Lecture 6- Water Acticity and mineral Flashcards
What is water activity?
Ratio of vapour pressure of water in a food and vapor pressure of water at the same temperature
aw=
p/p0
Is all water available for micorbial growth. physical properties and chemical/enzymatic reactions?
No especially bound water
What is the pathogenic microbial growth cut off point?
<0.85
Is water activity linear?
No but the higher the Aw the higher the water activity
Lipid oxidation is highest in which range of water activity?
0.75-0.85
Maillard Browning is most promenant in which range of water acitivty?
0.70
What is the cut off point for gorwth of yeast and molds?
0.62
What happens as a food product goes from a dry to intermediate moisture?
Powder caking
Loss of Crispness
What happens as food goes from intermediate moisture to dry food?
Hardening and dryinh
What happens when you increase the moistuer of a food product?
Noticable increse in molecular mobility
What is the range for a dry food?
<0.2
Intermediate moisture food?
0.2-0.85
How do you measure the water activity using a dessector?
Measuring the vapour pressure of the headspace after the close system attains equilibirum
How does the sample reach equilibirum?
WAter will migrate out of the sample into the container headspace
What are some detectors to determine when equilibrium has been reached?
- Dew Point
- Electric Hygrometer sensors
- Direct measurement pressure
- Tunable diode laser
- Freezing point determination
What temperature is the assay normally conducted?
25C
What samples require longer duration to attain equilibrium
dry and dense
Which detector is more accurate?
Electric (capacitance/electrolyte) , can determine more water due to the higjer conductance–> more specific
What are the compoenents of the dew point analyzer?
Fan
Infrared thermometer
Temeprature contolled mirror
Sensor that detects condensation on morror
What is the principle of the dew point analyzer?
Aw of the sample is the same as the RH of the headspace at eq –> instrument measures the RH in headspace
Why is the mirror cooled in the dew-point?
Until condensation from headpsace first appears, determine by a change in reflectance
What is recorded?
Temperatures of teh sample and the mirror
What is used to determine the vater vapour pressure P0?
Temperature of the sample
What is used to determine the vapour pressure of water (P)?
Dew Pount temperature
Why is temperature important for equilibration and aw?
Higher temp –> higher Aw reading
Temperature difference betwenen samples and containers could cause water condensation –> error
What is Ash?
Inorganic(mineral) residue remaining after combustion or complete acid-facilitated oxidation of organic matter in food
What is the principle of ashing?
Minerals are not destroyed by heating, low volatility compared to toehr food compoenents
What is the relevance of ash analysis?
Microbiological stability
Nutrition
Processing
Adulteration
Relevance of microbiological stability
High mineral content used to retard growth of certain imcroogranisms
Relevance of nutriton
Some minerals are essential to a healthy diet whereas some can be toxic
Relevance of Processing
Often important to know the mineral content of foods during processing and the amounts of minerals interfere with processing
Relevance of adulteration
Ash content can help determine if a certain mineral hs been used to cut costs
Ash content in ___ is very low
Oils/fats
What product does not vary in ash content?
Animal products
What is the ashing procedure?
- Food sample —(O)——–> Co2+N Oxides +H2O+ grey/white ash
What is the importance of pre-ashing?
Get ride of moisture and lipids
- -> evaportate water
- -> reduce the likelihood of sputter
What are the type of crucibles?
Quartz
Porcelain
Steel
Platinum
Which crucible is better?
Quartz
Pros and con of Quartz?
resistant to acids and halogens butno alkali
Stable unti 900C
Pros and cons for procelain crucibles?
Inexpensive
Will crack with rapid temperature changes
Most common
Pros and cons of steel crucibles
Inexpensive
Resistant to both acids and alkalines
Contain chromium and nickel
Pros and cons of platinum crucibles?
Far to expensive for routine use for large numberof samples
What are the four types of analytical procedures used to determine teh as content of foods?
Dry ashing
Wet ashing
Microwave
Low temp plasma dry ashing
What is the temperature of the frunace for dry ashing
Use high temperature furnace (500-600C
What are the compounds that are vaporize with dry ashing?
Water and volatile materials and organic copounds are burned in the presence of oxygen
–> CO2, H2O and N2
What are the minerals converted to in dry ashing?
Oxides, sulfates, phosphates, chlorides or silicates
What is the gerneral dry ashing protocol?
- Weigh a 5–10-g sample into a tared crucible. Pre-dryif the sample is very moist.
- Place crucibles in a cool muffle furnace. Use tongs, gloves, and protective eyewear if the muffle furnace is warm.
- Ignite 12–18 h (or overnight) at about 550OC.
- Turn off muffle furnace and wait to cool down to at ~250◦C. Open door carefully to avoid losing ash that may be fluffy.
- Using safety tongs, quickly transfer crucibles to a desiccator with a porcelain plate and desiccant.
- Cover crucibles, close desiccator, and allow crucibles to cool prior to weighing
What can the ash content be expressed as?
Either a dry or wet bases
How do you calculate the percent ash?
Weight after ashin/weight before ashing (dry) x100
What are the advantages of dry ashing?
Safe, few reagents, many sample can be analyzed simutaneously, not labour intensive
–> can analyze specific mineral content
What are the disadvantages of dry ashing?
Long time required
Costly (power consumption
Possibility of losing volatile minerals
What are black spots?
Some CHO traps inorganic materials leading to tiny black spots
- fused salts
Black Salts are evidence of what?
organic materials
How do you resolve black sports?
Adding a small amoun tof water or HNO3 and reashing
With which type of food is complete combustion hard to obtain?
High carbohydrate foods
What type of water is themost important in terms or microbiolog and other biological processes?
Free water
What is total water?
Aw + bound water
How do you determine bound water?
Distillation
What is the cut off point for pathogenic bacteria?
0.85
Does a higher Aw mean a higher moisture content?
Yes, becaue there is an increase of water in the system
Does a higher moisture content lead to higher Aw?
No, the linear relation is no linear
What reduces the water acitivity?
Adding solute to the water solution
Why is it omportant to determine the Aw
Quality and safety
Whta does an increaseof free water in the system lead to?
Distrupt Quality
What is at equilibirum in the headpace?
Aw of the food and the relative humidity of the headspace
How should you do when tou get dew?
Bring analyzer to a lower temp
Why is it important to handle the crucible with tongs?
Oil on you skin can contaminate the crucible
Would a dry or wet sample have a lower percentage?
Wet sample
What is the protocol for dry asking?
- Weigh a 5–10-g sample into a tared crucible. Pre-dryif the sample is very moist.
- Place crucibles in a cool muffle furnace. Use tongs, gloves, and protective eyewear if the muffle furnace is warm.
- Ignite 12–18 h (or overnight) at about 550OC.
- Turn off muffle furnace and wait to cool down to at ~250◦C. Open door carefully to avoid losing ash that may be fluffy.
- Using safety tongs, quickly transfer crucibles to a desiccator with a porcelain plate and desiccant.
- Cover crucibles, close desiccator, and allow crucibles to cool prior to weighing
What is wet ashing?
breaking down and reomival of organic matrix surrounding the minerals so that they are left in acidic solution
–> food sample is weighed into flask containing oxidizing agents and then heated
= oxidation of organic contents by acid and heat
What is the primary use for wet ashing?
preparation of samples for subsequent analysis of specific minerals
How much time does wet ashing take?
10mins–> few hours at 120C-300C
What is the resulting solution from wet ashing?
The solution can be assayed for metal ions using AAs or ICP-MS
–> ppb or ppm
Advantages of wet ashing?
little loss of volatile minerals due to low temperature
More rapid
Provides solution for determination
What are the disadvantages of wet ashing?
Labor intensive
Requires fume hood
Low sample
Wet ashing peocedure?
- WeighsampleandaddtoErlenmeyerflask(previouslyacidwashedanddried)
- Add3mlofH2SO4followedby5mlofHNO3
- Heatthesampleonahotplateat200oC(orboilinginblockheaters)
- Oncethebrown-yellowfumesceaseandwhitefumesfromdecomposingH2SO4areobserved,thesamplewillbecomedarker.Removetheflaskfromthehotplate
- Slowlyadd3–5mlofHNO3
- PuttheflaskbackonthehotplateandallowtheHNO3toboiloff.ProceedtothenextstepwhenalltheHNO3isremovedandthecoloriscleartostrawyellow.Ifthesolutionisstilldarkincolor,addanother3–5mlofHNO3andboil.Repeattheprocessuntilthesolutioniscleartostrawyellow
- Reducethevolumeappropriatelytoallowforeaseoffinaltransferanddiluteusingultrapurewater
What is microwave wet ashing?
Microwave source of heat, nitric acid used as the digestant
What are the advatanges of microwave wet ashing?
Faster to complete digestion (high temp and pressure
Programmable instrument that handles multiple samples at a time
Difference between microwave wet ashing and microwave dry ashin
with or without acid
What is low temperature plasma ashing?
Sample is place in glass chamber which is evacuated using vacuum pump, small amount of O2 pumped into chaber and browken down to nascent oxygen by applicaotnof electromagnetic readio frequency field
–> organic matter is rapidly oxidized and moisture evaporated (temp)
Advantages of low temperature plasma ashing?
Less chance of losing trace elements by volatilization
Disadvantages of low temperature plasma ashing?
Relatively expensice equipment and small small sample throughput
What are other methods of ashing?
Water soluble/insoluble ash: index of fruit content in fruit preserves
Alkalinity of ash: ash of fruits and vegetables
Ash insoluble in acid: index of contamination
Which ashis alkaline? acidic?
Acidic- meat and some cereals
Alkaline- fruit and vegetables
What is water insoluble ash a indication of?
Presence of siliceous matter
What is a lower water insoluble ash indicate?
higher fruit content of jam and jellies (lower % of skin, seed, stem)
What is acid insoluble ash indicative of?
surface contamination of fruits and vegetables and wheat and rice coatings–> generally silicates
