1. Chapter 5: Analysis of water content, total solids & water activity

2. Water is an essential constituent of many foods
Water is an essential constituent of many foods. It may occur as an intracellular or extracellular component in vegetable and animal products, as a dispersing medium or solvent in a variety of products, as the dispersed phase in some emulsified products such as butter and margarine, and as a minor constituent in other foods

3. The presence of water influences the chemical and microbiological deterioration of foods. Also, removal (drying) or freezing of water is essential to some methods of food preservation. Fundamental changes in the product may take place in both instances.

4. to measure the availability of water for chemical and biochemical reactions we use water activity, which is a property of aqueous solutions, is defined as the ratio of the vapor pressures of pure water and a solution

5. Definitions
Water content should refer to all water in food and nothing but water
Loss of weight on drying = ‘moisture content’
does not include all forms of water
may include non-water components
Chemical and spectroscopic methods may include almost all water
Total solids = dry matter after moisture removal
Water activity = a measure of free water
available for microbial growth and chemical deteriorative reactions
Tightly bound vicinal water - hydrogen bonded directly to hydrophilic molecules
Multilayer water some attraction to hydrophilic molecules
Entrapped water - water trapped by layers of fat
Free water - water not bound or trapped acts as pure water
Loss of weight on drying = ‘moisture content’
May not include some multi-layer and vicinal water
May include volatile non-water components such as organic acids, alcohols and aldehydes or decomposition of carbohydrates to give water or hydrolysis of sucrose to use up water
Chemical include Karl Fischer
Spectroscopic methods include NMR & NIR
Very important to distinguish water or moisture content from water activity

6. Water content of food Accurate determination of water is difficult;
not all water is free (freezes or evaporates)
may be bound by ionic and polar species in food
Therefore the keeping qualities of food should be measured by aw (range 0.1-1) rather than moisture content
Ionic: salts
Polar: amino acids
Where do sugars stand

7. aw activity ranges High moisture foods aw 0.97 to 1;
highly susceptible to deterioration by micro- organisms
milk, fruit, vegetables, meat etc
Intermediate moisture foods aw 0.6 to 0.9
shelf stable without refrigeration or heat processing
susceptible to enzymic browning & Maillard reaction
cheese, jams, jellies etc

8. aw activity ranges Low moisture foods aw < 0.6
stable to deterioration
except lipid oxidation
dried milk, cereals

9. Importance of water content in food
Measuring water in foods is important in ensuring:
preservation and stability
product quality
legal food standards are met
Preservation and stability: micro & chemical stability of dehydrated vegetables and fruits, milk powders
Product quality: certain water levels required for high quality jams and jellies (to prevent sugar crystallisation), minimum levels to produce crisp breakfast cereals. (bowl time/life)
Legal food standards are met: in some countries milk and fruit juices are required to have minimum total solids levels, there are maximum levels of added water permitted in processed meat products

10. Methods of water measurement -Oven drying
Loss of weight on heating used to calculate moisture content of sample
convection oven, oC, several hours - overnight, heat stable samples
forced draft oven
better air circulation
vacuum oven, approx. 70oC ( mmHg), several hours - overnight, heat unstable samples (sugars)
A simple method (AOAV), allows simultaneous analysis of a large number of samples
Convection oven: no fan, variation in temperature across oven (±10oC) can produce differing results
Fan-forced oven: Less temperature variation (±1oC)
Vacuum oven: used for fruits and high sugar products designed to minimize cold spots and remove moisture. They need a dry air purge plus temperature and vacuum control
Ideally handle pans with tongs

11. vacuum desiccators at room temperature)
infrared drying lamps.
incorporates direct reading balance
fast but lacks accuracy
distance from sample is important
sample thickness (curst formation)
not approved by AOAC
vacuum desiccators at room temperature
for products such as backing powder
Vacuum desiccators; Ammonium carbonate and bicarbonate decomposes during baking therefore does not need a leavening acid as does sodium bicarbonate
Also:
Infra-red heat lamp drying, mins. Incorporate halogen lamps ( K) or ceramic heating elements. Used for spot checks due to speed but they lack accuracy. Sample may burn or case harden while drying
Microwave drying , 2-5 mins

12. Pure water evaporates at 100oC at sea level.
boiling point increases with increased concentration
Sample preparation for oven drying;
liquids may be pre-dried over a steam bath before drying in oven
high moisture solids such as yoghurt and humus mixed with known weight of pre-heated and cooled sand and a small glass rod that remains in sample pan
But binding to dissolved material or food matrix increases boiling point more and more as concentration occurs
1g of solute dissolved in 1L of water, the boiling point would increase by 0.512oC. Therefore the boiling point increases as the moisture removal process occurs.
Water is retained in biological products to at least 365oC
Two stage process often used - liquids can be pre-dried over a steam bath before drying in oven. Foods that form skin on drying which could impede drying can be mixed with known weight of pre-heated and cooled sand and a small glass rod that remains in sample pan during evaluation

13. Sample preparation Sample pans mill to consistent particle size of 1mm
eliminate exposure to atmosphere
store in airtight small container with minimal headspace
Sample pans
aluminum with cover
pre-heated at 100OC for 3 hrs
cool and store in desiccators
Need to learn to work as a team

14. Oven drying Advantages/Disadvantages
simple, little expense and reasonably accurate
Disadvantages;
unsuitable for products
C6H12O6  6C + 6H2O (produce moisture)
sucrose hydrolysis (utilise moisture)
containing volatile constituents
acetic & butyric acids; alcohols, esters & aldehydes
variation between samples due to variation in sample particle size
Sample moisture loss is a function of time and temperature.
Decomposition occurs when time is too long and temperature is too high. As the temperature rises decomposition products are CO, CO2, CH4 and H20
Method is a compromise between time and temperature.
Loss of volatile matter is a less serious but is a source consistent error
alcohols, esters & aldehydes (flavour compounds

15. Distillation procedure
Immiscible solvent (xylene or toluene)
less dense than water with boiling point slightly higher
helps prevent charring of sample and assists in heat transfer and effective distillation
Sample and solvent in distillation flask
Heated to distill emulsion of water and solvent
Toluene boiling point is 110C and xylene boiling point of C. Toluene is the most widely used.
Causes less thermal decomposition than oven drying method.
When Toluene beings to boil a hazy cloud appears with is a vaporous emulsion of waterin toluene.

16. Emulsion condenses in condenser and runs into graduated tube (Bidwell-Sterling moisture trap)
emulsion separates and water layer can be measured on graduations under solvent layer
Condensation occurs at the condenser where water droplets from the emulsion inverts and becomes toluene dispersed in water. in the Bidwell-Sterling moisture trap

17. Distillation procedure

18. Distillation procedure, advantages / disadvantages
useful for foods containing low moisture content and volatile oils
cheap to run, no sophisticated equipment
Disadvantages;
under estimates water content (water droplet may cling to dirty apparatus)
requirement for flammable solvents
Useful for foods of low moisture content containing volatile oils such as herbs and spices since the oils remain dissolved in the organic solvent

19. Chemical procedures - Karl Fischer Titration
Ideal for low moisture foods showing erratic results by oven drying
Rapid & sensitive (no heat)
Based on reduction of iodine with SO2 in the presence of water
2H2O + SO2 + I2  C5H2SO4 + 2HI
Low moisture foods - easier than measuring small weight loss in oven drying
Useful for food that show erratic results when subjected to heat or vacuum - volatiles, degradation of sugars, sticky skins preventing moisture loss dried fruit and vegetables, oils and roasted coffee candies chocolate
For each mole of water present, 1 mole of iodine and 1 mole of SO2, 3 moles of pyridine and 1 mol of methanol is used

20. Difficulties and sources of error of Karl Fischer Titration
Incomplete water extraction (especially in solid food)
finely grind food
Atmospheric water (drying tubes)
Moisture adhering to unit
Interference
ascorbic acid
carbonyl compounds
unsaturated fatty acids
Incomplete water extraction - need to finely grind sample if not rapidly dispersible. May extract moisture with appropriate solvent (methanol). The methanol extract is titrated with KFR.
Atmospheric water – drying tubes - moisture must not be allowed to infiltrate the reaction chamber
Moisture adhering to unit - all equipment must be totally dry before use
Interference
ascorbic acid - oxidised by KFR to dehydro-ascorbic acid leading to overestimation
carbonyl compounds - react with methanol to form acetals and release water and therefore overestimate it (find out what are acetasl up to here
unsaturated fatty acids - react with iodine to overestimate water

21. Introduction to physical methods of water content analysis
Electrical methods
based on resistance to or conductivity of an electric current
the electrical resistance of wheat at 13% moisture is 7 times greater as that of 14% and 50 times at 15%
dielectric constant (measure ability to store charge)
for water (80.37 at 20C), dielectric constant of starch and protein is 10
% water is obtained from standard curve
Hydrometry
measures specific gravity (density=weight/volume)
alcoholic beverages, brines and sugar solutions using hydrometer
Dielectric how polar the compound. Water has high polarity and hexane has low polarity
Nielsen (2003 p93):
Dielectric (resistance) - used widely for cereal grains as water has dramatically different resistance to an electric current than other grain components (starch and proteins) therefore water content can be estimated against appropriate standards curves based on resistance readings for a particular grain
Hydrometry - used to measure water content of solution of one component in water.
Hydrometer is a standard weight on the end of a spindle - displaces weight of liquid equal to its own weight therefore in low density liquid will sink deeper than in high density liquid. Specially calibrated to measure specific gravity of milk, salt solutions, fruit juices, during processes such as concentration and fermentation. From which water content can be calculated.
Best applied to the analysis of solutions of only one solute in water

22. Hydrometers
Nielsen 2003, p. 94
Lactometer used to measure the specific gravity of milk to 1.040
Baume hydrometer designed to measure the density of salt
Brix hydrometer: used for sugar solutions such as fruit juice and syrups

23. Electrical methods - Dielectric constant Meter
Instruments need calibration against known samples
Sample density and temperature affect the reliability
Useful for continues measurements
Limited to food systems that contain no more than % moisture
These instruments need calibration against samples of known moisture content

24. Refractometry
refractive index of a solution increases with concentration of solute
refractometer calibrates in Brix used for carbohydrate-based solutions
Brix = g sucrose/100g sample
Refractoemeters may be installed in liquid processing lines to monitor brix of products such as carbonated soft drinks
Refractometer - sugar syrups, fruit products, tomato products, sugar concentration in foods

25. Refractometry Measures ‘refractive index’
Based on bending of light rays in solution containing component being measured
Dependent on concentration of solution
Used for rapid estimation of sugar content of jams during processing
When a beam of light is passed from one medium to another and the density of the two differs then the beam of light is bent or refracted.
Refractive index detectors to identify separated compounds in HPLC (see later info)
Used as detection method in CSIRO water activity kit
Display refractometer to students if possible

26. Refractometer
Bench top instruments are more accurate compared to hand held units due to temperature control. Bench top refract. Have water circulation through the head where the prism and sample meet.

27. Freezing point - Cryoscope
AOAC approved (method )
used to test for added water in milk
AOAC standard freezing point taken as C
the FDA rejects milk if freezing point > than C
When water is added to a food product many physical constants are altered, due to changing the concentration of solute particles as ions or molecules. These properties are vapor pressure, freezing point, boiling point, and osmotic pressure.
Measurement of any of the above may assist in determining the concentration of solutes in a solution.
Milk freezing point is important for economic reasons
Freezing point - uses an instrument known as a cryoscope that takes 1-2 min per sample using a pre-chilled sample

28. Expression of compositional data on ‘as is’ or ‘dry basis’
‘as is’, ‘as received’, ‘fresh sample’
refers to composition (eg fat content) of sample as a percentage of food as received from original source
‘dry matter basis’, ‘dry basis’
refer to composition (eg fat content) as a percentage of the total solids in the sample

29. Calculating composition on ‘dry basis’ from ‘as is’ analysis
The Bligh & Dyer method requires fat to be extracted from a fresh, ‘as is’ sample (not pre-dried)
% fat ‘as is’ is calculated as follows:
% fat (as is) = wt fat extracted (g) x 100
fresh sample wt (g)
to convert fat content on ‘as is’ basis to ‘dry basis’ the following equation is used
% fat (dry basis) = % fat (as is) / dry matter coefficient

30. The Soxhlet method requires fat to be extracted from a pre-dried sample of food
% fat ‘dry basis’ is calculated as follows:
% fat (dry basis) = wt fat extracted (g) x 100
wt of pre-dried sample (g)
to convert fat content on ‘dry basis’ to ‘as is’ basis (eg the % fat in the original sample before drying) the following equation is used:
% fat (as is) = % fat (dry basis) x dry matter coefficient

31. Measurement of water activity (aw)
A better indicator of food perishailiby than moisture content
Refers to water not tightly bound to other food molecules
aw = P / Po = ERH / 100
aw = water activity
P = partial pressure of water above sample
Po = vapour pressure of pure water at same temp.
ERH = equilibrium relative humidity surrounding product
Foods of same moisture content can differ in their perish-ability
Water activity above 0.6 supports microbial growth below will not
High water activity supports deteriorative reactions involving water such as hydrolysis and softening of fruits and vegetables during ripening
BUT - very low water activity increases rate of oxidation reactions such as lipid auto-oxidation and fat-soluble vitamin oxidation since if water present oxygen will tend to dissolve in it and not come in contact with immiscible lipid material - this protection is lost on drying - therefore dried foods have to be packaged very carefully to prevent oxidation damage
Depends on the levels of polar and ionic compounds such as sugars, polysaccharides, and salts in the food, which bind with the water and make it unavailable for microbial growth and many chemical reactions but which is still measured to some extent by methods such as oven drying and particularly Karl Fischer Titration.
Important to understand relationship between water activity and moisture content in food systems to ensure the microbial and chemical stability and satisfactory shelf-life of the food products.
Water activity represents the relative humidity that the foodstuff in question would produce, compared to that which pure water would produce if equilibrated in a sealed container at a constant temperature.
Thus a water activity of 0.7 would produce a ERH of 70% of that of pure water.