By BIRGER NORDMARK, technical writer, Wine Technology in New Zealand
Brix readings and the handy hand-held instruments that provide them are very useful aids when the time comes to make a decision about starting the grape harvest.
In the following, consideration will be given to both their great potential and their limitations in guiding the harvester to a wise decision.
What do Brix measurements actually mean? When looking for an answer, varying suggestions are offered by common sources of information. Let us look at a few.
American Heritage Dictionary: Brix scale - a hydrometer scale for measuring the sugar content of a solution at a given temperature.
Business Dictionary: Relative density scale that indicates the percent of sucrose by weight (grams per 100 milliliter of water) in a solution or unfermented grape juice.
Epicurean Wine Dictionary: Each degree Brix is equivalent to 1g sugar per 100g juice.
Kens WineGuide: The Brix (sugar content) is determined by a hydrometer, which indicates the liquid’s specific gavity. Each degree Brix is equivalent to 1 gram of sugar per 100 grams grape juice.
Merriam-Webster:
Brix scale, hydrometer scale for sugar solutions so graduated that its readings at a specified temperature represents percentages by weight of sugar in the solution – called also Brix.
Meyers Lexikon: Brix, mass unit for the must hydrometer.
Random House Dictionary: Brix scale, a graduated scale, used on a hydrometer that indicates the weight of sugar per volume of solution at a given temperature.
Shreve’s Chemical Process Industries: The degree Brix is the percentage, by weight, of sucrose in a sugar solution; commercially it is taken as the approximate percentage of solid matter in a liquid.
Webster’s Online: A hydrometer scale used to measure the approximative sugar content of grape juice, sweet wines, and sugar solutions. The scale is calibrated to indicate percentage by weight of sucrose.
Wikipedia: Degrees Brix (°B) is a measurement of the dissolved sugar-to-water mass ratio of a liquid. It is measured with a saccharimeter that measures specific gravity of a liquid
It is interesting to note the range of attributes different presenters have offered.
The most common is ‘hydrometer’ (including saccharimeter). It occurs in six statements, but only three writers have described the test as a “relative density” or “specific gravity” test, which it is.
Five answers report “per cent” of sugar as the purpose for the test. One of them informs that today the search for Brix “per cent” is not restricted to sugar but may apply to any soluble solids. Two descriptions include the word “approximate” in their characteristics, leaving one to assume that in other cases the result is considered solid fact.
The varying characteristics found in this survey have their roots in history. That is very clear in the statement in the Business Dictionary, which contrary to all others says that the test concerns “grams per 100 millilitres of water”. That was also the rule I learnt and practised when working with sugars many years ago. The scale has at some stage been ‘rationalised’ to show the more practical percentage, but no document related to that event has been found. The change is only one of the developments in industrial practice that have affected the Brix concept in the past.
The Brix scale got its name from the man who developed it. Adolf F.W. Brix (1798-1870) was born in Austria but worked most of his time in Germany.
He was an ingenious scientist with deep interest in botany and chemistry. Much of his work was devoted to study properties of plant juices and in particular the juice extracted from sugar beet, the source of sugar in Europe.
Several different methods existed at the time to measure the sugar content in sugar solutions. The most common was the Balling scale using hydrometer at 17.5 degrees Celsius. Adolf Brix considered that scale inaccurate and set about to improve it, first by recalculating it for the measuring temperature 15.5 and, later, for 20 degrees Celsius. He prepared standard sugar solutions by adding accurately weighed amounts of refined sucrose to 100mL water and measured the resulting relative density. It was, thus, not a percentage scale.
Relative density
This test method measures the density of liquids relative the density of water at the same temperature. It is measured by hydrometer, an instrument with a long history in the brewing and distilling crafts. It consists of an oblong cylindrical bulb of glass or plastic with a weight at one end to keep it upright when placed in a liquid. The other end carries a slender stem enclosing a graduated scale.
When placed in a cylinder filled to the brim with a liquid, it will sink, causing some liquid to spill over the edge of the cylinder, until it comes to rest. At that level it is held afloat by the buoyant force, which is equal to the weight of the displaced liquid. If the liquid is a light one like water, the bulb will sink quite deep to a point close to the end of the stem; if the liquid under test is a heavy sugar solution the bulb will sink much less, because the buoyant force is greater, and the displaced volume of liquid smaller. The depth of submergence is read on the scale at the point where the spindle breaks through the liquid surface.
The common, rationalised, version of the Brix scale is marked in degrees corresponding to the percentage by mass of sucrose in the tested solution. For example, 20°B implies a solution of 20 parts by weight of sugar dissolved in 80 parts by weight of water to make 100 parts of solution.
The degrees Brix correspond to per cent sucrose only in solutions of pure sucrose; for any other solutions the result will be only approximate, because other dissolved substances contribute to density as well. Further, in wine-making the interest is not centred on sucrose but on fermentable sugars that are usually monosaccharides.
Because the density of liquids varies with temperature, the test must be carried out at the specified temperature, 20 degrees Celsius, or reference made to correction scales.
Newer Brix methods
The hydrometer has been a very useful tool in every winery for a long time, but at present most large and middle-sized wineries would have followed the sugar industry to adopt one or another of the test methods - refractive index by refractometer, density by digital density meter, or infrared spectroscopy measuring energy absorption by molecular bonds.
The international sugar industry took a momentous step in 1974. The ICUMSA (International Commission for Uniform Methods of Sugar Analysis) with more than 30 member countries then decided to introduce a new definition of °Brix. It was no longer to be tied to the slow hydrometer but to refractive index. The move was a direct result of technological progress. In the sugar industry, the Brix concept appears to have lost its foothold. A European sugar manufacturer, Danisco Sugar, has this to say in the 2008 edition of its 144-page Sweetener Lexicon: “Brix is an old measure of the concentration of dried matter dissolved in a liquid and is normally used for sugar solutions. These days the concentration in percent of soluble dry matter in solutions is almost exclusively determined by means of a refractometer.” The °Brix is now identified according to the following tables.
Refractive index (1) Density (2)
°Brix nD20 °Brix d20
0 0 1.00000
5 1.34026 5 1.00965
10 1.34782 10 1.03998
15 1.35568 15 1.06104
20 1.36384 20 1.08287
25 1.37233 25 1.10551
30 1.38115 30 1.11898
35 1.39032 35 1.15331
40 1.39986 40 1.17853
45 1.40987 45 1.20467
50 1.42009 50 1.23174
55 1.43080 55 1.25976
60 1.44193 60 1.28873
65 1.45348 65 1.31866
70 1.46546 70 1.34956
75 1.47787 75 1.38141
80 1.49071 80 1.41421
85 1.50398 85 1.44794
90 - 90 1.48259
95 - 95 1.51814
(1) According to 16th Session (2) According to 109 of NBS
of ICUMSA 1974 Circular 440
Credit for Chart: Mettler-Toledo GmbH, Analytical
Refractive index
Light travels at different speeds in different media. The denser the medium is, the slower the speed of the light in that medium. When light passes from one medium to a different one at any angle other than 90°, it changes speed, and as a result of that it also changes direction at the boundary between the two media.
The refractive index (symbol n or nD) of a medium is defined as the speed of light in air divided by the speed of light in the medium. The refractive index is a specific characteristic of each medium.
The refractive index of fluid materials is read in refractometers, and there are several different types available. The basic one, the traditional hand-held refractometer is the one that the vineyard manager needs for assessing the grape readiness for harvest. It is an analogue instrument in the form of a short tube fitted to a plain-sided section with a hinged lid.
Under the lid is a prism, onto which a drop or two of sample is placed. The instrument works on the critical angle principle by which lenses and prisms project a shadow line onto a small glass reticle inside. This is then viewed by the user through a magnifying eyepiece.
Hand-held refractometers come also in digital form. They have a source of light that is directed to the underside of a sapphire prism on which the sample is placed. Some of the light passes through the sample and is lost, while the rest is reflected onto a linear array of photodiodes that make up a shadowy line. The refractive index is directly related to that shadowy line. The more elements there are in the photodiode array, the more precise the readings will be.
Digital refractometers for bench-top use are advanced instruments for winery laboratories, where they would be of great assistance for blending wines to precise requirements. They normally have automatic temperature correction and offer a range of scales for read out.
Special refractometers are available for fitting into pipelines or tanks to serve automatic winery process systems.
From the reading of a number of different manufacturers’ product specifications one was found to state that its Brix scale was based on the ICUMSA refractive index, two manufacturers hold on to the original Brix scale based on 100 mL water. Others have not made any statement, so their Brix values could be either of these two or, more likely, the common, rationalised (Brix = per cent) scale.
This section needs, therefore, to finish with a reminder of the necessity to ensure that a purchased instrument is supplied with a clear statement of which scale it adheres to.
Digital density
“Digital density meter replaces hydrometer” was announced in April, 2005, by the German company that in 1967 developed the first instrument to employ the oscillating U-tube principle discovered by Dr Hans Stabinger and Professor Hans Leopold.
Essentially it consists of a glass tube into which the fluid sample – gas or liquid – is either held or continuously passing through. The tube is shaped into a narrow ‘U’. When set in vibration at right angle to the flat level of the U, it vibrates at its own inherent frequency, “eigenfrequency”. When a sample is placed between two measuring points along the tube, or flows between them, the frequency will change to a degree that is proportional to the mass of the sample. Higher mass gives lower frequency.
The new frequency is electromagnetically transformed into an alternating voltage of the same frequency.
As the volume between the two measuring points is constant and known, the density of the sample will be displayed.
The density of samples can be determined to very high accuracy by digital density meters. The relationship between internationally adopted Brix values and the density of wine and grape juice are given in the table (see above). Instruments for winery use are available.
Infrared spectroscopy
This technique of analysis could hardly have been developed without the parallel development of the computer.
The Australian Wine Research Institute gave in August, 2004, a non-commercial description of the method and the equipment, which inferred that the procedures offer to simplify testing to save time and enable several sample attributes to be tested simultaneously.
They are based on the principle that most materials have characteristic electromagnetic energy-absorbing bonds within their molecules. In the infrared region, the absorbing patterns depend on the molecular structure of the material, and they serve as “fingerprints” of the chemical constituents of the sample.
Before use, the instrument is calibrated with samples of known composition which are entered into the computer. When the analyser examines a sample it sends its findings to the computer that then builds up a body of statistics concerning the relationship between energy absorbing bonds and numerical data. When the instrument is presented with an unknown sample it will search the statistics to find out what kind of substance and how much would produce the same energy absorbing pattern, and thereby identify it.
The result of an infrared test is not a result of an analysis. It is merely a prediction of the result a chemical analysis would find. However, the agreement between predictions and analytical results is historically so high that for daily use the predictions are accepted.
A new generation of dedicated infrared wine analysers has reached the industry in the last few years. They have removed all confusion, because sugars and acids are measured separately as are several other parameters, all from a drop size sample with a report returned within two minutes.
What’s worth remembering?
1. There are at least four Brix scales in use. Watch the calibration of your instrument.
2. None of them can measure the sugar content exactly, all results are approximate.
3. The vineyard manager and wine-maker are advised to check the level of fruit acids in the fruit appearing ready for harvest. An undue proportion of the Brix could be due to acids.
4. The wine industry is spanning a vast era of testing technology.
5. With all these reservations, Brix is a tremendous concept!
(Copyright: Wine Technology in New Zealand magazine)