Lecture 6 - Water Activity Flashcards
what is water activity
ratio of the vapor pressure of water in a food (P) to the vapor pressure of water (Po) at the same tempeature and barometric pressure
Aw = P/Po
aw below ___ is the cut off point for ___ pathogenic microbial growth in canned food
0.85
pathogenic
the relationship between moisture content and aw is…
no linear
higher moisture content = ____ aw
higher
how to measure water activity?
- measure vapour pressure of headspace after the closed system containing the food sample attains equilibrium
- water migrates out of the sample into the container headspace (or vice versa) until the aw of food and relative humidity of headspace equilibrates
3.
what are detectors?
dew point measurement, electric hydrometer sensors (including capacitance and electrolyte sensors), direct measurement of manometric pressure, tunable diode laser sensor, freezing point determination
the assay is conducted at what temperature?
25 deg C
this is important because vapour pressure is a function of temperature
___ and ___ samples require longer duration to attain the equilibrium
dry and dense
What is Ash?
the inorganic (mineral) residue remaining after the combustion or complete acid-faciliated oxidation of organic matter in food
what is the principle of ash analysis based on?
- minerals are not destroyed by heating
2. they have a low volatility compared to other food components
relevance of ashing in microbiological stability?
high mineral contents are used to retard growth of certain microorganisms
some minerals are essential to a ____ ___ and others can be ___
healthy diet
toxic
importance of ash analysis in food processing?
it is important to know the mineral content b/c excessive amount of minerals can interfere with processing (decolorization and crystallization)
importance of ash analysis in adulteration?
eg. it is useful to distinguish fruit vinegar from synthetic vinegar
ash content in oils and fats are usually …..
very low
cured meat may have ash up to ___
11%
ash content of animal products _____
don’t vary too much
black ash means….
combustion is not complete. Finished ash should be grey
steps in the ashing procedure?
- crucible
- rice
- reashing in fume hood
- muffle furnace (incineration 500-600 deg C) for 2 hr)
what is the ash content?
crucible weight = 18g
crucible + ash weight = 18.04g
ash wt = 18.04 - 18g = 0.04g
ash content = wt of ash/ wt of sample *100
=0.04 / 2 *100
=2%
why does temperature need to be between 500-600 deg C?
some metals volatilize above 600 degC
components of a food sample after combustion?
- CO2 + N oxides (NO, (N2O, NO2) + H2O
2. gray- white residue (ash)
what are the 4 types of crucibles used for ashing?
- quartz crucibles
- porcelain crucibles
- steel crucibles
- platinum crucibles
what are quartz crucibles resistant to? what temp are they stable until
crucibles resistant to acids and halogens, but not alkali, at high temp
stable until 900deg C
what are porcelain crucibles?
- inexpensive
- same properties as quartz crucibles
- will crack with rapid temp changes
- most common
what are steel crucibles?
- inexpensive
- resistant to acids and alkalies
- contain chromium and nickel
what are platinum crucibles?
- for larger number of samples
- too expensive for routine use
why is there a preashing step before combustion?
because some foods have lots of organic matter
what are 4 procedures used to determine the ash content of food?
- ry ashing
- wet ashing
- microwave ashing
- low temp plasma dry ashing
describe dry ashing.
- uses a high temp muffle furnace (500-600 deg C)
- water and other volatile materials are vaporized. Organic compounds are burned in presence of O2 to Co2, H2O and N2
- most mineral are converted to oxides, sulfates, phosphates, chlorides or silicates
- the sample is weighed before and after ashing
in dry ashing, what compounds are vaporized and what is burned?
water and other volatile materials are vaporized. Organic compounds are burned in presence of O2 to Co2, H2O and N2
in dry ashing, most minerals are convert to…
oxides, sulfates, phosphates, chlorides or silicates
what is the protocol of dry ashing?
- weigh 5-10g sample in a tared crucible. Predry sample if moist
- place crucible in cool muffle furnace
- ignite overnight
- turn off muffle furnace. Wait to cool down at 250deg C.
- transfer crucible to desiccator with a porcelain plate and desiccant
- cover crucibles, close desiccator, allow crucibles to cool prior to weighing
% ash (dry basis) = ?
% ash (Wet basis) = ?
% ash (dry basis) = (wt after ashing)/(wt before ashing (dry) *100
% ash (Wet basis) = (wt after ashing)/(wt before ashing (wet) *100
advantages and disadvantages of dry ashing?
adv:
- safe
- few reagents required
- many samples can be analyzed simultaneously
- not labor intensive
- ash canbe analyzed for specific mineral content
dis
- long time required
- costly to run due to power consumption
- possibility of losing volatile minerals at high temp
combustion is not easy to obtain in ____ foods?
why? what do they form?
high carbohydrate foods
at high temp, some CHO can be trapped in inorganic materials, leading to tiny black spots (which show evidence of organic materials)
This is due to fused salts (NaCl, Kcl).
Phosphate salts and sulfate salts don’t form fused salts
what do black spot formation show?
how can you solve this problem?
an evidence of organic materials due to CHO being trapped in inorganic materials (because of fused salts)
can be solved by adding a small amount of water and re-ashing
what wet ashing primarily used for?
primarily used in preparation of samples for subsequent analysis of specific minerals
how does wet ashing work?
breaks down and removes organic matrix surrounding minerals so that they are left in an aqueous solution
a dried ground food sample is weighed into a flask with strong acids and oxidizing agents, then heated
time and temperature of wet ashing?
10 minutes to a few hours
350 deg C
advantages and disadvantages of wet ashing?
advantages
- little loss of volatile minerals due to low temp
- faster than dry ashing
- provides solution for determination of individual minerals
disadv:
- labor intensive
- requires a special fumecupboard
- low sample throughput
procedure of wet ashing?
- weight sample and add to erlenmeyer
- add 3ml of H2SO4
- add 5ml of HNO3
- heat sample on hot plate at 200deg C
- once brown-yellow fumes cease and whie fumes from decomposing H2SO4 are observed, sample becomes darker. Remove flash from hot plate
- add 3-5ml of HNO3
- heat flash and allow HNO3 to boil off. Color is clear to straw yellow
- reduce volume to allow for ease of final transfer
describe microwave wet ashing
source of heat?
temp and pressure?
what is used as digestant?
how many samples can it handle at once?
- source of heat: microwave
- faster and complete digestion at high temp: 250 deg C and high temp
- digestant: nitric acid
- can handle multiple samples at a time
difference between microwave wet and dry ashing?
dry ashing: without acid
describe low temp plasma ashing
- sample placed into glass chamber with a vacuum pump
- O2 is pumped into chamber and broken down to nascent oxygen by an electromagnetic radio frequency field
- organic matter in sample is rapidly oxidized by nascent oxygen. Moisutre is evaporated b/c of elevated temp
advantages and disadvantages of low temp plasma ashing
adv: less chance of losing trace elements by volatilization
dis: relatively expensive equipment. small sample throughput
what are other ash measurements?
- water soluble ash: used as index of fruit content of jelly and fruit preserves
- water insoluble ash
- alkalinity of ash: ash of fruits and veg is alkaline
- ash insoluble in acid: index of contamination
- sulfated ash: applied to sugar and syrups:
water insolbule ash is an indication of…
presence of siliceous (silica) matter
lower water insoluble ash indicates…
higher fruit content of jam and jellies
ash of fruits and veg is ___
ash of meats and cereals is ___
- alkaline
2. acidic
acid insoluble ash indicates…
surface contamination of fruit and veg used
