food chemistry & analysis i
TRANSCRIPT
❖ Determination of moisture content
❖Water Activity
❖ Food Deterioration Rate & Water Activity
❖ Limitation of Water Activity
❖ Determination of Water Activity
Outline
The presence of water in food is described as the moisture
content or water activity (aw) of the food.
Moisture refers to the amount of water present in food.
while
Water activity (aw) refers to the form in which water exist in
the food.
Water in Food
❖ There are various methods of measuring moisture content in
food:
1. Distillation
2. Dielectric method
3. Hydrometry
4. Infrared spectroscopy
5. Refractometry
6. Chemical analysis
7. Oven drying
Determination of Moisture Content
❖ The ease at which H2O is removed from a food by
evaporation depends on its interaction with other
component present.
❖ Free water is most easily removed from foods by
evaporation, whereas more severe conditions are needed
to remove bound water.
Some methods include:
❖ Oven drying methods
❖ Vacuum oven
❖ Distillation methods
Determination of Moisture Content
Oven Drying Methods
❖ The Sample is weighed. It is usually weighed into a flat
bottom shallow dish made up of aluminium or similar
material which will not react with the food nor pick up
water readily.
❖ Weighed samples are placed in an oven for a specified
time & temperature. The oven temperature is usually set at
100°C or 105°C & the time varies depending on the
sample
❖ They are dried until they reach constant mass.
❖ The difference in weight is the water which has evaporated
Determination of Moisture Content
%𝑴𝒐𝒊𝒔𝒕𝒖𝒓𝒆 =𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒘𝒆𝒕 𝒔𝒂𝒎𝒑𝒍𝒆 − 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒅𝒓𝒚 𝒔𝒂𝒎𝒑𝒍𝒆
𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒘𝒆𝒕 𝒔𝒂𝒎𝒑𝒍𝒆𝒙 𝟏𝟎𝟎
1
Vacuum oven methods❖ Weigh the sample.
❖ Place it under reduced pressure (typically 25 – 100 mm
Hg) in a vacuum oven for a specified time & temperature.
❖ Weigh the sample after drying.
Note:
❖ The boiling point of water is reduced when it is placed
under vacuum.
❖ The thermal energy used to evaporate the water is applied
directly to the sample through the metallic shelf.
❖ There is an air inlet & outlet to carry the moisture lost
from the sample out of the vacuum oven, which prevents
the accumulation of moisture within the oven.
Determination of Moisture Content
2
Advantage of vacuum oven over conventional oven drying
techniques:
❖ If the sample is heated at the same temperature, drying can
be carried out much quicker.
❖ Lower temperature can be used to remove the moisture
(e.g., 70º C instead of 100º C).
❖ Using low temperature can reduce the problems
associated with degradation of heat labile substances.
Determination of Moisture Content
Distillation Methods
❖ Distillation methods are based on direct measurement of
the amount of water removed from a food simply by
evaporation.
❖ Distillation methods are illustrated by the Dean & stark
method.
❖ A known weight of food is placed in a flask with an
❖ organic solvent such as xylene or toluene.
➢ Toluene with a B.P of 110.6 ºC
➢ Xylene with a B.P of 138.4 ºC
Determination of Moisture Content
3
Distillation Methods
The organic solvent must be:
❖ Insoluble with water,
❖ Have a higher boiling point than water,
❖ Be less dense than water,
❖ & be safe to use.
Determination of Moisture Content
3
Distillation Methods
❖ The flask containing the sample & the organic
solvent is attached to a condenser & a graduated
glass tube.
❖ Then the mixture is heated.
❖ The water in the sample evaporates & moves up
into the condenser where it is cooled &
converted back into liquid water which is
collected into the graduated tube.
❖ When no more water is collected in the
graduated tube, distillation is stopped & the
volume of water is read from the tube
Determination of Moisture Content
3
Water is most abundant substance in plant and animal matter.
The water content of animals and plant varies widely.
Water content in foods :
Water is as much a part of all, foods as a carbohydrates, fats
and proteins. Cellular material contains a abundance of water.
In leafy green there is 90% or more fruits and vegetables
contain plenty of moisture to the extent of 70, 80 percent.
Water, which is present in foods, may be held as
1. Free water
2. Bound water
Role and Type of Water in Foods
❖ Free water is present in cells, and in circulating fluids of
tissues as in cell sap.
❖ It contains dissolved and dispersed solutes in the cell.
❖ It is easily lost by drying the food.
❖ The bound water in foods is held by proteins
polysaccharides and fats in the living cells.
❖ Bound water may also be absorbed on the surfaces of solids
in foods.
❖ The removal of bound water from tissues is very difficult.
Bound water is resistant to freezing and chilling.
Role and Type of Water in Foods
Role of Water in Food Preparation:
❖ The role of water in food preparation is of great importance.
1. As a cooking medium
❖ This is perhaps the most common and important its many
uses in cookery.
❖Water has been universally used as a medium of cooking.
❖ The ubiquitous nature of water, its free availability and its
low cost of supply are some of the factors which influence
the use of water as a cooking medium.
Role and Type of Water in Foods
1. As a cooking medium
❖ Dry foods absorb water and swell before they get cooked.
❖Water acts as a medium of heat transfer from the surface
area to the different parts of the food.
❖ Therefore, foods which have moisture content take a longer
time than foods with greater moisture content.
Role and Type of Water in Foods
2. As a solvent
❖Water is a universal solvent for many food substances.
❖Water not only dissolve flavors, but also color pigments in
fruits and vegetables like anthocyanins and odors.
❖ Thus, the solvent action of water is responsible for the
palatability of the food cooked in it.
Role and Type of Water in Foods
3. Water absorption
❖ Dry foods cooked in water absorb water, expand in volume
and increase in weight.
❖ Foods like cereals and pulses when cooked in water, gain
weight to the extent of 2 – 3 times.
❖Water also functions in food preparation as a dispersing
medium and helps to produce smooth texture.
❖ It helps to distribute particles of materials like starch and
protein.
❖When flour is used to thicken liquids, the particles need to
be dispersed through out the liquid phase as in a starch gel.
Role and Type of Water in Foods
3. Water absorption
❖ Dry foods like cereals, millets, pulses are generally first
soaked for a period of time before they are cooked as they
take a longer time to cook than foods with a greater moisture
content.
❖ This helps to decrease the cooking time, very often rice, dals
and legumes are cooked under pressure to hasten the
cooking process.
❖Water acts as a leavening agent in food preparations.
❖When batters and doughs are exposed to heat, the water
present is converted to steam.
❖ The steam expands and is responsable for the leavening
effect.
Role and Type of Water in Foods
❖ On the basis of their stability during storage, foods can be
❖ divided into 3 categories:
1. Non-perishable
2. Semi perishable
3. Perishable
1. Non-perishable:
❖ It may be noted that cereals, dals and legumes with a
moisture content below 13 percent are nonperishable if
stored in a cool, dry place.
❖ It is important to store dry foods like sugar, salt, coffee
powder in very dry containers.
❖ For these foods pick up moisture readily from the
atmosphere and may deteriorate.
Role and Type of Water in Foods
2. Semi perishable :
❖ Semi perishable foods can be stored for a week to a month at
room temperature without any undesirable change in flavor
or texture
❖ Example: biscuits, roasted chana dal etc.
3. Perishable foods
❖Which have high moisture content can be kept only for a
short period.
❖ They have to be stored at refrigeration temperature, if their
shelf life is to be perishable.
❖ Example: milk, paneer, meat, fresh fruits & vegetables.
Role and Type of Water in Foods
The presence of water in food is described as the moisture
content or water activity (aw) of the food.
Moisture refers to the amount of water present in food.
while
Water activity (aw) refers to the form in which water exist in
the food.
Water in Food
Water Activity
❖ In 1952, Scott came to the conclusion that the storage
quality of food does not depend on the water content, but on
water activity (aw).
❖ Water activity tells you how active the water is in a food
system.
❖ Is the water free to move around or is it bound?
❖ Water can be bound by other food components like
carbohydrates, salt, proteins and lipids.
❖ The freer the water is, the more it is available for use by
microorganisms, and for chemical and metabolic reactions
such as enzymatic activity.
Water Activity
❖ Water Activity is the measure of the availability of water
molecule to enter into microbial, enzymatic or chemical
reactions.
❖ It can be represented by the symbol (aw).
❖ The availability determines the shelf life of food.
❖ Regarding the forms of water, bound water is inversely
related to water activity.
❖ As the % of bound water in a food increases the aw
decreases.
Water Activity
❖ Often, moisture content alone may not be enough to predict
shelf life.
❖ Moisture content may be high but the product could be very
shelf stable.
❖ For example, jams and jellies may have a very high
moisture content (up to 90%), but the water activity may be
relatively low (0.7).
❖ Because it has sugar and sugar has the ability to bind to
water.
Calculating of Water Activity
❖ (aw) is calculated as the ratio of the water vapor pressure of
the substance divided by the vapor pressure of pure water
at same temperature.
❖ Vapor pressure can be measured by using a manometer.
❖ (aw) is calculated using the following equation:
Water Activity & Relative Humidity
❖ In simpler terms (aw) is a measure of relative humidity (RH)
❖ Relative humidity (RH) is the ratio of the vapor pressure of
air to its saturation vapor pressure.
❖ The equilibrium relative humidity (ERH) of a food product
is defined as relative humidity of the air surrounding the food
that is in equilibrium with its environment.
❖ When the equilibrium is obtained, the ERH (in percent) is
equal to the water activity multiplied by 100.
❖ ERH (%) = aw × 100
❖ When a food is exposed to a constant humidity, the product
will gain or lose moisture until the ERH is reached.
Water Activity & Relative Humidity
❖ The moisture migration significantly affects the physical and
chemical properties of the food, as previously described.
❖ So, the ERH of a product is defined as the relative humidity
of the air surrounding the food at which the product neither
gains nor loses its natural moisture & is in equilibrium with
the environment.
𝐸𝑅𝐻 = 𝑅𝐻 % = 𝑎𝑤 𝑥 100
𝑎𝑤 =𝐸𝑅𝐻
100
Water Activity of some Foods
❖ Water activity ranges between 0 to 1.
❖ Some foods are stable at low moisture content whereas
others are stable at relatively high moisture content.
❖ For example: Peanut oil deteriorates at moisture content
above 0.6% whereas potato starch is stable at 20% moisture
Food aw
Pure water 1
Fresh meat 0.985
Milk 0.97
Bread 0.96
Potato chips 0.80
Flour 0.72
Raisins 0.60
Macaroni 0.45
Water Activity
❖ Water activity is related to moisture content in a non-linear
relationship known as moisture sorption or isotherm
curve.
❖ The relationship between water content & aw is indicated by
the sorption isotherm of a food.
Sorption isotherm
Water Activity
❖ The plotting of the uptake termed adsorption or the loss of
water termed desorption provides a record of aw of a
particular food at a particular temperature over varying
levels of humidity in the environment.
Sorption isotherm
❖ Water activity has an important role in food preservation.
❖ Each microorganism has a critical aw below which the
growth cannot occur.
❖ For example: Pathogenic microorganisms cannot grow at aw
below 0.86, Yeast & molds are tolerant & usually no growth
occurs at or below 0.62.
Food Deterioration Rate & Water Activity
Relationship of
Food Deterioration
Rate as a Function
of Water Activity.
❖ So aw is important in foods and it is a major factor in food
spoilage & safety.
❖ Decreased aw retards the growth of microorganisms, slows enzyme
catalyzed reactions & retards non enzymatic browning.
❖ In contrast, the rate of lipid autoxidation increases in dried food
systems. Lipid oxidation rates are high in aw values from a
minimum at 0.3 – 0.4 to a maximum at aw 0.8.
Relationship of
Food Deterioration
Rate as a Function
of Water Activity.
Food Deterioration Rate & Water Activity
❖ With aw at 0.3, the product is most stable with respect to
lipid oxidation, non enzymatic browning, enzymatic
activity & the various microbial parameters.
❖ As aw increases towards the right the probability of the food
product deterioration increases.
Relationship of
Food Deterioration
Rate as a Function
of Water Activity.
Food Deterioration Rate & Water Activity
❖ For decreasing aw & thus improving the shelf life of food is
by the use additives with high water binding capacity
(humectants).
❖ In addition to common salt, glycerol, & sucrose have the
potential as humectants.
Relationship of
Food Deterioration
Rate as a Function
of Water Activity.
Food Deterioration Rate & Water Activity
Humectants: sucrose, propylene glycol, glycerol.
Be careful of:
➢ Solubility, MW
➢ Flavor
➢ Crystallization on storage
➢ Chemical reactivity
➢ Toxicity
Food Deterioration Rate & Water Activity
❖ Water activity is only of limited use as an indicator for the
storage life of foods with a low water content.
❖ A new concept based on phase transition, is better suited to
the prediction of storage life which takes into account the
change in physical properties of foods during contact
between water and hydrophilic ingredients.
Limitation of Water Activity
❖ The physical state of fresh (metastable) foods depends
on their:
1. Composition,
2. Temperature,
3. Storage time.
❖ Foods become plastic when their hydrophilic components
are hydrated.
❖ Thus, the water content affects the temperature Tg, for
example in the case of gelatinized starch.
❖ For example: depending on the temperature, the phases
could be glassy, rubbery or highly viscous.
❖ When food is heated, its phase changes from glassy to
rubbery (plastic) and the temperature called Tg.
Limitation of Water Activity
❖ When the food frozen, most of the water will freeze.
❖ The dissolved components will transfer to the unfrozen part
so the concentration of it will increase so the melting point
will decrease.
❖ The food at this temperature is a rubber-like state.
❖ Food contents affect physical state of the food.
Limitation of Water Activity
❖ The viscosity of a food is extremely high at temperature Tg.
❖ As the temperature rises, the viscosity decreases.
❖ So the drop-in food quality will accelerate.
Limitation of Water Activity