This chapter will present some of the most widely applied procedures in home-made or small-scale processing systems.
The general concept underlying the preservation of foods aims to prevent the development of micro-organisms (bacteria, yeasts and mould), to avoid food spoilage during storage. At the same time, the chemical and biochemical changes that bring about the deterioration process must be controlled. This way, it will be possible to obtain a food whose typical organoleptic characteristics will have remained unchanged (colour, flavour and aroma), and which can be safely consumed within a certain period of time (at least a year).
Recently, there have been many innovations in industrial food processing. The techniques employed today to preserve foods are characterized by different levels of complexity compared to ancient fermentation and sun drying methods. These include irradiation and freeze-drying. However, when considering relevant food preservation techniques in small-scale industrial systems, the discussion should be limited to the simplest methods.
Such methods include:
These operations include washing, sorting, peeling, cutting or grinding and blanching, among others.
The raw material must be processed as soon as possible (between 4 and 48 hours after it is harvested), to prevent spoilage. These preliminary operations are required for the processing of all fruits and vegetables, which must generally be washed before anything else takes place (onions and cabbages, for instance, will be washed after the removal of the dry outer layers and external leaves, respectively).
Washing is an operation that generally is the point of departure of any fruit and vegetable production process. In a small-scale operation, this activity is normally carried out in basins with recirculating water, or simply with still water that is continuously replaced.
The operation consists of eliminating the dirt sticking to the material before it enters the processing line, thus avoiding complications deriving from the possible contamination of the raw material. The washing must be performed using clean water, which should be as pure as possible, and if necessary should be made potable by adding sodium hypochlorite, 10 ml of 10%, solution for every 100 litres of water.
It is advisable to use implements that allow for an adequate cleaning of the material, so that no traces of dirt are left in the subsequent phases.
Once the raw material is clean, it must then undergo the selection phase. At this stage, the material that will really be used in the process will be separated from material presenting some sort of defect, which will become second-choice and will be used for a different purpose, or will simply be eliminated.
In the case of a semi-mechanized small-scale plant, the selection is carried out on a table suitable for this process or on a conveyor belt. Such a process will entail the removal of all of the fruit and vegetables that do not have uniform characteristics compared to the rest of the lot, in terms of ripeness, colour, shape and size, or which present mechanical or microbiological damage.
Sometimes, to appreciate the uniformity or quality of a material, it is necessary to cut it in half to verify its inner contents. Uniformity is a significant quality factor, since it is of utmost importance for the material to be even and uniform. The function of the sorting process is precisely that of securing such a homogeneity.
This operation too is performed on a regular basis. It consists of the removal of the skin of the fruit or vegetable. It may be performed by using physical devices like knives or similar instruments, by using heat or chemical methods. Such methods basically aim to bring about the decomposition of the walls of the external cells of the skin, so that the skin is removed as a result of the tissue's loss of integrity.
Peeling is an operation that allows for a better presentation of the product, and at the same time fosters sensory quality, for the material with a firmer and rougher texture is eliminated. Moreover, the skin often presents a colour that has been affected by the thermal processes normally used in processing methods.
Cutting is an operation that is usually included in the different preservation processes. This operation makes it possible to achieve different objectives, like an even penetration of heat in thermal processes, uniform drying and a better package appearance, since the packed material is more even in terms of its shape and weight. In the specific case of drying, cutting enhances the surface/volume ratio, which increases the efficiency of the process.
When performing the cutting operation, special care must be taken to fulfil two conditions. First of all, the cutting tools or devices must produce clean and clear cuts not involving more than a few layers of cells, to the extent possible. In other words, they must not cause excessive damage to the tissue, to avoid detrimental effects like a change in colour, and subsequently a change in the product's flavour. Moreover, the cutting must be performed in such a way as to allow for a viable industrial performance. A way must always be found for the cutting operation to supply the greatest possible amount of usable material.
This is another widely employed operation in fruit and vegetable processing. It is a form of thermal treatment, the aim of which is to condition the material in several ways: to soften it to facilitate the filling of the containers and to inactivate enzymes which cause an unpleasant smell and flavour, as well as defects in the natural colour of the product.
This operation requires great care, that is, it must be properly controlled and the temperature and time of application are to be closely monitored. Also, the treatment must be rapidly followed by means of efficient cooling. A high-temperature treatment for a brief period of time is always preferable. Furthermore, it is best to use steam rather than hot water in the blanching process, mainly to avoid the loss of soluble solids, like water-soluble vitamins, which occurs when hot water is used.
The most common method used to perform the treatment is the immersion of the product packed inside a metal basket in a bath of boiling water or in a pot in which a small portion of water forms an atmosphere of high-temperature saturated steam. In a more automated system, a steam tunnel may be used, with a continuous line or a chain conveyor which is submerged in a hot water bath. In both cases, jets of water are used for cooling.
The operations described above may be applied on a general basis, in different processes. However, some procedures are intended for more specific applications, such as the removal of pits, coring, pulp extraction and others, which must be carefully studied on a case-by-case basis to determine the best way to proceed. In view of the limited scope of this manual, it would be impossible to provide a detailed description of each of these techniques. It is thus recommended that the general quality criteria described previously be used to implement such specific operations.
Food preservation may be defined as the set of treatment processes that are performed to prolong the life of foods and at the same time retain the features that determine their quality, like colour, texture, flavour and especially nutritional value.
Food preservation processes require a varied and broad time scale, ranging from short periods needed for home cooking and cold storage methods, to much longer periods of time required by strictly controlled industrial procedures such as canning, freezing and dehydration.
When microbial stability is considered, short-term preservation methods like refrigeration are inadequate after a few days or weeks, depending upon the raw material, for accelerated microbial development occurs.
In the case of industrial processes, in which the preservation is achieved by commercial sterilization, dehydration or freezing, microbial development is controlled until the processed food may be safely consumed. It should also be borne in mind that the use of appropriate containers is extremely important, for the processes would be completely useless if the containers employed were not able to prevent subsequent contamination.
The preservation of fruit and vegetables entails the integral or partial utilization of the raw material. In some cases, during the process it becomes necessary to add a packing medium, syrup or brine, while in others the raw material is used alone, as in frozen products. The raw material may be processed differently, depending upon the product to be obtained, as vegetables in sauce, soups, jellies, pickles and juices, for instance.
The same raw material may be processed in different ways, as a result of which, different products will be manufactured.
The case of the pineapple is a good example, for the same raw material may be processed into canned slices or rings, pulps or juices.
In general terms, processing methods may be divided into three groups:
Short-term Processing Methods
- Cold storage with modified atmosphere
- Superficial chemical treatments
- Special storage conditions
- Packaging systems involving modifications in atmosphere
Preservation Methods by Chemical Action
- Preservation with sugar
- Addition of sulphur dioxide
- Preservation by fermentation and salting
- Treatment with acids (addition of vinegar)
- Use of chemical additives for microbial control
Preservation Methods by Physical Treatment
- Use of high temperatures
- Use of low temperatures
- Use of ionizing radiation
Most of these methods entail a combination of techniques. For instance, there is a procedure combining freezing, dehydration and preservation, fermentation and pasteurization. It will also be necessary to be provided with appropriate containers and packages protecting the food from microorganisms.
The preservation methods that will be mentioned in this manual are the following: canning, pasteurization, preservation by the addition of soluble solids (sugar), the addition of acid (vinegar) and the natural drying of fruits and vegetables.
The processes that use high temperatures as a way to preserve foods include canned and pasteurized products (juices, pulps). Such thermal processes involve sterilization or pasteurization in jars, bottles or other containers serving the same function. Other containers include tin cans. The bulk sterilization of products and their packaging in aseptic containers is another procedure based on the utilization of ultra high temperatures.
Sterilization as a preservation method may be applied to any product having been peeled, cut or having undergone some other preparation procedure, provided that it has been packaged in an appropriate container and sealed hermetically to prevent the penetration of micro-organisms and oxygen.
The purpose of canning, which is based on commercial sterilization, is to destroy any existing pathogenic microorganisms and prevent the development of those that may cause the product to deteriorate.
Sterilization prevents the survival of pathogenic or disease-causing organisms whose presence in the food and accelerated multiplication during storage may be a serious hazard to the health of consumers. Micro-organisms may be destroyed by heat, but the temperature required varies. Many bacteria exist in two forms: vegetative bacteria, which are less resistant to temperatures, and sporulated bacteria, which are more resistant. An analysis of the micro-organisms present in food products has led to the selection of the presence of certain types of bacteria as indicators of a successful process.
"Indicator" micro-organisms are the most difficult to destroy by means of thermal treatment. This means that if the treatment is successful in destroying them, it will be all the more so in the case of other more heat-sensitive microorganisms.
One of the micro organisms mostly used as indicators in commercial sterilization processes is the Clostridium botulinum, which causes serious intoxications in low-acidity foods, a condition which elicits toxin production by the micro-organism.
Heat destroys the vegetative forms of micro-organisms and reduces the security level of the spores, that is, the resistant forms of micro-organisms, making sure that the product may be consumed safely by humans.
The products that may be subjected to preservation by means of commercial sterilization are quite varied. Fruits in general may be processed this way, pineapple and guava being two examples. They are acidic and very safe in terms of the presence of Clostridium botulinum, for this degree of acidity does not provide the micro-organism with suitable conditions to produce the toxin, which is highly dangerous and deadly to humans. Low-acidity products like most vegetables, may be contaminated by the micro-organism and produce the toxin during storage.
Due to the afore-mentioned reasons, it is not advisable to process low-acidity vegetables in home settings which do not allow for an appropriate control of the process.
The application of this method is crucial to the products covered in this course, like pulps or juices.
Pasteurization consists of a thermal treatment that is less drastic than sterilization, but sufficient to inactivate the disease-causing micro-organisms present in the foods. Pasteurization inactivates most of the vegetative micro-organism forms but not the spore-bearing forms, which is why it is suitable for short-term preservation. Furthermore, pasteurization fosters the inactivation of enzymes that may cause the food to deteriorate. As with sterilization, pasteurization is performed according to an appropriate combination between time and temperature.
The processing of fruits and of some vegetables into juices and pulps extends their storage life. This is made possible by pasteurization, which considerably reduces the number of fermentative micro-organisms that contribute to the acidification of juice, at the expense of sugars.
The pasteurization of clear and pulp-containing juices and of fruit pulp, provides for their stabilization so that they can be preserved in combination with other methods like chilling or freezing. All of these procedures will contribute to guaranteeing the quality and shelf life of the product over time.
The preservation of foods through the removal of water is probably one of the oldest techniques. In the past, the process was simplified by directly exposing the product to sunlight. The crop was spread on the ground directly or over sacks or mats made from plant leaves.
Today, the quality of dried products has improved thanks to a number of factors, including the following.
- The use of dehydrating equipment for solar and artificial drying, which increases the efficiency of dehydration.
- The use of chemical pre-treatment to better preserve the colour, aroma and flavour of the products.
The fundamental principle on which dehydration is based is that at low moisture levels, the water activity drops down to levels at which neither micro-organisms nor deteriorating chemical reactions can develop.
Generally speaking, vegetables with less than 8% moisture and fruits with less than 18% residual moisture are not favourable substrates for the development of fungi, bacteria or important chemical or biochemical reactions.
There are reactions, such as non-enzymatic browning, which may develop at slower rates in low water-level environments, but that requires high temperatures. Other reactions include fat oxidation, which may occur at very low water levels, but which is accelerated by light and temperature. The container and environment in which the dehydrated products are kept are thus extremely important to guarantee good preservation.
Fruits and vegetables can be dried using simple apparatus, as shown in pictures 8 and onwards. The quality of the products dried according to these methods is much better than that of products that are simply spread over the ground to dry in the sun.
It is very important to avoid contamination by dust and other substances that may be the carriers of micro-organisms which are resistant to low moisture levels, as for example the excrements or urine of rodents or domestic animals, chemical products, pesticides and others. The sites used for the drying process must also be very carefully chosen. All of such risks are greatly reduced when equipment as that illustrated in pictures 8-12 is employed.
The drying time and the product's final moisture level will depend on the location of the dryer, the climatic conditions of the place and the characteristics of the product. Material cut in small sections and with a greater drying surface will dry more rapidly.
The drying process must be handled with great care, if one wishes to obtain a quality product. Often, the drying must take place in the shade in order to preserve the product's sensory qualities, like colour, aroma and texture.
Preservation by the addition of sugar
Sugar is generally added in the processing of jams, jellies and sweets. The fruit must be boiled, after which the sugar is added in variable amounts, depending upon the kind of fruit and the product being prepared. The mixture must then continue to boil until it reaches such a level of soluble solids, which allows for its preservation.
The addition of sugar plus certain fruit substances produces a gel-like consistency, which characterizes the texture of jams and jellies. To achieve this, appropriate acidity levels and sugar contents are necessary. Some fruits do not contain sufficient amounts of the substance known as pectin to form a proper gel. In such cases, exogenous pectin must be added. There is a difference between apples or citrus fruits and berries, like raspberries or Chilean strawberries. The level of pectin is high in the former and low in the latter.
While the fruit is boiling, after sugar has been added, sucrose - which is the aggregate sugar - breaks down into its components, fructose and glucose. This determines two major effects on the product: greater solubility, which prevents crystallization, and a sweeter taste. This process is known as sucrose inversion.
Jams and the other products mentioned are preserved on the basis of a principle named water activity. It refers to the availability of water that is free to react with and allows the development of micro-organisms. The lower the level of water activity, the lower the incidence of deteriorating chemical reactions and micro-organism development.
The level of water in jams may permit the development of old. Therefore, if the product is to be preserved, it must be packaged under vacuum by means of the heat filling method, or else fungistatic chemical substances, like sodium benzoate and potassium sorbate may be used, which inhibit the development of fungi. Whenever possible, it is always best to opt for the first alternative, although it requires the use of glass containers, which are more expensive.
Preservation by means of pH regulation
Most foods may be preserved by heat treatment when the medium has a pH lower than 4.0. It is for this reason that several methods have been developed which seek to control the pH through the endogenous production of acid, or the exogenous addition of some organic acid, like acetic, citric and even lactic acid.
The acidification of low-acidity vegetables for commercial sterilization-based processing, with brief sterilization periods at temperatures around 100°C, is a very practical method to employ in small-scale and even home processing.
The preparation of pickles from different vegetables, by means of natural fermentation with the production of lactic acid, is also a very suitable method for the preservation of cucumbers, small onions, carrots, peppers and other crops that are regularly marketed in large volumes throughout the world.
It is important to make sure that the pH is kept at a level of about 3.5, so that the product will have a pleasant flavour and not taste like lactic acid. Lactic acid is naturally produced by the fermentation of substrates constituting the material, carried out by micro-organisms.
The acidity of a pickle having been prepared by the addition of acetic acid or vinegar, must be around 4% and not over 6%, expressed as citric acid. In addition to acid, pickles are also prepared with salt, which is known to possess antiseptic properties, and at appropriate concentrations preserves the quality of the product for a long time, as well as enhancing the product's sensory qualities, like its texture and flavour.
It should be stressed that these natural fermentation processes in brine are produced by micro-organisms that thrive in anaerobic conditions. Therefore, to obtain a good product, the system must be characterized by a low oxygen content.
The product is immersed in brine, or a small amount of dried salt is added (as in fermented cabbage) and anaerobic conditions are provided in a polyethylene bag or in a container that should be as hermetic as possible.
Temperature is an important factor in this type of process. It should be no lower than 15°C, with best results obtained at 25°C.
As established previously, small-scale industrial processing does not differ greatly from home processing, as far as the main principles are concerned. The great difference lies in the procedures and equipment employed in a low-level industrialized plant.
The processes are similar to those analyzed previously, but their volume is greater, which makes it necessary to have greater control over the components to be able to manage any problem that may arise during the process.
All of the products that are illustrated may be employed in the same way in a small-scale process, except for the fact that pots will have to be replaced by double-bottom large kettles normally made of stainless steel, heated by steam. The process is more efficient thanks to the advantages afforded by the steam heating system, the preparation time is shorter and inspections will also require less time.
On the other hand, the amounts of raw material will be larger, and this will require greater promotional efforts in the case of home processes. However, a sound home-processing system also requires planning in terms of raw materials and goods. As a result, the difference is not so significant after all.
In a small-scale industrial process, the equipment fixed in a solid premise has the inconvenience of not being very flexible, especially when small quantities of raw materials are involved.
This is a priority concept when considering food processing, even if home or small-scale industrial processes are involved. The concept of quality is rather complex although common sense has instilled some idea of this basic principle in our heads.
Quality may be defined as the set of attributes or characteristics which describe the nature of a given good or service. This means that quality is not synonymous with good quality, as is often thought. Quality is nothing but quality, with no adjectives; it is a set of characteristics that must be defined more accurately when describing a given product or service.
The determination of quality is just important a process as the proper food preparation. To determine quality, one must rely on a system, a defined and systematic methodology. The best way to go about it is to focus on quality production, that is, apply the good quality concepts to each and every step of the process that leads to the final product.
The control of the product's quality, as the only quality control method, is totally outdated. The idea today is to produce properly once and for all. In other words, one should seek to avoid having to go back to the production line to correct the mistakes made in the previous steps. Having to go back is very expensive in terms of current expertise standards.
It is for these reasons that quality should be an assimilated concept, so that emphasis is placed on the production of goods that will always be acceptable to the consumers, whose demand is predictable.
Quality control must be intended as a planned activity or as a complete system, with written specifications and standards calling for the revision of raw materials and other ingredients, the inspection of critical process control points, and finally the revision of the entire system, including an inspection of the finished product.
An integral quality control program must contemplate a series of operations, which are listed in the paragraphs below:
- Inspection of the inputs to prevent damaged raw materials or faulty containers from reaching the processing area.
- Process control.
- Inspection of the finished product.
- Monitoring of the product during its storage and distribution. This point is generally disregarded, as a result of which all of the previous quality control efforts may turn out to be fruitless.
It is important to bear in mind that to achieve a good quality product, the following points must be considered:
Processing instructions for each product:
- Specific processing equipment.
- Processing temperatures and times.
- Packaging materials.
- Weight or volume limitations per package.
- Product labelling.
Specifications for each ingredient and finished product, including the measurement of chemical parameters:
- Soluble solids.
Sampling and analysis regulations to ensure the fulfilment of standards.
The production plant must be inspected at regular intervals to ensure the following:
- Sound processing and health standards.
- Compliance with industrial regulations.
- Compliance with safety standards.
- Implementation of environmental control measures
- Promotion of energy conservation.
In the following paragraphs, two examples of the implementation of quality systems applied to fruit and vegetable processes will be presented.
Selection and inspection: one of the most important factors in the achievement of the end product is the selection of the raw material, which in the case of fruit will have to be firm and mature, free from insect or rodent bites and free from all signs of deterioration.
Washing: this operation must be performed with abundant water, to eliminate soil or any other source of contamination. The water must be potable and contain some sort of disinfectant, such as chlorine in low concentrations.
Pasteurization: in the case of juices contained in glass bottles, this operation will have to be carried out at a temperature of 70°C for 30 minutes.
Pulp extraction: in this process, the size of holes in the sieve placed in the pulping machine will have to be controlled, as it will determine the quality of pulp that is obtained. For instance, an excessively fine sieve will retain a lot of fibre, which will decrease the yield of the finished product.
Soluble solids: the concentration of soluble solids will be determined by a refractometer, and will be no higher than 18° Brix.
Product storage and labelling: the labels must be clean and firmly adhere to the container. Labels must not be placed on other existing labels, except for the cases in which they complement the already existing information.
Labels must contain the following information:
a) Name of the product, in big letters.
b) Type, class and grade.
c) Production area.
d) Net contents.
e) Indication of the product's origin.
f) Name or corporate name and address of the manufacturer or distributor.
g) Certification of standard compliance, if pertinent.
h) Additives used.
i) Authorization by the health authorities.
Selection of the received fruit: the fruits used to make preserves must not be over ripe. Rather, they should be firm or they will not tolerate sterilization temperatures and will cause the preserves to have an unpleasant appearance. Fruit selection must be performed according to homogeneous criteria; in the case of pineapple, for example, the slices must be of the same size.
Fruit peeling: this operation must be carried out in such a way as to avoid the excessive loss of pulp, for this would have a significant influence on the yield of the finished product.
Packaging: the packaging must be performed in such a way that a minimum head-space is left to produce the vacuum and allow the product to expand at the different temperatures which it undergoes during the process. The package must have a head-space of 5 mm after the hot product is filled.
Sealing: this is one of the critical and most important stages of the process, for it determines to a great extent the quality of the finished product. After sterilization and chilling, it must be checked that the lids of the jars have a concave shape, for if they are lifted it means that the product has not been properly sealed. As a result, the product is not safe for consumption because it is exposed to contamination by microorganisms, mainly yeasts and fungi. Therefore, the product cannot be stored and must be reprocessed.
Sterilization: the sterilization of preserves will be performed by means of an autoclave at a temperature of 100°C for 15-22 minutes.
Yield of the finished product: to assess the yield of the product, one has to proceed in the following manner:
- Weigh the raw material.
- Weigh the fruit eliminated in the sorting stage.
- Weigh losses like peel, seeds and fibre generated in the peeling and cutting processes.
- Sum up all of the previous weight values.
- Obtain the weight of the fruit pieces ready to be packaged.
On the basis of these assessments, it will be possible to obtain the yield by calculating the percentage of finished product obtained and the percentage of waste in relation to the processed raw material, considering that the raw material to be processed has a value of 100%.
Tests will be carried out on the following parameters:
c) Soluble solids
To perform such tests, a laboratory will have to be equipped with the following instruments and materials:
- A 50 cc burette.
- 100 and 250 cc precipitation beakers.
- A burette support.
- A nut fixing the support.
- A potentiometer (pH meter).
- A magnetic agitator.
- 10 and 20 cc pipettes.
- A refractometer.
- A 250 cc glass flask.
- Distilled water.
- Sodium hydroxide
pH determination: this test will primarily be performed on juices and jams, but may be carried out on pickles as well.
- The pH value will be determined by means of the potentiometer (pH meter), which will have to be calibrated with buffer solutions 4 and 7 before every group of determinations.
- If a potentiometer is not available, Litmus paper may also be used to determine the pH.
Determination of the acid content:
Using a potentiometer:
The method is based on the titration of the sample with a sodium hydroxide solution, using the potentiometer to control the pH.
- Decinormal sodium hydroxide solution (NaOH; 0.1 N)
- Buffer solutions of a known pH, 4 and 7.
- Potentiometer with glass electrodes.
- Magnetic agitator.
- Calibrate the potentiometer by means of buffer solutions 4 and 7.
- Repeat the determination twice.
- Place 25 to 100 cc of the sample in a pipette, according to the expected acidity.
Introduce the potentiometer electrodes into the sample. Add 10 to 50 cc of sodium hydroxide solution from a burette, until a pH near 6 is reached.
Then slowly add the sodium hydroxide solution until a pH of 7 is reached.
Continue the titration with the sodium hydroxide solution, by adding 4 drops at a time and reading the volume of sodium hydroxide spent as well as the potentiometer, until a pH of 8.3 reached.
By interpolation, work out the exact volume of sodium hydroxide solution that corresponds to a pH of 8.1; record volume V.
Express the level of acidity as the acid content per sample mass or volume. If there is no express indication, the acidity will be expressed on the basis of the acids presented below.
citric acid for citrus fruit products or berries;
malic acid for products derived from pit-bearing fruits;
tartaric acid for grape products and others.
Work out the acidity content from the following formulas in meq/kg
A = acidity, in meq/kg.
V = volume in cc of NaOH used.
N = normally of the NaOH solution.
m = mass in g, of the sample.
- in g/1
A = acidity.
V = volume in ml of NaOH used.
N = normality of the NaOH solution.
n = number of replaceable H+ (Hydrogen ion) of the acid used to express the acidity.
M = molecular weight of the acid used to express the acidity.
v = volume of the sample, in cc.
Note: the factor (M/n) for the considered acids will be as follows:
malic acid 67
citric acid 64
tartaric acid 75
Note: As a result take the average of two measurements performed on the same sample. Round off the result to the first decimal figure.
If the difference between two determinations performed on the same sample is greater than 1%, repeat the tests.
Determination of soluble solids: the soluble solids content is determined by means of the index of refraction. This method is widely used in fruit and vegetable processing to determine the concentration of sugar in such products.
Sugar concentration is expressed in °Brix. At a temperature of 20°C, the °Brix is equivalent to the percentage of weight of the sucrose contained in an aqueous solution. If at 20°C a solution has 60° Brix, then it means that it has a 60%, sugar content.
In products like juices and jams, the presence of other solid substances influences the refraction of light. Nevertheless, the index of refraction and the °Brix are sufficient to determine the content of soluble solids in the product.
As it is extremely handy, a great use is made of the portable refractometer, as the one shown in pictures 17-20 and drawn in Figure 6, which usually has a scale in °Brix. Its most important parts are the following:
(1) Light refracting prism
(2) Measuring prism
(3) Entry of light
(9) Calibrating light
(5) Focus button
(6) Focusing and determination mechanism
Figure 6. Diagram of a typical refractometer.
To determine the °Brix of a solution with an Abbe-type refractometer, the prism temperature must be maintained at 20°C. Then the prism is opened and a drop of solution is placed inside. The prism is then closed. The light entry is opened. In the field of vision, it will be possible to see a transition from a light to a dark field. The limit between fields is set with the compensation button, as accurately as possible.
Figure 7. Measuring the Brix°.
How to proceed:
1. Place one or two drops of sample on the prism.
2. Close the prism with great care.
3. The sample should now be evenly distributed over the surface of the prism.
4. Draw the device near a source of light and look through the field of vision.
5. The line between the dark and the light fields will be observed in the field of vision. Read the corresponding number on the scale. This number expresses the percentage of sugar in the sample.
6. Now open the prism and remove the sample with a piece of paper or clean and wet cotton (use only distilled water).