Sugars are a natural source of energy that the body can easily digest and absorb. The sugars found in fruit are typically Fructose, Glucose and Sucrose. Other sugars may also present in some fruits, but generally in much smaller quantities. Sugar content of fruit products is generally measured using the Brix scale.
Fructose is a simple monosaccharide found widely in nature, and is one of the most basic units of carbohydrate (hence the name - mono=single, saccharide=sugar). In isolation, fructose is usually a colourless crystalline solid that readily dissolves in water. It is commonly known as fruit sugar and indeed this is reflected in the origins of the name, Fructus being the Latin root of the word Fruit.
Glucose, also known as Dextrose or Grape Sugar, is also a simple monosaccharide molecule found widely in nature. It is produced by most plants during photosynthesis and is used in further biological processes in a large number of species of both plants and animals, as it is a fundamental basis for both energy production and more complex processes such as the creation of proteins and metabolisation of lipids (fats). Glucose is a readily useable source of energy as our bodies can absorb it directly into the bloodstream, hence its popularity as a component of many sports nutrition products. This same fast-acting energy boost may be obtained from many fruits.
Sucrose, also known as Saccharose or Table Sugar, is perhaps the most familiar of sugars due to the widespread availability and use in culinary applications. It is a crystalline solid and generally what one pictures when one thinks of "sugar". Sucrose is a disaccharide molecule that can effectively be viewed as comprising a linked Fructose molecule and Glucose molecule, into which it is readily broken down by the digestive process. Due to its widespread use, Sucrose is often used as a reference point by which to define the relative sweetness of different sugars. On such a scale sucrose is usually assigned a value of 100, compared to 74 for glucose and 173 for fructose, i.e. we perceive fructose as being over twice as sweet as glucose.
Most fruits contain fructose, glucose and sucrose in varying proportions. Whilst individual examples of any specific fruit will differ in terms of the exact sugar breakdown, one may still broadly classify the sugar composition of types of fruit based on large data sets. For example Grapes typically contain approximately 50% Fructose and 50% Glucose, with relatively small amounts sucrose present, whereas apples typically contain over 60% Fructose, around 10% Sucrose and 30% Glucose.
Degrees Brix (°Brix), named after the German chemist Adolf Brix, is a relative measure of the amount of sucrose in a solution. Generally speaking, 1 °Brix is equivalent to a water-based solution containing 1% sucrose by weight. In actual application within the food industry, the measure is really an approximation - As discussed above, fruit juices contain different types of sugars, and also further dissolved nutrients besides sugars, so in routine application the measure is less strictly related to actual sugar content by weight. It may also therefore be referred to as soluble solids since the general method of measurement (see below) is not able to differentiate between any of these dissolved components.
Other sugar components exhibit behaviour broadly similar to sucrose so the Brix scale may still be used with a reasonable degree of accuracy, provided it is fully understood that one is not solely measuring dissolved sucrose content. Acids do behave differently to sugars, and are present in most fruit products; consideration does therefore need to be given to the effects of acid on the measurement and this is discussed in more detail below. Other dissolved nutrients are present in solution but in quantities significantly lower than sugars and acids, so the effect does not need consideration at the levels of accuracy generally required in applied situations.
The Brix value is usually determined using a Refractometer. When light moves between two different substances (e.g. air and water) it is refracted - it bends relative to its original trajectory as the refractive index, and indeed the velocity of light, differs in the two materials. In this case, we are considering the interface between the air and a volume of juice which we approximate as a sugar solution. The degree to which the light bends is a function of the difference in refractive indices of these two materials, and since we can measure how much this bending occurs at different known strengths of sugar solution, we can then measure the sugar content of an unknown solution by observing the degree to which a light beam is refracted and cross-referencing with the data for known sugar solutions. This is the principle by which a Refractometer operates.
The standard can also be related to other common measures used for characterising liquids such as Specific Gravity, for which a conversion table is provided here.
It should be noted that the presence of acid in fruit also affects the Brix value as measured by Refractometer, as this changes the refractive index such that light tends to bend in the opposite direction to that caused by sugar. This therefore has the effect of lowering the measured value relative to the actual sugar content of the product. By measuring the acidity, one can correct for this effect by again comparing with known values for acid (usually considered to be Citric acid) at various concentrations. These values are available in reference tables, such as the acid correction table found here.
If we squeeze or press a fruit, we can measure the Brix value of the juice using a Refractometer to determine the typical soluble solids level present in the fruit in its natural state. By performing this analysis on a large number of samples of a given fruit, we are then able to determine a typical Brix value for the juice of any given fruit. From this, a minimum Brix may be estimated, allowing easy identification of any products that may have been diluted to increase profitability. It should be noted that there are also natural phenomena related to growing conditions that may cause unusually low Brix (for example, unusually high rainfall during fruiting).
This is also particularly useful when it comes to concentrated fruit products - if we concentrate to a specific Brix level, we are then able to relate this to an equivalent volume of unconcentrated juice based on minimum acceptable values that are obtained via measurement of large numbers of samples. Furthermore, when reconstituting this concentrate, we can set a minimum Brix value for the finished product to ensure it is not overly diluted beyond what would be typical of the natural fruit. For many fruits, these minimum values are now included within regulations such as The Fruit Juice and Fruit Nectars (Amendment) Regulations 2011, and directive 2001/112/EC. A table of values for common fruits is provided here.
Brix values can also be useful for characterising the perceived sweetness of a juice, but only when considered in combination with the acidity. For further information on this, please refer to the section on measurement of acids and ratio in fruit juices here.