L1 Water Activity Flashcards
What is moisture content of foods?
Water content, or moisture content, is a measurement of the total water contained in a food. It is usually expressed as a percentage of the total weight:
Mw = moisture content on a wet per cent basis
Mw=[w-d/w]x100
Hows is dry weight found in foods?
Dry weight is the weight of food after drying. Drying is normally achieved in hot air oven at temperatures higher than 100 degree c for more than 24 hours. Every food product has a different drying protocol provided by official associations managing food research.
Explain the types of moisture content in foods.
In foods there is bound water and free water.Bound water is usually defined in terms of the ways it is measured; different methods of measurement give different values for bound water in
a particular food. Many food constituents can bind or hold onto water molecules, such that they cannot be easily removed and they do not behave like liquid water.
Some of the characteristics of bound water are:
It is not free to act as a solvent for salts and sugars.
Water tightly Bound.
It can be frozen only at very low temperatures (below freezing point of water).
Not available to bacteria for enzymatic reactions
Free water has loose association and is trapped physically.
What is water activity in Foods?
Water activity is a measure of how tightly water is “bound” and related to the work required to remove water from the system. Water that is “bound” should not be thought of as totally immobilized.Water activity describes the energy status or escaping tendency of the water in a sample. It indicates how tightly water is “bound”, structurally or chemically, in products.
Water activity is a measurement of the availability of water for biological reactions.
Water activity (aw) is expressed as the ratio of the vapour pressure of water in a food (P) to the vapour pressure of pure water (P0). It predicts whether water is likely to move from the food product into the cells of micro-organisms that may be present. The higher the water activity the more available water is.
aw= P/P0
What is an alternative way to represent water activity?
It can also be expressed as the equilibrium relative humidity (ERH) of the air surrounding the food at the same temperature.ERH can be calculated as water activity multiplied by 100. For example, if water activity of product is 0.90, ERH will be 90%, meaning at 90% RH, there will be no exchange of water vapors between food and surroundings.
What is the different significance of water activity and moisture?
Water activity: To determine the shelf life stability of foods
Moisture content: To know the yield of foods/ dry weight of food products
Why does a sponge absorb water from a glass?
Water activity is the force that causes the water to move into the sponge. To understand it better, think about how the water in the sponge is different from the water in the glass.
The water in the glass is free, but the water in the sponge is anything but free. It’s bound by hydrogen bonds, capillary forces and van der Waals–London forces. These are called matrix effects. The water in the sponge has a lower energy state than the water in the glass. Water will flow into the sponge, but to get it back out, we must do work by squeezing the sponge.
The water in the sponge has a lower vapor pressure, lower freezing point, and higher boiling point than the water in the glass. They are different in ways we can measure and quantify.
What is the relationship between moisture content and water activity?
The relationship between water content and water activity is complex and related to the relative humidity of the food and its water content.
This relationship must be determined for each specific food item.
What is a moisture sorption isotherm?
The relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm.Each product has its own unique relationship with aw and moisture content.In 2 foods even at the same water activity point, moisture content may vary.
How are products able to be stable?
As long as water activity of two food components (for example, sponge cake and icing) remains same, there will be no migration of water and product remains stable in terms of texture.
What are the implications of water activity on texture?
Water activity and moisture content have implications on food texture too. A combination of low water activity and moisture content provided hard and crispy (less than 0.25 aw around 25% mositure) texture as in cookies, intermediate water activity and low moisture lead to dry and firm texture, (around 0.5 aw and around 25% moisture) whereas moisture in the range of low to high and high water activity leads to soft and moist texture (0.75 aw and higher above 50% moisture).
What aw is best for lipid oxidation, non enzymatic browning,hydrolytic reactions,moisture content,enzyme activity,bacteria,yeast and mold growth?
Lipid Oxidation is highest around 0-0.1 and 0.6-0.8
Non-enzymatic Browning increases from 0.2 and peaks at 0.6-0.7 and decreases.
Hydrolytic Reactions increases from 0.2 to 0.7
Enzyme activity increases from 0.3 and peaks at 0.9
Moisture content starts at 0 increasing slowly till 0.7 and from 0.7 onwards it increases sharply.
Mold growth starts at 0.7 and increases
Yeast growth starts at 0.75 and increases
Baterial growth starts at 0.8 and increases
What is the requirements of growth for most gram negative bacteria,SA toxin production,most gram positive bacteria,most yeasts,SA,most moulds,Halophillic bacteria,Xerophillic molds and Osmophillic yeasts.
Most gram-negative bacteria 0.97 SA TOXIN 0.93 Most gram-positive bacteria 0.90 Most Yeasts 0.88 SA 0.86 Most Moulds 0.80 Halophillic Bacteria,Xerophillic Moulds,Osmophillic Yeasts. 0.75
What are ways to measure water activity and what is the process+ Adv Disadv?
Loss on drying: These methods rely on measuring the mass of water in a known mass of sample. The moisture content is determined by measuring the mass of a food before and after the water is removed by evaporation.
Convection and forced draft ovens: Weighed samples are placed in an oven for a specified time and temperature (e.g. 24 hours at 100 o C) and their dried mass is determined, or they are dried until they reach constant mass. The thermal energy used to evaporate the water is applied directly to the sample via the shelf and air that surround it. There are often considerable temperature variations within convection ovens, and so precise measurements are carried out using forced draft ovens that circulate the air so as to achieve a more uniform temperature distribution within the oven. Samples that contain significant quantities of carbohydrates that might undergo chemical changes or volatile materials other than water should not be dried in a convection or forced draft oven. Many official methods of analysis are based on forced draft ovens.
Advantages: Precise; Relatively cheap; Easy to use; Officially sanctioned for many applications; Many samples can be analyzed simultaneously
Disadvantages: Destructive; Unsuitable for some types of food; Time consuming
Karl-Fisher method
The Karl-Fisher titration is often used for determining the moisture content of foods that have low water contents (e.g. dried fruits and vegetables, confectionary, coffee, oils and fats).
After this lecture, you should have understand the :
The meaning of moisture content and water activity
Significance of both the parameters in food quality and safety
Requirement of different microorganisms in terms of water activity
Meaning of moisture sorption isotherm
Methods for determining moisture content in foods
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