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- ersT 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 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 the Science in School websitew2. weight and measured in Oechsle Image courtesy of Rennett Stowe; image source: Flickr (°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. • Pipette increased alcohol content? • Paper towels Alcohol content (vol%) • Distilled water Procedure Must (filtered) 1. Filter 20 ml must through a coffee Wine filter to remove any remaining yeast cells. Table 2: Alcohol content determined with a vinometer Image courtesy of Edward Stevens; image source: Wikimedia Commons Total acid content Fruit juices can contain several be as high as 15 g/l. It must always Alkaline different acids, including tartaric, be considered in conjunction with malic, citric and oxalic acid, in differ- the amount of remaining sugar (see ing ratios, depending on the type of ‘Determining sugar content’). fruit. The predominant acid in wine is Tartaric acid (molecular weight 150 tartaric acid, which has a pH between g) is a diprotic acid (containing two 3 and 4. However, due to the complex hydrogen atoms per molecule that can mixture of different acids and bases, dissociate in water as protons) that proteins and salts, the total acid con- can be fully neutralised with sodium tent of wine cannot be estimated from hydroxide. Because 1 mol NaOH neu- the pH value alone. Instead, it is de- tralises 0.5 mol tartaric acid (75 g/l), termined by titration to neutrality and 1 ml 0.1 M NaOH neutralises 7.5 mg Acidic expressed as total equivalent amount tartaric acid. of tartaric acid in g/l. The acid content + - + of wine is typically 4-8.5 g/l but can HOOC-CH(OH)-CH(OH)-COOH + 2NaOH → Na -OOC-CH(OH)-CH(OH)-COO Na + 2H2O

50 I Science in School I Issue 24 : Autumn 2012 www.scienceinschool.org Science education projects

Student activity 3: Determination of acidity by titration

All wines contain a certain amount of acid. The winemaker is interested Must Wine in the total acidity, caused mainly by tartaric acid. The total acidity is deter-

pH at start Chemistry mined by titration with diluted sodium hydroxide. Starting volume NaOH (ml) Materials • pH meter End volume NaOH (ml) • Magnetic stirrer • Beaker (250 ml) Volume NaOH used (ml) • Two measuring cylinders (10 ml, 100 ml) Concentration of acid (g/l) • Burette • NaOH solution (0.1 M) Table 3: Determination of the total acidity • Distilled water • 10 ml must (filtered, from activity 2) sides of the beaker or the magnetic Safety note: Wear safety goggles • 10 ml wine stirrer flea. and gloves. See also the general safety 4. Add NaOH solution dropwise note on the Science in School website Image courtesy of Experimenta until neutral pH is reached. Take a (www.scienceinschool.org/safety) and reading on the burette and enter it on page 65. in table 3. Questions • Did you observe any colour In activities 1-3, you analysed the changes? three major factors that determine the • If so, at what pH value? quality of the final product: sweet- • What could be the reason for a ness, alcohol and acid content. Now it colour change? is time to evaluate your product. 5. Calculate the amount of NaOH 1. Is the total acidity within the limits used and the acid concentration. for wine production? Example: We used 14 ml 0.1 M 2. Did the starting grape juice contain NaOH to neutralise 10 ml solution. sufficient sugar to produce the The concentration is therefore (14 x expected alcohol content? 7.5 mg/ml x 100) = 10.5 g/l acid. Figure 3: Set-up of the titration experiment 3. How long would you expect fermentation to take before the process is finished? Procedure Image courtesy of I .. C U; image source: Flickr For each sample (must or wine): 1. Fill the burette with NaOH solution. Enter the starting volume in table 3. 2. Measure 10 ml of your sample and place it in the 250 ml beaker. Add 100 ml distilled water. 3. Start the magnetic stirrer and insert the pH electrode so that the tip is in the sample, but not touching the

www.scienceinschool.org Science in School I Issue 24 : Autumn 2012 I 51 Acknowledgements · Activity 1a: to determine the sugar The author would like to thank content of grape juice using density the wine laboratory of Pfäffle GmbH measurement Experimenta in Heilbronn, Germany, for sup- · Activity 4: to quantify CO2 levels port during the development of over the course of the reaction Experimentaw1 in Heilbronn the activities. He would also like to · Activity 5: to use light transmittance is the largest informal learn- thank in particular Christine Dietrich to investigate the difference that fin- ing and interactive science and Karsten Wiese from the teacher- ing makes to the cloudiness of the centre in southern Germa- training college in Heilbronn for their finished product ny. In addition to the inter- collaboration. active exhibitions and sci- · Activity 6: to examine fermenting ence garden, Experimenta Web references yeast under the microscope. offers more than 30 labora- w1 – Visit the Experimenta website: Resources tory-based programmes for www.experimenta-heilbronn.de school classes and individu- A basic guideline for common experi- w2 – The following materials can al pupils, from kindergarten ments in wine analysis: be downloaded from the Sci- level up to upper secondary ence in School website (www. Schmitt A (1975) Aktuelle Wein­ school. These programmes scienceinschool.org/2012/issue24/ analytik, Ein Leitfaden für die Praxis. address technology and all wine#resources): Germany: Heller Chemie. ISBN: life sciences as well as pro-

· Instructions for preparing the 978-3-9800498-3-2 BACKGROUND viding teacher training. materials – ideally at least a week in For a comprehensive overview of advance topics that are relevant to the hobby winemaker, see the Fruchtweinkel- Image courtesy of def110; image source: Flickr ler website (www.fruchtweinkeller. de; in German) and the Fruchtwein website (http://met-und-fruchtwein. de, also in German) If you found this activity useful, why not browse the other teaching activi- ties in Science in School? See: www.scienceinschool.org/teaching

Thomas Wendt received his PhD on structural biology from the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany, in 1998. Dur- ing his postdoctoral research in the USA and back in Germany, he then focused on protein biochemical and molecular biology methods. After su- pervising numerous students, Thomas decided to concentrate on encourag- ing young people to consider a sci- entific career. Since 2009, he has been the educational head of the teaching laboratories at Experimenta.

To learn how to use this code, see page 65.

52 I Science in School I Issue 24 : Autumn 2012 www.scienceinschool.org