Image courtesy of Digimist; image source: Flickr Science education projects Analysing wine at school European countries produce more Chemistry than half of the world’s wine – and drink a lot of it too! These hands- on activities for schools reveal the science behind the perfect wine. By Thomas Wendt C6H12O6 + 2 ADP + 2 Pi paratus. In activity 2, students can = 2 C2H5OH + 2 CO2 + 2 ATP use the equipment used by hobby winemakers – a vinometer – to he age at which it is legal to The three main factors that deter- measure the alcohol content of the drink alcohol varies from coun- mine the quality of the final product must and wine. try to country, but most teach- are sweetness, alcohol content and 3. A well balanced wine needs a cer- Ters would agree that drinking wine acid content. Using standard meth- tain amount of fruit acid; the total during chemistry lessons is inappro- ods of a commercial wine laboratory, acid content is a very important priate (and potentially dangerous!). these three activities for the school measure, because it directly affects However, producing and analysing laboratory explore how the quality of the flavour. In activity 3, the acid wine at school can be fun and edu- the starting grape juice and the must content is determined by pH titra- cational. These activities, developed (fermenting grape juice) affect the tion. at the science centre Experimentaw1, final product. Each activity takes ap- Four further activitiesw2 can be invite students aged 15-18 to become proximately 20-30 minutes. Image courtesy of def110; image source: Flickr Image courtesy of def110; image source: downloaded from the Science in School vintners for a day, using analytical 1. In activity 1, students can deter- website: techniques to explore the changes that mine the sugar content of grape · Activity 1a: in an alternative to take place during the wine-making juice using refractometry. Activity activity 1, students determine the process. 1a (see below) offers an alternative, sugar content of grape juice using Wine is produced by fermenting based on density measurement. density measurement instead of grape juice (which has particularly 2. The exact determination of the alco- refractometry. high levels of sugar) using specialised hol content in commercial wines is · Measuring levels of carbon dioxide, yeast cells. The sugar is converted into performed by distilling the ethanol, one of the products of the fermenta- ethanol and carbon dioxide under then measuring the viscosity of the tion, is a useful way to monitor the anaerobic conditions: distillate using sophisticated ap- progress of the reaction. In activity www.scienceinschool.org Science in School I Issue 24 : Autumn 2012 I 47 Image courtesy of RobW_; image source: Flickr Image courtesy of tomek.pl; image source: Flickr Image courtesy of JB London; image source: Flickr Grapes fermenting to make wine Barrels of wine being stored in a cellar to mature 4, students quantify CO2 levels over Determining sugar content The must weight is calculated by: the course of the reaction by liter- The sweetness of the wine is must weight = (density – 1) X 1000 ally shaking the gas out of solution. determined by the amount of sugar · In activity 5, students use light remaining after fermentation, together Where must weight is measured in °Oe transmittance to investigate the dif- with the total acidity of the wine. A and density in g/l. ference that fining (the addition of dry wine has up to 9 g/l sugar and an As a rough estimate, 1°Oe corresponds substances that clarify the wine by acidity level that is at least 2 g/l lower to 2.37 g/l sugar (i.e. about 0.237 °Bx). precipitation) makes to the cloudi- than the sugar content. A medium-dry Therefore, the sugar concentration can ness of the finished product. wine has a sugar content of 9-18 g/l be estimated as: · In activity 6, students examine and an acidity level that should be no fermenting yeast under the micro- more than 10 g/l lower than the sugar sugar concentration = scope. content. A sweet wine has 18-45 g/l of must weight X 2.37 To supply the must used in these sugar. To ensure the correct balance of Where sugar concentration is experiments, you will need to set up sugar, acidity and alcohol in the final measured in g/l. a simple grape juice fermentation at wine, it is important to determine the least one day in advance, using red starting sugar concentration; if neces- The fermentation of all fermentable grape juice (e.g. from the supermar- sary, limited amounts of sugar can be sugar in a solution of 100 °Oe (sugar ket). You will also need some basic added before fermentation. concentration 237 g/l or 23.7 °Bx) yields chemistry laboratory equipment, plus The increased density of the must approximately 100 g/l ethanol (or 10 a vinometer for measuring alcohol (compared to water) is mainly due to wt% alcohol). Because ethanol has a content, a pycnometer (also known as fermentable sugar. Density measure- density of 0.79 g/ml, this converts to a specific gravity bottle) and a refrac- ments or refractometry can be used 12.67 vol% ethanol. Thus: tometer. A description of how to set to measure the sugar content, which alcohol concentration (in % volume) = up the fermentation can be found on in Germany is expressed as the must alcohol concentration (in g/l) X 0.1267 Image courtesy of Rennett Stowe; image source: Flickr image source: Image courtesy of Rennett Stowe; the Science in School websitew2. weight and measured in Oechsle Flickr image source: Image courtesy of Rennett Stowe; (°Oe). In the English-speaking world, the sugar content is expressed in Brix (°Bx), which represents the concen- tration of dissolved sugar, in weight percent (wt%). 48 I Science in School I Issue 24 : Autumn 2012 www.scienceinschool.org Science education projects Student activity 1: determining sugar content using a refractometer The amount of sugar in the grape Materials 5. Calculate the missing numbers in juice will determine both the alcohol • Refractometer table 1 using the equations above. content and the sweetness of the • 20 wt% sucrose solution finished wine. In this activity, you will • Grape juice Questions Chemistry use the refractive index to estimate • Paper towels 1. How accurate was your result for sugar content. • Pipette the sucrose solution compared to Refraction is the change in direction the expected value? of light when it passes from one Procedure 2. How reproducible were your medium to another (e.g. from air 1. Pipette 2 drops of the sucrose measurements? Compare them into water). The light-scattering solution onto the glass surface of to that of other groups. behaviour of a solution changes as the the refractometer and close the lid. 3. If you carried out activity 1a concentration of solutes (dissolved 2. Take a reading through the as well, how comparable were substances) increases. A refractometer eyepiece and enter the data in your results for the two methods uses this principle to determine the table 1. (density versus refractometry)? concentration of dissolved particles 3. Using a paper towel soaked in 4. A typical wine has about 12 vol% in a solution. In wine, these are distilled water, clean the glass alcohol. Estimate how much sugar principally sucrose. surface, and then dry it. needs to be added to the grape Most handheld refractometers give juice to obtain 12 vol% alcohol. the concentration of the dissolved 4. Repeat the measurement with substance either in Brix (°Bx), a scale grape juice. defined in terms of sucrose content, or in Oechsle (°Oe). A solution of 20 wt% 20 wt% sucrose Grape juice sucrose in water is 20 °Bx. Oechsle can be converted approximately into Brix by multiplying by 0.237. Must weight (°Oe) Sugar concentration (°Bx) Possible alcohol yield (vol%) Table 1: Calculation of sugar content of samples Image courtesy of Maksud_kr / iStockphoto Image courtesy of Experimenta Figure 1: Refractometer and scale www.scienceinschool.org Science in School I Issue 24 : Autumn 2012 I 49 Student activity 2: determining alcohol content The amount of alcohol obtained by 2. Place a small amount of filtrate in Image courtesy of Experimenta fermentation depends on the sugar the funnel of the vinometer (figure content of the grape juice and the 2B) and wait until the capillary is alcohol tolerance of the yeast strain: full. Keep the remaining filtrate for most yeast strains tolerate up to 16 % activity 3. alcohol. The amount of alcohol can 3. Carefully invert the vinometer be measured quite accurately using a onto a layer of paper towels, then vinometer, a simple device developed observe the level of liquid while it for hobby winemakers. It is based on slowly drops (figure 2C). Once it the principle that the surface tension stays constant, take a reading and Figure 2: Using the vinometer falls as the alcohol content increases. enter it in table 2. In this activity, you will measure the 4. Rinse the vinometer with alcohol content of your must. distilled water, then repeat the Questions measurement using the wine. Materials 1. You determined the sugar Note: The alcohol content of concentration of the grape juice in • Coffee filter the must is probably much lower activity 1. Based on the available • Funnel than that of the wine. This may be sugar in the grape juice, did you • Beakers because the fermentation process is expect a higher alcohol content in • Must not finished. It can also indicate that the wine? • Wine remaining sugar has increased the 2. If the fermentation continued • Vinometer surface tension and is affecting the for longer, would you expect an reading.