Lecture 4 - Moisture Analysis Flashcards
what are the 5 components in a proximate analysis?
moisture ash protein fibre fat
why is CHO usually not included in proximate analysis?
it exists in different forms
difficult to analyze
how is %CHO measured?
by difference of measured other components
%CHO= 100- (%H2O+ %Ash+ %Protein+ %Fibre+ %Fat)
what is food broken down into?
dry matter + H2O
what is dry matter broken down into?
organic + inorganic (ash)
what is organic matter broken down into?
non nitrogenous components and N containing components (crude protein)
what is non-nitrogenous components broken down into?
non fatty components and fatty components (fat)
what is non fatty components broken down into?
CHO and fiber
role of water in food produce?
- provides structure
- standard of identity (need to have certain amount of water)
- cheap filler (economic relevance)
- undesired in dry products
role of dry matter in food?
- often the expensive part
- defines the nutritional part
- convenient for transport
define moisture determination
most widely used analytical measurements in processing and testing of food products
why is it important to measure moisture in food materials?
- food safety (mold growth produces aflatoxins)
- shelf life and stability
- food quality
- economic considerations
- government regulations (ie standardized foods)
common levels of moisture in food materials?
- high moisture foods
- intermediate moisture
- low moisture
4 forms of water in food?
- free water/bulk water
- capillary or trapped water
- physically bound water
- chemically bound water
define free/bulk water
- water in food that is free from other constituents
- each h2o molecule is surrounded only by other h2o molecules
- physicochemical properties that are the same as pure water (ie mp, bp, density, compressibility, heat of vaporization, electromagnetic absorption spectra)
describe capillary/trapped water
- water held in narrow channels formed by physical barriers by capillary forces
- has physicochemical properties similar to bulk water
- ie water in a plant cell
describe physically bound water
- water that is physically bound w/ other molecules (ie proteins)
- has different physicochemical properties than free water
describe chemically bound water
- ie salts such as Na2SO4 . 10H2O
- different properties to free water
4 methods for measurement of moisture in foods
- drying methods
- distillation methods
- chemical methods
- physical methods
describe drying methods
removal of water in form of water vapour
the lost of weight is taken as a measure of moisture content
describe distillation methods
removal of water by distillation process
describe chemical methods
uses a chemical rxn in which water is involved
describe physical methods
takes advantage of physical properties of water (ie electrical or dielectric properties)
cons of drying methods?
indirect
less specific to water
simple skills
3 types of drying methods?
- oven drying
- freeze drying
- infrared drying
according to Raoult’s law, the BP of water increases ___ for every 1 molecular weight of a solute is dissolved in 1.0L of water
0.512 deg C
ie the boiling point increases as the removal of water increases = solute conc increases
moisture loss is a function of ___ and ___
time and temperature
types of ovens used in oven drying?
convection
air forced
microwave
vacuum
in the temperature and %moisture relationship, the rise in temperature shows…
%mass loss (% moisture)
CHO breaks down at ___ and ____
higher temp and releases water
oxidation of FA could ____
increase weight of sample
this occurs in high fat foods
temperature and time that food stays in convection oven?
105-110 deg C
22-24 hours
disadvantages of convection oven drying?
longer analysis time
loss of volatile components other than water
larger temp gradient
describe forced air oven drying method
- air circulated by a fan
- temp can be higher and time can be shorter (135degC for 4h)
- temp differential not >1degC
- process is faster, efficient and water vapour will not condense due to fan
what is a possibility that occurs in forced air oven at high temp?
lipid oxidation leading to weight gain
describe microwave analyzer
- rapid technique. allows in process analysis
- has in-built microbalance that monitors weight loss
- microwave energy and time have to be defined to ensure accurate measurement
- pan used should not absorb microwave energy (fiberglass and quartz fiber)
- not approved method for moisture analysis
describe vacuum oven
- drying under reduced pressure (22-100mm Hg)
-fast rate of evaporation
-removal of water with out decompoisition
-temp 95-102 degC
temp for high sugar products: 60-70degC
define boiling point
temp at which the vapor pressure of the liquid equals pressure surrounding the liquid
vacuum oven is based on the fat that…
water boils at lower temp under vacuum
what is the purpose of vacuum pump?
to reduce the oven’s pressure to between 25-50 mmHg
role of drying agents in the vacuum oven method?
to dry the air for the vacuum pump to operate normally
components of vacuum oven?
vacuum oven
vacuum pump
drying agents
weight of a empty lab pan in the vacuum oven method?
20 grams
using the vacuum method, what is the moisture%?
wt of empty pan = 20g
wt of wheat sample = 5g
wt of wheat and pan after drying = 24g
loss of wt = 25-24 = 1g
moisture % = 1/5g x 100 = 20%
4 common drying agents for vacuum oven?
- anhydrous CaCl2 (most safe)
- anhydrous So3
- phosphorus pentoxide P2O5
4 concentrated H2SO4 (most effective)
what is the most safe and most effective drying agents for vacuum ovens?
most safe: anhydrous CaCl2
most effective: concentrated H2SO4
practical advantages of vacuum oven?
- reduced temp in drying chamber = reduction in loss of volatiles (non water components) and less effect on degradation of some components (ie sugars, especially fructose)
- shortened analysis time (3-8h)
factors that affect oven drying methods?
- temp
- time
- particle size
how does particle size affect drying?
smaller particles = surface area will be larger = loss of water will be effective
in small particles, the distance that water molecules will migrate are smaller than surface where they will be lost
describe infra red drying
- penetrating infrared rays dries the sample
- short drying time: 10-25min
- heat of infrared lamp: 1730-2230 deg C
- infrared drying ovens can be equipped with an analytical balance to read moisture content directly
- not an approved drying technique by AOAC international
what is a hygroscopic substance?
substance that absorbs moisture from air
describe rapid moisture analyzer tehcnologies
- quick decision making
- uses halogen or ceramic heaters
- uses aluminum pans
usual temp: 25 to 275 deg C - uses digital balance and automatic weights to calculate %moisture
- con: increases possibility of error
describe thermogravimetric analysis
- mass of sample continuously measured as it’s heated at a controlled rate in a controlled atmosphere
- inert gas (ie N) is used to avoid oxidation and gain mass
- can acquire higher temp, thus can be used for measuring decomposition and weight % ash
