food chemistry & analysis i

FOOD CHEMISTRY & ANALYSIS I

Lecture 2

By:Dr. Ismail Elhaty

Importance of Water

❖ 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


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


%𝑴𝒐𝒊𝒔𝒕𝒖𝒓𝒆 =𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒘𝒆𝒕 𝒔𝒂𝒎𝒑𝒍𝒆 − 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒅𝒓𝒚 𝒔𝒂𝒎𝒑𝒍𝒆

𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒘𝒆𝒕 𝒔𝒂𝒎𝒑𝒍𝒆𝒙 𝟏𝟎𝟎

1

Oven Drying Methods


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.


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.


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


3


The organic solvent must be:

❖ Insoluble with water,

❖ Have a higher boiling point than water,

❖ Be less dense than water,

❖ & be safe to use.


3


❖ 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


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 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.


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.


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.


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.


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.


❖ 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.


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.


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

Sorption isotherm

aw

❖ Water activity is directly proportional to moisture content.

Water Activity

Sorption isotherm

aw

❖ 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.


❖ 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.


❖ 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.


Humectants: sucrose, propylene glycol, glycerol.

Be careful of:

➢ Solubility, MW

➢ Flavor

➢ Crystallization on storage

➢ Chemical reactivity

➢ Toxicity


❖ 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.


❖ 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.


❖ 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.


Determination of Water Activity

food chemistry & analysis i

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