Chapter 3 Simple methods for quality control

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The methods described below have been included because:

It should be noted that many of the methods described in this chapter are comparative and not absolute methods. In other words the results may be used to give a comparison with other results obtained by the same method but they cannot be compared to results found using a different method. For most purposes this is acceptable for routine quality control but it should be emphasised that careful attention is needed to ensure that exactly the same procedure is used on each occasion.

 

Acidity measurement

To measure the amount of acid (such as citric acid, acetic acid, lactic acid, etc.), it is necessary to titrate a sample of the food with sodium hydroxide solution. It is not sufficient to measure the pH of a food (see pH measurement below) as this does not tell you the amount of acid present.

The method involves the following steps:

1 take a 10 ml sample of liquid food or 10 g of solid food

2 if solid, the food should be liquidised to a fine pulp

3 mix the sample with 90 ml of distilled water, making sure that it is completely mixed

4 add about 0.3 ml of an indicator solution (eg 1% phenolphthalein in 95% solution of ethanol)

5 fill a burette (Fig. 37) with 0.1 M sodium hydroxide solution (obtainable from larger pharmacies) and titrate the sample until there is a pink colour that does not change.

6 calculate the amount of acid as " % acid per ml of liquid food" or "per g of solid food" using the formula: % acid = number of ml of sodium hydroxide x one of the conversion factors below:

acetic acid (vinegar) 0.060

citric acid 0.070

tartaric acid 0.075

lactic acid 0.090

It is necessary to know what is the major acid present in a type of food before selecting the conversion factor.

Figure 37: A burette used for titration

 

Chlorine measurement

The determination of levels of chlorine in water is usually carried out using a calorimetric test in which a chemical dye, which reacts with chlorine, produces a colour that is proportional to the amount of chlorine present.

The test requires the use of a 'Lovibond Comparator' (Figure 38: A comparator, used to test chlorine levels in water). This is supplied with a number of discs of coloured glass that are calibrated for different chlorine levels (eg 1 to 10 ppm, 10 to 50 ppm etc).

In use, a few drops of the dye are placed in test tube. The water under test is added and a colour develops. The tube is then placed in the comparator and the colour is matched with one of the calibrated discs.

These chlorine test kits are available from a number of suppliers and it is suggested that those considering testing chlorine levels in water should consult the local Water Department which will be able to advise on the nearest source of supply and possibly provide training in the method.

 

Fill-weight measurement

In most countries there are legal requirements which state that a container has the weight of its contents written on the label and that the net weight of food inside is not less that this weight. This should be routinely checked by taking regular samples of filled containers and placing them on a scale (Fig. 24). On the other side of the scale there is the heaviest empty container from a batch (see 'weight of glass containers' below) plus a metal weight that is equal to the net weight shown on the label. Filled containers that are underweight should be removed and re-filled. The results of these checks should be recorded on a chart and related back to the worker who is filling the containers or operating a filling machine to ensure that accuracy of filling increases with experience gained.

It should be noted that this is the "minimum weight" method of checking fill-weights. It is intended to ensure that all packs contain more than the net weight shown on the label and it is a simple system for small scale producers to operate. There is however another system, known as the "average weight" system, which relies on the statistical probability that a known proportion of packs will be above the net weight. This system was developed in Europe for automatic filling operations and is un-necessarily complex for small scale producers. However if exports to Europe, USA or some other countries are contemplated, then it is recommended that details of this system should be obtained from a local Export Development Board or its equivalent.

 

Flour infestation

1 100 grams of flour is weighed onto a flat surface.

2 Flour is flattened using a ruler.

3 Flour is examined after two minutes for evidence of pimpling, i.e. surface disturbance.

Pimpling indicates the presence of flour mites breaking the surface for air.

 

Glass container measurement

Glass containers have more variable dimensions than either plastic or metal containers because of the nature of their manufacture. It is therefore important to check particular dimensions to ensure that a) a container has the expected capacity; b) that the neck is properly formed and will allow the lid to fit properly; c) that the container is vertical to prevent it breaking in a filling machine, and d) that the weight of a sample of empty containers is checked to find the heaviest in a batch for use in check-weighing.

Glass pieces are also a particular danger to customers if they become mixed into the product. It is therefore essential that the checks described below are routinely performed on all glass containers.

Finally glass containers are often re-used and may have become contaminated by kerosene, pesticides or other materials. They should be thoroughly washed and inspected by looking and smelling to ensure that no residues remain before the food is filled.

 

Weight of containers

Take a random sample of empty containers from an incoming batch (for example 1 in 50) and weigh them, together with their lids. The required check-weight is calculated as the weight of the heaviest container plus the net weight of product.

 

Capacity of containers

Weigh a dried container and then fill it to the top with distilled water at 20C. Reweigh the filled container and the difference in weight (in grams) is equivalent to the capacity in ml. This capacity should be great enough to allow sufficient food to be filled to meet the net weight declared on the label.

 

Headspace

A space is required between the surface of a hot-filled product in a jar and the underneath surface of the lid. This allows a partial vacuum to form when the product cools and thus helps to prevent spoilage. The volume of the headspace does not normally exceed 10% of the capacity of the container. Measuring the depth of the headspace is a quick method of assessing the volume, but the depth varies according to the capacity of the container (larger containers require a deeper headspace).

A headspace gauge (Figure 39 : A headspace gauge used to check the depth of a headspace) is a cheap and simple way of routinely checking that product has been filled to the correct level. It consists of a series of prongs of different lengths fixed onto a bar and it is placed on the rim of the jar before fitting the lid. The level of product may then be seen where it touches one of the prongs. Alternatively a device (Figure 40: A simple way of ensuring a consistent headspace) can be fitted into the production line to maintain a uniform headspace.

 

Vacuum

The partial vacuum in hot-filled glass jars may be measured using a Bourdon tube vacuum gauge (Figure 41: A vacuum gauge for measuring headspace vacuum). The gauge is fitted with a sharp needle, surrounded by a rubber seal. The needle is pushed through the lid of the container and the moistened rubber seal prevents air from entering. The partial vacuum may then be read directly from the gauge as 'mm of mercury' or 'minus kPa'. As the product is not saleable after this test, it is usually only applied to a small sample or when a problem arises.

 

Dimensions of containers

The important routine checks are to measure the height of containers, their neck diameter and outside diameter, and their ovality (to ensure that they are round and not oval). Simple equipment may be manufactured to perform these checks (Figure 42: Simple equipment for checking glass containers: a) height measurement, and b) go/no-go rings). A vertical ruler on a stand is used to measure height. Different "go/no-go" rings can be made for each size of container that is used. Rings are slipped over the neck of a container to quickly show whether the diameter is too large or too small for the intended lid and also to show if the neck is not circular. Different sized rings can similarly be used to check the outside diameter of the container and its ovality.

 

Faults in glass

It is essential that all glass containers are visually checked to make sure that no glass splinters or cracks are present. Common faults in glass are bubbles, cracks and strings (Figure 43: Serious faults in a glass jar). A light-box, in which a light bulb is placed behind a translucent plastic screen, is useful to view glass containers clearly. Operators who check glass containers should be fully trained in which faults to look for and they should be moved from inspection after 3060 minutes to prevent tiredness and lack of concentration.

 

Gluten measurement

1 Weigh 10 + 0.01 g of flour and place in a basin.
2 Add 6ml of water to the basin, (5 ml will be sufficient for weak flour).
3 Using a spatula, mix the flour and water into a dough. Form the dough into a round ball by rolling between the palms of the hands.
4 Replace the dough in the basin and cover it with water. Leave for a time, at least 10 inutes, preferably 45-60 minutes.
5 Holding the dough ball in one hand under cold running water, wash out the starch. Squeeze the dough frequently between the fingers and the palm to help the process.
6 When all the starch has been removed the wash water will run clear and the remaining gluten will be free from lumps.
7 Remove the excess water with blotting paper.
8 Weigh the wet gluten and record this as a percentage of the flour weight.
9 The gluten may be dried in an oven at 103C to determine dry gluten.

The gluten obtained at step 7 may be physically examined for strength and elastic properties by pulling the gluten apart.

 

Label measurement and quality checks

It is often forgotten that labels are an integral and important part of a food product. They need to be tested and checked in the same way as any ingredient. On delivery, samples should be taken from each pack of labels for examination as faults may develop in a print run. All packets of labels should then be repacked and sealed.

Label faults may be divided into major and minor faults. Major faults include:

Labels showing major faults should not be used and should be returned to the printer.

Minor faults include:

Labels with minor faults may be used but the problems should be discussed with the printer and possibly a reduction in price negotiated.

 

Loaf volume measurement

It is usual to use a simple device in which the displacement of rapeseed or mustard seed is measured. This is accurate because the individual seeds are hard and quite round, flow easily, do not disintegrate and a given weight always occupies the same volume. There are two rectangular compartments, connected by a graduated cylinder made of glass or transparent plastic. The equipment may easily be inverted, allowing either compartment to be uppermost. An adequate amount of seed is placed inside and flows from one compartment to the other as the apparatus is inverted. With the seed in one compartment in the lower position, the loaf under test is placed in the top compartment which is then closed and the apparatus is inverted so that the seed fills the space around the loaf and levels off in the tube. The bigger the loaf, the higher up the tube will be the surface of the seed layer. The actual volume is read off from the graduations on the tube, which is previously calibrated. Loaf volume is usually expressed in cc's and the volume of 1 lb loaves may vary between 1,400 cc's and 1,600 cc's, depending on the flour used.

An indication of volume may quickly be obtained by measuring the maximum height of the loaf. This is useful but because of the irregular shape involved, it is not very accurate.

 

Moisture content measurement: grains

With experience, an operator may assess the correct moisture content of grains by placing them on a hard surface and tapping them with a metal or stone weight. The hardness (or softness) of the grain indicates the approximate moisture content.

A more accurate but more time consuming method is to dry a weighed sample of grain in an oven at 100C for 5 hours (or 104C for 2 hours) and re-weigh. Certain items of equipment are needed to determine the moisture content: a balance accurate to three decimal places (ie 0.001 g), a thermostatically controlled oven and a laboratory desiccator. A sample of material is dried to constant weight and the loss reported as moisture content.

Approximately 2 g of the material under test is accurately weighed (to 0.001 g) into a small dish. This is then placed in the oven for 1 hour, removed from the oven and put in the desiccator to cool. It is then weighed. The dish is replaced in the oven for 30 minutes and the process repeated to constant weight.

The moisture content is found using the following formula:

A faster but more expensive method is to use a moisture meter. This measures the conductance of electricity through a sample of grain to indicate the amount of water it contains. The instrument is expensive and therefore likely to be affordable only by larger scale millers.

 

Moisture content measurement: fruits.

This is measured by oven drying as described above for grain moisture content measurement.

 

Moisture content measurement: spices.

This is measured by oven drying as described above for grain moisture content measurement. However, it is likely that volatile oils from the spices will also be evaporated and care should therefore be taken when interpreting the results of such tests. Drying at a lower temperature using a vacuum oven is likely to be too expensive for most producers.

 

Solids content measurement

The method involved is the same as that described for moisture content above, but the result is expressed as ' % solids'. This is calculated using the following formula:

 

Packaging film measurement

Made-up plastic bags and rolls of film need to be checked and there are a number of simple tests that a small food processor may carry out. It should be remembered that there is no way of checking for faults inside a roll of film as only the outer part may be seen. Rolls thus need to be examined during use. Typical faults in plastic bags and films include:

 

pH measurement

pH is a scale that is used to describe acidity (pH 1-6), neutrality (pH 7) or alkalinity (pH 814). There are two methods of measuring the pH of a sample of liquid food: the simplest and cheapest is to dip a piece of pH paper into the sample. The paper is impregnated with chemicals that change colour and the colour may be compared to a chart supplied with the paper to give the pH of the sample. This method is often sufficiently accurate for routine Q checks.

If greater accuracy is required a pH meter should be used. These may be mains powered bench models or battery powered portable models (Figure 44: Bench mounted and portable pH meters). In general bench models are more accurate than portable types, although newer equipment has reduced this difference. Bench types are more expensive than portable types and, when properly maintained, may have a longer working life. If voltage fluctuations are a problem, bench models require a voltage regulator to be fitted.

Modern portable pH meters are fitted with a container filled with buffer solution when they are delivered. This should be replaced as directed in the suppliers instructions. New electrodes for bench models should be soaked for several hours in distilled water or buffer solution. Afterwards they should be stored with their tips in one of these solutions. Older electrodes may be cleaned by placing in 0.1 M sodium hydroxide solution for 1 minute and then in 0.1 M hydrochloric acid for 1 minute, repeated twice. They are then rinsed in water and carefully blotted (not wiped) with a soft cloth or tissue paper.

pH meters should be standardised against buffer solutions which have a known pH. The standardisation and later pH measurements should all be done at the same temperature (between 20-30C) to avoid errors in the results.

In use the instructions supplied with the equipment should be carefully followed. For example the instrument has a temperature compensation control that should be set to the local ambient temperature.

The general procedure for measuring pH is as follows:

  1. Standardise the instrument using a commercially supplied pH 4.0 buffer or freshly made 0.05 M potassium acid phthalate solution (10.12 g of the chemical in 1 litre of distilled water). Dip the electrode into the buffer and adjust the standardisation control so that the scale reads pH 4.0.
  2. Rinse the electrode in water and carefully blot (not wipe) clean with a soft tissue.
  3. Check the pH of a commercially supplied pH 7.0 buffer or 0.025 M potassium dihydrogen phosphate solution (3.387 g of the chemical dissolved in 1 litre of distilled water).
  4. Repeat step 2.
  5. Place the electrode in a sample at the same temperature as the buffers (between 20-30C) and allow the instrument to stabilise for 1 minute. Repeat step 2 and take a second reading. The two results should be within 0.1 pH units of each other.
  6. Store the cleaned electrode in distilled water or buffer solution and switch the instrument to 'standby' when it is not being used.

 

Plastic container measurement

There are fewer checks that are needed on plastic containers, compared to glass containers. This is because the method of manufacture results in more uniform dimensions, the weight of the container is small compared to glass and variations are therefore less important. The main faults are likely to be splits, punctures, a badly formed neck and the use of non food-grade plastic. With the exception of the last fault, each can be checked visually by operators involved in filling the containers. There are no simple checks to ensure that a container is made from food grade plastic and if there is any doubt the processor should consult a reputable supplier for advice. They may be checked to ensure that the seal or cap is water-tight by simply inverting a sample of filled containers to detect leaks.

 

Salt measurement

There are three methods that can be used for measuring the salt concentration in foods: hydrometry, refractometry or salt titration. In the context of this book, the main application is checking the salt concentration of brines and hydrometry is the recommended method. Refractometers are expensive and the titration method is more complex and requires training and laboratory chemicals.

Hydrometers are hollow glass rods with a bulb at one end (Figure 45: Hydrometer for measuring salt concentration in brine). They are sealed at both ends so that they float when immersed in a liquid. The bulb is weighted so that the hydrometer partially sinks to a level that depends on the specific gravity of the brine (the more salt that there is in a solution, the higher the specific gravity). A scale on the stem of the hydrometer is calibrated and may read from 0-100 degrees, where 0 is pure water and 100 is saturated salt solution (26.5%). It is important that the measurements are made at the reference temperature for the hydrometer (usually 20C) because the specific gravity of the brine changes at different temperatures.

The method of measurement involves placing a sample of brine at the correct temperature into a large clear glass or plastic cylinder and gently lowering the hydrometer into the liquid. When it has stopped moving, the scale is read at the surface of the liquid and the reading is converted to % salt using a conversion table supplied with the hydrometer.

It is important that a salt hydrometer is specified as there are other types that are calibrated for alcohol or for sugar solutions.

 

Sieving tests (flours and spices)

A 500 g sample of flour or ground spice is sieved through a stack of metal sieves with the largest mesh at the top of the stack and the smallest at the base. Typically the range of sieve aperture sizes is 1.6 mm to 0.038 mm. The sieves may be placed on a shaker to achieve a consistent amount of shaking. The amount of material that is collected on each sieve is weighed and expressed as a percentage of the total weight. Further details are given in (Dichter, 1978) in Appendix 1.

This method can also be used to detect gross contamination with stalks, stones, string, cigarette ends, leaves etc as these are retained on the larger aperture sieves and can be examined, recorded or weighed.

 

Filth test

This is a modified method that allows detection of insect parts, rodent hairs or ground faeces in milled spices or flours. The sample of food is mixed with petrol and thoroughly stirred. The insect parts, etc. are preferentially wetted by the petrol and when the suspension of particles settles, these may be seen floating on the surface of the petrol. If required they may be filtered through a filter paper and examined or identified.

 

Sodium benzoate measurement

Although it is possible to measure the amount of sodium benzoate in a food by measuring the benzoic acid content, this is a fairly complex method that requires laboratory facilities and it is unlikely to be routinely done by a small scale producer. The method is described in publications by Board (1988) and Egan et al (1981) in Appendix 1.

 

Sodium metabisulphite measurement

The amount of sulphur dioxide that is produced from sodium metabisulphite is approximately two thirds. For example if 1.5 g of sodium metabisulphite is added to one litre of juice it will form 1 g of sulphur dioxide. (la per litre = 0.001%. This is equivalent to 1000 ppm). The amount of sulphur that is required to produce sulphur dioxide in a sulphur cabinet is usually only estimated approximately because of the large number of variables that influence the absorption of sulphur dioxide by fruits. As an approximate estimate, 350400 g of sulphur can be used per 100 kg of fresh fruit.

Although it is possible to measure the sulphur dioxide content of a food item, this requires relatively sophisticated laboratory equipment and is not usually done by small scale producers. A method is described by Board (1988) in Appendix 1.

Starch gelatinization measurement (modified 'Falling Number' method)

  1. 100 g of flour is mixed with 900 g of hot water in a pot.
  2. The pot is heated until the flour mixture has gelatinised.
  3. The mixture is poured into a 1 litre measuring vessel.
  4. The measuring vessel is stood in hot water.
  5. A steel ball is dropped into the mixture and the time to drop 200 ml recorded.
  6. The time (in seconds) is compared against the standard batch.

 

Sugar measurement using soluble solids content measurement (by refractometer)

Fruit jams, juices, sauces, confectionery, etc. contain sugar as the main soluble solid. For these products the sugar content can be measured directly using a refractometer. Although this equipment is relatively expensive for a small scale producer, it does give an accurate measurement of sugar concentration which is a vital control point for many products. Two types of refractometers are available: the bench type (or 'Abbe' refractometer) and a hand held type (Figure 46: A hand-held refractometer). For quality control purposes the hand held type is cheaper and it is usually sufficiently accurate.

The method involves taking a small sample of the food and placing it on the lower glass prism of the instrument. The upper prism is then closed and the refractometer is held against the eye, pointing in the direction of a window or bright light. It is focused until the scale can be read against a clearly defined division between black and orange colours. The reading is recorded as degrees Brix which corresponds to % sucrose.

 

Sugar measurement using hydrometry

Simple sugar syrups may also be measured using hydrometry. The hydrometers are similar to those described for salt, but they are calibrated for sugar (sucrose). The method used is the same as that described for salt and the scale is read as % sucrose. The samples should be at the reference temperature (usually 20C) for the hydrometer.

 

Sugar measurement using temperature

The sugar content of products such as jams and confectionery can be estimated in a less accurate way by measuring the temperature of boiling. As the sugar content increases the temperature of boiling also increases (Table 10). Note that the boiling temperature also changes according to the amount of invert sugar or glucose syrup in the boiling mixture and experience of making the product is needed before using boiling temperature as a control measure. The boiling point also changes according to height above sea level and this should be checked if a producer is operating in a mountainous region.

Table 10: Boiling temperatures of different sugar syrup concentrations

Sugar concentration (% sucrose) Boiling point (C)
40 101.4
50 102
60 103
711 105.5

A special thermometer that reads up to 150C is required and the bulb of the thermometer should be protected by a metal casing to protect it against breaking (Figure 47: A thermometer suitable for measuring the high temperatures in jam boiling). In general mercury thermometers should not be used in food premises.

A crude estimate of the solids content of jam and confectionery products may be made by placing a sample on a jar lid which is floating in cold water and noting the texture of the product after it has cooled to see if a firm gel is formed. With experience this may be used as a simple check to ensure that products have been boiled to the correct consistency, but a more accurate measurement using a refractometer is recommended to ensure uniform product quality. A summary of the methods and expected results is given in Table 11.

 

Water absorption measurement (flour)

1 100 g of flour is weighed into a small mixer.

2 Water is slowly added from a burette until a standard dough is made. This is judged by the processor by adding water until it 'feels right'.

3 The amount of water added is recorded.

New batches of flour are tested alongside the existing material. This can be used as a comparative test to indicate wrong grade of flour.

 

Yeast activity measurement

1 A 5% suspension of fresh yeast is made.

2 3 g of flour is weighed into a pot.

3 1.8 g of suspension is added and mixed with a spoon.

4 The mass is moulded into a ball using the hand and dropped into a narrow 200 ml beaker containing 150 ml of water.

5 The beaker is placed in a water bath at room temperature (25-30C).

6 The time from placing in beaker until final break-up of the ball is the fermentation time.

Table 11: Simple tests for sugar boiling

Approx Temperature (C) Test Name Result
103 A Thread Thin strands
105 B Small pearl Forms small droplets
105 C Jam set Forms a strong gel
106 B Large pearl Forms large droplets
111 B Feather Forms hard feathery strands
116 B Small ball Forms soft ball
120 B Large ball Forms hard ball
129 B Light crack Forms thin sheet
133 B Medium crack Sheet forms, slightly brittle
143 B Hard crack Sheet forms rapidly
180 B Caramel Brown brittle sheet forms

 

Methods:

A - Place a sample of cooked syrup between two wetted fingers and open them.
B - Dip a spatula in water and then in the cooked syrup, return it to cold water.
C - Place a sample on a jar lid in cold water, test by feeling after it has cooled.
(Adapted from Lees and Jackson, 1985. see Appendix 1)


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