Physical methods

Food Standard and Quality Control

Lesson 07 : Determination of quality of foods- objective methods

Physical methods

Weight : Weight of a food indicates the quality like in the case of apple or egg. There are various digital weighing scales with different ranges of weight.

Volume : Liquid volumes can be measured by using measuring cups.
Solid food volume can be found by displacement method. In this method the volume can be calculated by subtracting the volume of seeds held by a container with a baked product from that of volume of seeds without the baked product. Usually mustard seeds are used.

Specific volume: The determination of specific volume of any product should be done with care and average of replicates are to be taken since experimental 'errors are likely to be large. Measurement of bulk volume in a porous and spongy product like idli is difficult. The volume may be measured by displacement with solvents like kerosene. The idli is given a momentary dip in molten wax to seal off the pores. Increase in volume is taken as the measure of its bulk volume.

Specific volume = Bulk volume
Wt. of the substance

Index to volume :It can be found by measuring the area of a slice of food with a planimeter. It is important to use a slice that is representative of the product such as a centre slice.
Index to volume is a measurement made by first tracing detailed outline of a cross section of the food. This tracing can be done with a sharply pointed pencil or a pen or by making a clear ink blot of the cross section. The ink blot is made simply by pressing the cross section of the sample lightly onto an inked stamp pad and then making the imprint of the inked sample on paper. •
A planimeter can also be used to trace the entire outline of the sample, being careful to follow all indentations and protrusions so that the final measure corded on the planimeter represents the circumference of the slice.

Specific gravity: It is a measure of the relative density of a substance in relation to that of water. The measurement is obtained by weighing a given volume of the sample and then dividing that weight by the same volume of water. This technique is used for comparing the lightness of products physically unsuited to the volume measurements e.g., egg white foams. Potatoes with low specific gravity (waxy type potatoes) have cooking characteristics different from those of potatoes with a comparatively higher specific gravity.
Moisture:Press fluids: Initial weight of the sample is noted. After the appropriate pres­sure has been applied for a controlled length of time, the sample is again weighed. The difference between the two weights represents the amount of juice contained in the original sample e.g. juiciness of meats, poultry and fish.
The weight of the original sample is determined and then the food is dried until the weight remains constant.

Moisture content = initial – dried weight x 100=%
Initial weight

Karl Fischer Titration: In 1990 Karl Fischer showed that food to be ana­lysed by this method is homogenised in a high-speed blender at speeds up to 7,500 rpm to release the water and the water is titrated with Karl Fischer rea­gent until all the water has reacted with the reagent. The calculation for water content is handled by a microprocessor, which is built into the machine. It is costly but gives quick results.

Wettability : Baked products can be tested for moisture level by conducting a test for wettability. For this test, the sample is weighed before being placed for 5 seconds in a dish of water. Immediately at the end of the lapsed time, the sample is removed from the water and weighed again to determine the weight gain. High moisture retention is synonymous with good wettability, a sign that a cake probably will be considered to be appropriately moist when judged subjectively.

Cell structure: Cell structure of baked products is an important characteristic to measure the Uniformity, size and thickness of cell walls.
Photocopies of cross-sectional slices give this valuable information. This techniques gives third dimensional view into the cells on the cut surface of the sample and gives the actual size clearly.
Size of the grain: This can be found by using photography or ink print with stamp pad or sand retention e.g. idli. Retention of sand is more if the grains are course. Cut the idli into two pieces and take one piece and press it on the stamp pad and take an impression on the paper. Ink prints may be less clear but satisfactory for some purposes.
Photography: This may be colour or black and white. They may not represent the sample size so a marked ruler should be kept adjacent.

Non destructive methods of texture measurement:
Most of the methods used for measuring texture alter the food sample being tested, so that it cannot be returned to a production batch. It is quite obvious that there are correlations between color and texture in some instances, there are applications where color may be used as an indication of acceptable texture.

Eg: Under controlled conditions automatic color measurement may then be used as a nondestructive measure of texture; this is done in the evaluation of the ripeness of certain fruits and vegetables moving along .conveyor belts in a processing unit.

Another example for nondestructive measurement of texture is in cheese making. Thumping the outside of a cheese and listening to the sound. This gives a rough indication of the degree of eye formation during ripening of Swiss cheese. One of the newer methods of nondestructive texture measurement makes use of sonic energy, which is absorbed to different extents depending on the firmness of an object.

Colour Dictionaries: The dictionary of Maerz and Paul is most commonly used. The dictionary consists of 56 charts. Seven main groups of hues are pre­sented in order of their spectra. For each group there are 8 plates. In place of colour dictionary, colour reproduced on secondary standards such as painted test panels, rings, discs or plastic models may be used.
A mask of neutral grey having two openings is used. The size of each opening should be equal to the size of the individual colour patch in the sheet. An opening should be placed over the sample and the other over different patches on the chart until a match is achieved and the colour is noted.
Disc colourimeter: Here the discs have radial slits so that a number of them may be slipped together with varying portions of each showing. The discs are spun on a spindle at about 2700 rpm so that the colours merge into a single hue without flickering. The test sample is placed adjacent to the spin­ning disc under controlled illumination and both are viewed simultaneously.

Coloured chips: A simple method is to match the colour of the food with the colour chips or colour glass, chart or colour tiles. This method is not very satisfactory as it is difficult to match the food with one small block of colour or the chart. The data are difficult to tabulate and analyse also

Last modified: Thursday, 16 February 2012, 9:55 AM