The Refractometer, How It Works and Role in the Food Industry

Jun 13, 2023

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A refractometer is a device used to measure the refractive index (n) of a substance. This physical property can be used to make a variety of assessments, such as to determine the purity of a substance (by comparing its n value with a standard value) or to assist in identifying an unknown substance by comparing its n value to reference values.1 Refractometers are versatile in their design and use, and are able to provide results as refractive indices, or in other cases, as direct measurements of parameters such as soluble solids, sugar concentration or salinity.

How does a refractometer work, the index of refraction and role of Snell’s law

The refractive index of a substance is determined by the ratio of the speed of light in a vacuum to the speed of light traveling through the test substance, as described in Equation 1.

Refractive index (n) = (Speed of light in a vacuum) (Speed of light in the test substance)

Equation 1: Definition of the refractive index of a substance using light speeds.

When light travels from one medium to another, it changes not only in speed but also in direction, resulting in refraction. This phenomenon is represented by Figure 1, where a light beam travels through media A and B at the velocities vA and vB respectively.

According to Snell's law, the relation between the sines of the angle of incidence and angle of refraction is equal to the refractive index of the second medium.2 Considering a perpendicular plane to the interface, the incident and refracted beams form angles θA and θB, and the refractive indices of media A and B (nA and nB), the following relation can be established (Equation 2):

sin⁡θA = nB = VAsin⁡θB nA VB

Equation 2: Snell’s law formula.

The above equation shows that the refractive index of a substance can be found by measuring the angles of incidence and refraction rather than measuring the speed of light.

To understand the working principles of refractometers better, there is another fundamental concept to consider, the critical angle. As the angle of an incident light beam increases, so does the refraction angle (see Figure 2A). At a specific angle of incidence, the refraction angle reaches 90°, which is the maximum refraction angle possible. At this particular point, known as the critical angle, the light beam travels parallel to the interface and any additional increase in the angle of incidence will result in reflection of the light.3 If we now consider an illuminated prism in contact with a liquid (Figure 2B), the beams traveling below the critical angle are refracted and the beams traveling above the critical angle are reflected. When observed from the right position, the critical angle can be visualized as a transition between dark and light regions. By knowing the refractive index of the prism (nA) as well as the refraction angle θB at the critical angle (i.e., 90°), the refractive index of the liquid (nB) can now be determined. This relation establishes the working principle of most refractometers.4

To determine the refractive index of a substance accurately, additional considerations must be made, in particular due to the dependency of this property on the temperature and the wavelength of light used to measure it. Therefore, care must be taken to control or compensate for temperature variations and wavelength.1

The refractive index is specific to a substance, making this property an easy and efficient method to characterize materials and evaluate their purity. When the test sample is a simple mixture of two components, such as water and sugar or water and alcohol, the first choice for their characterization is a refractometer. Although more complex mixtures may produce less accurate determinations, refractometers can still give acceptable approximations of solute concentration, suitable for quality control. For this reason, refractometers are used in the laboratory, in industries and in the production fields as they are simple to use, low maintenance and provide fast results. The refractive index of a solution is usually directly proportional to its concentration by volume (m/V), which can be converted to the concentration by mass (m/m) when multiplied by a density factor. This relationship enables the mutual comparison of refractive index and specific gravity data. The specific gravity, or relative density, of a substance is the ratio of the density to that of a standard substance, usually water at 4 °C. The relative density of a liquid can be determined by using a hydrometer, which is a device made out of a sealed glass tube with a bulb attached to the bottom, acting as a ballast. These devices have a specific density and, based on a hydrometer´s buoyancy, the specific gravity of the liquid in which it is placed can be read on a scale printed on it. Alternatively, the specific gravity of a liquid can be determined using a pycnometer, which is a glass container with a specified volume and a special stopper that allows the container to be filled completely and the gas bubbles to escape. This device allows a very accurate volume to be measured, thus enabling the specific gravity of a fluid to be determined.6 Both specific gravity and refractive index can be used to determine the composition of binary mixtures, which is essential in process control and quality evaluations. Hydrometers are inexpensive and simple to use, making them ideal for homebrewers, for example. However, refractometry enables faster and more direct determinations, requires much smaller samples than specific gravity measurements and can be more portable, making it the preferred choice for most industrial and food production applications.

The most common use of refractometers is the determination of dissolved solids in a liquid, such as sugar concentration in water. Commonly, the results are expressed in Brix degrees (°Bx), °Bx 1 being equivalent to 1 gram of sucrose in 100 grams of solution, representing then the percentage by weight (% w/w) of sugar content.5 The Brix scale is frequently used to check the sugar content of grape juice for wine production, fruit juices, jelly and jam, honey, milk and many other kinds of beverages. Other comparable scales are used to express the content of solids in solution such as Oechsle, Plato and Baumé. The origin of these scales was in fact the concept of specific gravity. The Oechsle scale was established to measure the density of grape must for the winemaking industry. This scale expresses the difference of density between a test sample and water, with 1 degree Oechsle (°Oe) corresponding to a one gram difference between one liter of must and one liter of water. Baumé (B°or Bé°) actually combines two scales based on hydrometer determinations, measuring the density of fluids denser and lighter than water, respectively. Baumé degrees were established by the percentage of sodium chloride mass dissolved in water, with 0 B° corresponding to pure water and e.g., 15 B° being the density of 15% sodium chloride in water. Plato is also a unit of mass fraction developed for the brewing industry, representing the concentration of dissolved solids in a brewery wort. Degrees Plato (°P) quantifies the concentration of extract as a percentage by weight. The main difference to the Brix scale is that this considers only sugars, while Plato includes other soluble materials in wort. Therefore, a 10 °P wort will contain 10 grams of extract per 100 grams of total weight.

Other units can be given by specific refractometers, such as salinity in permille (‰) or the relative density at a specific temperature.

The Abbe refractometer was the first commercial refractometer, invented by Ernest Abbé in 1869. Although they have a long history and several modifications have been made to improve their performance, the basic layout of these refractometers is still the same (see Figure 3). In this configuration, the sample is placed between two prisms, the illuminating prism and the refracting prism, both attached on a rotary mount. The refracting prism is made of high refractive index glass or sapphire and has a smooth surface. The illuminating prism surface is roughed so light beams dissipate at all directions according to each surface point. The light then passes through the sample, is refracted at the interface with the smooth surfaces of the refracting prism and enters the instrument's telescope. The telescope is equipped with two adjustable Amici prisms, preventing light dispersion by collecting divergent critical light beams into directed white light. To perform a measurement, the prisms are rotated until the boundary between the illuminated and dark regions is aligned with the center of the field of view. Then, the values of the refractive index can be read on the scale. The modern versions of the Abbe refractometer have improved design features, such as thermo-stabilized prism mounts, fixed telescope mounts and a combined view of the shadow boundary and scale, allowing for an easier and faster reading.4 The equipment is fairly large and thus they are used as bench laboratory refractometers. They can be used to measure the refractive index of liquids and solids, providing highly accurate measurements.

Figure 3: Representation of an Abbe refractometer. Credit: The Author.

Handheld refractometers are the most common and popular version of the critical angle refractometers. They are compact, analog instruments, easily transported, providing quick and easy operation. They are reliable, require only a few drops of sample and provide almost instant results, which make them the first choice for in-field determinations.4, 7 They are commonly used by winemakers to assess the ripeness of the grapes or by brewers to control the Plato degrees of wort.

Contrary to the Abbe refractometers, handheld instruments do not have an illuminating prim. Instead, they have an illuminating flap that produces light at a grazing angle and holds the sample in place. After passing through the samples, light enters the measuring prism and other lenses, reaching the field of view equipped with a reading scale (see Figure 4). Dependent on the application, the scale can be graduated in e.g., Brix, Plato, Baumé degrees or directly in percentage of solutes (e.g., alcohol or glycol). These devices can be easily calibrated through a calibration screw (using distilled water as a reference), allowing for temperature compensations. Some versions have a built-in temperature compensation system, based on a bimetallic strip that bends according to the temperature, adjusting the scale.

Digital refractometers function by using an LED to illuminate the sample placed over the refracting prism. The refracted light is then measured at an image sensor that determines the refraction angle (Figure 5). This reading is then temperature-corrected through a series of specialized algorithms and converted into the refractive index or a specified parameter.8 In relation to the analogue devices, digital refractometers yield the advantages of easy use, automatic corrections and adjustable scales. They can be miniaturized as handheld digital refractometers, allowing portability, or specifically designed for target applications, improving their performance but limiting the range of capabilities.

Digital refractometers can be operated manually or designed to make semi-automatic or fully automatic readings. Among the different variations, the following types can be highlighted:

Specialized refractometers are no more than equipment designed for a specific task or measurement. They operate under the same principles as the previously described devices, yet they are calibrated and display units according to their intended use. The most common variations are Brix refractometers and salinity refractometers.

Refractometry is a versatile and reliable technique, used in several stages of the food production chain. In the production sector, refractometry is used to monitor crop growth, ripeness of fruits and vegetables and determine optimal harvest times.10 The most notorious examples are its application in the sugar industry, where sugar cane or beets are evaluated for ripeness and to estimate production yields, and in the winemaking industry to determine optimal harvest time of the grapes.12 In the livestock industry, refractometry can be used to evaluate milk and colostrum quality. This allows in-field and instant evaluation of the product’s quality, the adequacy of the nutritional parameters for feeding the calves and detection of possible adulteration, such as milk dilution.13, 14 Similarly, refractometry is used by beekeepers to check honey quality and screen for product adulteration.15 In such examples, handheld refractometers play an essential role due to their low cost and capability to be carried into the fields, providing instant measurements. However, the portable and simple refractometers usually only cover a limited range of refractive indices. Therefore, these models are often specific to a particular use and display a limited range of units. For example, measuring the Brix in fresh fruits and vegetables requires a refractometer with an operating range of 0° to 12° Brix and measuring the brix of honey requires a refractometer with an operating range of 60° to 90° Brix.

In transformation and food processing, refractometry is also a fundamental tool in process monetarization and quality control. During fermentation of wine and beer, producers check the sugar and alcohol content with refractometers specialized to provide readings of the specific gravity of must and alcohol scales. Portable or bench-top equipment are also used in production lines of all kinds of food products (e.g., juices, yogurt, jams, baby foods and syrups), measuring the total solids content. Specialized refractometers are also used to measure the concentration of salt in brines, sauces and canned food. These quality control checks are essential to maintain production specifications and allow operators to approve, reject or rectify production batches.

The high precision and versatility of the measurements provided by the bench-top refractometers, such as Abbe refractometers and automatic refractometers, are of extreme importance in food innovation, quality control and combatting fraud. These units are more expensive and more complex to operate, however, a single unit can be used to measure the refractive index across the entire range of possible values and help to reduce the variability between measurements. A particular example for the need of precise measurements is in the screening of fats and oils.16 For the food industry, it is fundamental to know the origin and composition of its ingredients. For example, oil refineries rely on many raw products whose origin varies and often is unknown. Therefore, it is necessary to find out the identity of the product and its purity, often achieved through the determination of the refractive index. Likewise, screening can help to identify food fraud such as in the adulteration of premium olive oils with cheap substitutes. Using refractometry, the ratio of substituted oils can be easily detected.17

Overall, refractometry is an important tool for assessing and controlling the quality and purity of raw materials and final products. It is a technique capable of providing high sample throughput, requires low sample volume, is easy to handle and operate and is easy to clean. Through the determination of the refractive index, it is possible to determine the sugar concentration in fresh fruits, vegetables, juices and beverages, and to determine the alcohol or extract content in beer, wine or spirits. It is possible to perform quality control of dairy products and honey and validate process specifications in a vast array of foods. It also enables the compliance of ingredients to be tested with standards, detecting fraud, controlling quality, ensuring the purity of production samples and assisting in the research and development of acids, bases and solvent solutions.

References

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