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2010

Determination ofmoisture

B.K.K.K. Jinadasa

0 9 T H J A N U A R Y 2 0 1 0
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1.1. Introduction

Moisture determination is one of the most important and most widely used measurements in the

food industry. Since the amount of dry matter in a food is inversely related to the amount of

moisture it contains, moisture content is of direct importance to the processor and the consumer.Moisture determination is important in following areas.

Legal and Labeling Requirements. There are legal limits to the maximum or minimumamount of water that must be present in certain types of food.

Economic. The cost of many foods depends on the amount of water they contain - wateris an inexpensive ingredient, and manufacturers often try to incorporate as much as

possible in a food, without exceeding some maximum legal requirement. Microbial Stability. The propensity of microorganisms to grow in foods depends on their

water content. For this reason many foods are dried below some critical moisture content.

Food Quality. The texture, taste, appearance and stability of foods depend on the amountof water they contain.

Food Processing Operations. Knowledge of the moisture content is often necessary topredict the behavior of foods during processing, e.g. mixing, drying, flow through a pipeor packaging.

Foods are heterogeneous materials that contain different proportions of chemically bound,physically bound, capillary, trapped or bulk water. In addition, foods may contain water that is

present in different physical states: gas, liquid or solid. The fact that water molecules can exist ina number of different molecular environments, with different physicochemical properties, can be

problematic when determining the moisture content of foods.Measured value of the moisture content of a particular food may depend on the experimental

technique used to carry out the measurement. Methods for the determination of moisture can bedivided into drying methods, distillation procedures, chemical assays, and physical procedures.

Moisture determination methods:

Oven drying method Sand Pan Technique: Mix sample with sand to increase surface area and prevent crusting

on surface. Microwave Drying: More rapid than oven techniques and more expensive, but speed

makes it economical for the food industry. Problems of uneven heating. Infrared Drying: Penetrating heat goes into the sample for drying. Problems with uneven

heating and poor accuracy. Reflux Distillation: Moisture distilled from an immiscible solvent. May be used for very

low moisture samples. Problems of emulsions and water droplets sticking to glassware.
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1.2. Determination of moisture by oven drying method1.2.1. Material

Moisture dish made of stainless steel

Oven maintained at 105CWeighing balance

Sample Cinnamon balk

1.2.2. Procedure

5 g of crushed cinnamon was weighed into three moisture dishes (previously dried in the oven

and weighed) separately and kept in the oven for drying with the lids alongside for three hours.Drying was carried out till the samples obtained constant weights.

1.2.3. Calculation

1.2.4. Results

Wt of empty

container (g)

Wt of sample

(g)

Wt after drying (sample

+ container) (g)

Wt of sample

after drying (g)

Moisture content

(%)

68.8049 5.0031 73.2592 0.5488 10.969270.0481 5.0133 74.5002 0.5612 11.1942

71.0111 5.0238 75.4450 0.5899 11.7421

Moisture content 11.30 %

1.2.5. Discussion

The rate and extent of moisture removal depends on the size and shape of the sample, and how

finely it is ground. The greater the surface area of material exposed to the environment, the faster

the rate of moisture removal.

Some samples tend to clump together or form a semi-permeable surface crust during the drying

procedure. This can lead to erroneous and irreproducible results because the loss of moisture isrestricted by the clumps or crust. For this reason samples are often mixed with dried sand to

prevent clumping and surface crust formation.

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Under normal laboratory conditions pure water boils at 100o

C. Nevertheless, if solutes arepresent in a sample the boiling point ofwater is elevated. Consequently, the rate of moisture loss

from the sample becomes slower than expected.

Free water is most easily removed from foods by evaporation, whereas more severe conditions

are needed to remove chemically or physically bound water. If the temperature of drying is toohigh, or the drying is carried out for too long, there may be decomposition of some of the heat-sensitive components in the food. This will cause a change in the mass of the food matrix and

lead to errors in the moisture content determination. Foods that are particularly susceptible tothermal decomposition should be analyzed using alternative methods, e.g. chemical or physical.

It is often assumed that the weight loss of a food upon heating is entirely due to evaporation of

the water. In practice, foods often contain other volatile constituents that can also be lost duringheating, e.g., flavors or odors. For foods that do contain significant amounts of volatile

components (e.g. spices and herbs) it is necessary to use alternative methods to determine theirmoisture content, e.g., distillation, chemical or physical methods.

It is important to use appropriate containers to contain samples, and to handle them correctly,when carrying out a moisture content analysis. Typically stainless steel pans are used because

they have a high thermal conductivity. These pans usually have lids to prevent spattering of thesample, which would lead to weight loss and therefore erroneous results. Pans should be handled

with tongs because fingerprints can contribute to the mass of a sample. Pans should be dried inan oven and stored in desiccators prior to use to ensure that no residual moisture is attached to

them.

Advantages: Precise; relatively cheap; Easy to use; Many samples can be analyzedsimultaneously

Disadvantages: Destructive; Unsuitable for some types of food; Time consuming
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1.3. Determination of moisture by Dean and Stark distillation method1.3.1. Materials

Dean and Stark distillation unit

Measuring cylinderBeakerDrying oven

Weighing balance

Amyl alcohol and Toluene

1.3.2. Method

All the glass ware were thoroughly cleaned and dried in the oven before starting the practical. 75mL of amyl alcohol and 50 mL were mixed then 50 mL solution and few pumice stones were

added to the distillation flask and the calibrated arm was fixed to the flask. Flask was heated withthe contents for about 30 minutes till the graduated tube obtained a constant volume of water.

Then 2 mL of water was added to the flask and boiled until a constant volume obtained. (Thiswas done to calibrate the apparatus).

After the calibration step 2 g of crushed cinnamon sample was placed in the flask containingtoluene and allowed to distill until a constant water level was obtained.

1.3.3. Results and calculations

Weight of sample = 2.0186 g

Amount of water obtained from 2 ml of water = 2.0 mLWater level after sample distillation = 2.15 mL

Water gained from the sample = 0.15 mL

Amount of water collected from the sample = 2 0.15 = 0.152

Moisture content of the sample = 0.15 1002.0186

= 7.43 %

1.3.4. Discussion

This method is used to measure the moisture content of samples which contain low moisture

level and high volatile content. The organic solvent must be insoluble with water; have a higherboiling point than water; be less dense than water; and be safe to use.

The results obtained in oven drying method and distillation method are 11.30 % and 7.43 %respectively. As the results show the value obtained from the distillation method should be lower

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than the value obtained from the oven drying method since in the oven drying method volatilecompounds may also remove while drying and that will counted as moisture loss.

There are a number of practical factors that can lead to erroneous results:Emulsions can sometimes form between the water and the solvent which are difficult to separate;

water droplets can adhere to the inside of the glassware, decomposition of thermally labile

samples can occur at the elevated temperatures used.

Advantages:Suitable for application to foods with low moisture contents; Suitable for application to foodscontaining volatile oils, such as herbs or spices, since the oils remain dissolved in the organic

solvent, and therefore do not interfere with the measurement of the water; Equipment isrelatively cheap, easy to setup and operate; Distillation methods have been officially sanctioned

for a number of food applications.

Disadvantages:Relatively time-consuming; Involves the use of flammable solvents, difficulties in reading the

meniscus, adherence of moisture droplets to the glass, over boiling, incomplete evaporation ofwater and under estimation of moisture contents and distillation of water soluble compounds.

Adherence of water to the walls of the condenser tube or sides of the receiving tubes can

generally be prevented by using thoroughly cleaned glass ware.Incomplete recovery of water due to the formation of an emulsion can sometimes be avoided by

adding small amounts of amyl or isobutyl alcohol.Foods in powder form tend to bump during the distillation through overheating on the bottom of

the flask. This can be overcome by introducing pumice stones.Dispersing the tested material on diatomaceous earth is useful with many viscous foods rich in

sugar or protein.
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