Lecture 4 Flashcards
How is water important?
major part of most foods influencing strcture, aperance, taste and degradation susceptibility
What is measured by finding the water content?
amount of water lost by the susbtance when in true equilibrium with zero vapor pressure (humidity=0%)
What is the importance in measuring the water content in food?
- legal and label requirements: since there are legal max and mins for water in certain foods
- economic: water is inexpensive, so incorparting the most is best econcomicaly speaking
- microbial stability: growth of bad guys depends on water content
- Food quality
- Food processing operations: knowledge required to know it will affect behaviour of food during processing
What are the four states of water and define them.
- free (bulk) water: free, so only surrounded by water, retains physical properties and acts as a dispersing agent
- capillary/trapped water : kept in tight spaces inside food surrounded by a physical barrier like a droplet, or cell, properties similar to free water
- physically bound water: in molecular contact with other parts of food, different properties than free water
- hydration/chemically bound water: bound to other molecules forming water crystallization/hydrates, stronger bonds so different properties than free water
contrast and compare direct vs indirect methods
D: based on determination of water itself by physical method or chemical methods
I: determine property of sample that is very correlerated to water in sample/ response of the water chemicals to a physical influence. Done by either methods that are highly dependent on moisture content or methods based on the response of water molecules
Describe a physical direct method used
Thermogravimetric method:
- determines water/total solids in food
- principle is weighing sample and removing water and volatile matter by drying with ovens or furnances or other equipment until mass is constant then weighing sample
- advantages: pretty cheap, percise, easy to use, approved, can test many samples at once
- disadvantges: destructive, time consuming, not good for all types of food
Describe the principle of chemical direct methods
- uses chemical reagents to react with water to make a change that can be measured in the system properties like mass, pH, volume, color etc.
- better for foods that have thermally labile substance or volatile componments
what is one of the most common chemical direct methods and explain it.
Karl-Fisher Titration
- used for foods typically with low water contents
- has a measurble color change since HI is colourless and the reagent it is titrated with has iodine which is dark brown
- when all the water is used the solution will turn that dark brown, so this information can be used in a calibration curve
Describe an indirect method based on the response to water molecules
spectroscopic methods:
- NMR: is used to make it possible to distinguish molecules according to molecular mobility
- can indicate free vs bound water since free water has a higher mobility than bound water
What are factors the influence precision and accuracy of results?
- sample containing volatile organic matter: could evaporate at the same time giving inaccurate results
- heat gel forming sample: if forms gel during heating will give inaccurate results
- hygroscopic dried sample can reabsorb moisture from the air during handling
- lose of water by evapartion during weighing
- too many samples in oven
- distrubtion of sample in dish
What is dry matter?
what is obtained when removing water from product , the ration between weight of dry matter and non-dry matter is the percentage of dry matter
What is the total ash content?
inorganic material of a food sample that remains after sample has been burned at 600 degrees for 2 hours, mostly calculated on a dry basis
what are factors the influence the results of ash content determination?
- foods rich in fat/ volatile subtances: slpahes
- foods rich in sugars: foaming
- incomplete incineration
- handling and weighing of ashes:very light
