydration of food cause swelling of protein and carbo -> exposure of oxidizable sites -> increased rate of oxidation
