Principles of Food Analysis
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Where is food analysis required/used?
- In quality control: for safety and quality
- In the government: for regulation, consumer protection, health and food safety, to protect against fraud
- For research: to measure (quantities), analyze characteristics
How are the results of food analysis done for research used?
Need to find/use the more reliable methods
How are the results of food analysis done for the government used?
Lead to regulations, have legal implications - need high degree of reliability
How are the results of food analysis done for quality control used?
- Rapid results needed sometimes - rapid methods (especially for process control).
- Although may have some legal implications, all depend on the contract with the customer/supplier
What is reliability?
A combination of the specificity, accuracy, precision (or variability), and sensitivity of a set of results
What is specificity?
the ability of a method to measure a component of interest in the presence of other components without interference
What is accuracy?
the ability of a method to measure the true value of a component (where true = near standard)
What is precision (variability)?
The ability os a method to give a value from a singe measurement that comes close to the average from a large number of measures
What is sensitivity?
The number of decimals when measuring trace components
What is proximate analysis?
- A way of measuring the major components of foods.
- It is based on assumptions.
- This method is used most of the time for nutritional labels.
What are the 2 most basic components of food?
How can we determine liquids in food?
- Total solids (TS) vs Dry matter (DM)
- we can determine liquids by using TS - DM
What are the 3 types of non water components?
- Organic nitrogenous: (nitrogen-containing components are assumed to be proteins, so we count this as "protein content")
- Organic non-nitrogenous: (either lipids for lipid content calculations, or carbs and fiber)
- Inorganic: (ash)
How do we determine % carbs?
%water + % ash + % Crude proteins + % crude fat + % dietary fiber = 100% - % carbs
How do we calculate the amount of crude proteins?
Nitrogen amount (g)/16% = protein amount
What is crude fat?
non polar extractible material
Why is H2O measurement important?
- Can help prevent food spoilage
- Gives us an idea of food safety
- Economics- some foods are purchased by weight
What are the methods for determining H2O content?
- Drying methods
- Physical methods (electrical properties)
- Chemical methods (sensitive and specific)
- Others (spectroscopic)
What are the different drying methods?
- Oven-drying (convection oven, forced-air oven, vacuum oven)
- Freeze drying (very low pressure, very low temperature, water evaporates)
- Spectroscopic (IR, NMR)
What does it mean for a method to be indirect?
- Does not directly measure water, simply a loss of weight
- not-specific--> anything lost while heating will be included
What are the properties of oven-drying?
- Uses weight differences (before and after drying)
- A gravimetric method
- Indirect method
- Inexpensive, easy, short time
What is the convection oven method?
- Air moves through the oven- no fan, air stays in chamber
- Reaches vapor pressure equilibrium
What is forced-draft/forced-air oven?
- Contains a fan
- Temperature can be higher and time shorter
What is the vacuum oven method?
- If you lower the pressure, you need to lower temperature to evaporate water
- The heated chamber is connected to a vacuum pump (with a pressure controller)
- The vacuum also pumps the water evaporating from the sample
- If all the air is pumped out, noting can heat, so some air has to be put in and it needs to be dry
- Drying agents are often used such as CaCl2, SiO2, P2O5, H2SO4
What are 3 important factors when analyzing moisture content with indirect methods?
- Particle size- loss of water takes place at the surface of a particle, grinding the sample increases the surface- reduces the migration distance that water needs to travel before evaporating
What is the distillation method?
- It is a physical method - uses the boiling point
- Direct method - water is physically removed and collected and the amount of water is then measured
- This is done by: (1) heating the sample, (2) water vaporizes, (3) water condenses
What is one drawback to the distillation method?
Volatile compounds will still be present in the H2O distillate
What can be done to make sure the temperature doesn't vary much during the distillation method?
Do the distillation in the presence of a heat transfer medium
What is a good heat transfer medium?
- Unmiscible liquid in water
- Free of water
- example: toluene, aromatic hydrocarbon, mineral oil
Why is toluene a good choice for a heat transfer medium?
- Boiling point is 110.5oC
- Volatile, but unmiscible
- Preferred because it take 2-3 hours instead of 8 hours for vacuum-oven drying
- If you pour the sample into toluene, both will evaporate, but will form 2 separate layers which can then be separated
What are some drawbacks to using toluene in distillation methods?
- Fire safety (can be flammable)
- Toxic vapors
- Environmental concerns
What is the principle of doing chemical methods of water determination?
Takes advantage of a chemical reactive substance (with a high specificity)
What is the most widely used chemical method for water determination?
- The Karl Fischer method
- Most widely used because of its high degree of specificity and high sensitivity with water
- Used mostly with low moisture foods
How is the Karl Fischer method done?
- Use Bunsen reaction to develop the KF reaction (2H2O + SO2 + I2 --> H2SO4 + 2HI)
- Use pyridine as the solvent (non-aqueous solvent, basic)
- Pyridine equilibrates the acidic H2SO4 which pushes the reaction more towards the products
- BI2 + BSO2 + B + H2O --> 2BH+I- + BSO3, then BSO3 + ROH --> BH + ROSO3-
Wht are the KF reagents?
- SO2, I2, pyridine, methanol (to dilute pyridine)
- Pyridine is hazardous
What are the 2 electrical properties of water used as physical methods of drying?
- Conductance (1/resistance)
- Capacitance (high dielectric constant)
What is the basic assumption of using the electric properties of water as a method of drying?
Assume all of the electrical properties of a sample are due to the water
What is the preferred method for calibrating before electrical analysis?
Oven drying is the reference method for calibrating
What is ash?
The inorganic residue afer complete combustion of the organic material
How is ash determined?
- Food material is oxidized at 500-600 degrees for 2 hours
- CO2, H2O, nitroxides (NO, N2O, NO2) escape, residues remain (such as Cl-, SO4-, PO3-, silicate)
- Residues contain mixtures of metal oxides and salts (like chloride sulfate, phosphate, silicate)
Why is ash important?
- For nutrition: essential minerals need to be measured individually (such as Ca, Na, Fe)
- Undesirable minerals: (Pb, Hg, Cd, etc)
- Food safety/food quality: useful in wheat flour where higher ash content in bran and endosperm may not be good, also in raw sugar where there are specifications. Ash in sugar comes from the soil which has silicate, higher ash content in raw sugar means poorer quality
What are the steps of the ashing technique?
- Sample --> preashing --> furnace --> residues
- After incineration the sample should be grayish-whiteish (if all organic matter is combusted). Sometimes even at more than 530 degrees for 2 hours, the sampe does not get completely combusted (black spots). You don't want to go up to 550 degrees where some minerals may become volatile (such as lead or mercury)
What is preashing?
- To remove some gases, smoke the sample initially
- Does not come from furnace
- Care required by analyst to prevent loss of sample
What are the non-combusted "black spots" in ashing?
- Chlorides and phosphates fuse with salts (like Na and K) at high temperatures and trap organic material, rendering them unavailable for combustion. The organic material can be released by dissolving the sample/salt in a small amount of water
- Black spots are quite common with high CHO/high sugar samples
What is the sulfate ash technique?
Sample + diluted H2SO4 (just a few drops) --> preash/ash --> grey/white residue (with no black spots)
Why are there no black spots in sulfate ashing?
- Most of salt in residue is in metal sulfates - does not form fused salts
- In these cases, "sulfated" ash determination is done
What is the dry ashing technique?
- Ashing process is dry, meaning no solvent or liquid is used during the ashing process
- Result gives total ash content
What is the wet ashing technique?
- Remove all organic material via oxidation
- Objective: obtain a solution with all the minerals, and then analyze them one by one
- This provides a clear solution which has all minerals dissolved in it
- Extremely strong oxidizing agents are used (i.e. concentration HNO3, H2O2 + heat are used if the oxidizing power needs to be increased)
- Na, K, Ca, Fe, Cl, P, S contents can be measured by flame photometry, atomic absorption, or other techniques
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