12 minutes reading time (2441 words)

    The 1999 Nyholm Youth Lecture: Closet Chemistry - Is Your Home a Health Hazard?

    Assoc Prof Margaret M Harding
    University of Sydney

    Introduction

    Many people, both with and without any scientific background, suffer from chemophobia. The word "chemical" invokes images of pollution, smells, toxic compounds and hazardous substances. Hence, if you asked someone on the street the following questions, most people would probably be horrified and answer "no" or "none" to all four questions.

    • How many chemicals have you touched today?
    • Have you drunk any acids?
    • Have you eaten any polymers?
    • How many chemical reactions have you carried out today?

    The aim of this Nyholm Youth Lecture was to explain the chemistry behind everyday items that people routinely use, touch and eat, but do not think of as chemicals.

    Copies of the overheads, full descriptions of the demonstrations and additional resource material is provided at the web address given at the bottom of the page.

    Olive Oil, Butter and Magic: All about fat
    Everyone is familiar with olive oil, canola oil, safflower oil, margarine, butter and ice magic chocolate ice cream topping. In the media, low-fat products and products high and low in poly-unsaturated, mono-unsaturated and saturated fats are widely advertised and current advice is that reduction of our consumption of saturated fats has health benefits. These food products are all made of the same type of organic compound, called a fat, which is formed from a tri-alcohol (glycerol) and a carboxylic acid which contains >12 carbon atoms (a fatty acid) (Figure 1a). However, we all know that there is "good" fat and "bad" fat which refers to the level of unsaturation or double bonds in the carbon chains of the fat (Figure 1b). Well-labelled products list the relative amounts of each type of fat present with some examples shown in Figure 1c. Butter is high in saturated fat, olive oil and avocados are high in mono-unsaturated fat, while grapeseed oil is high in poly-unsaturated fat.

    The level of saturation has important health implications, but also affects the physical properties of fats and oils. For example, butter is hard (or solid) at room temperature but can be melted with heat. In contrast, margarine (containing some unsaturated fat) is soft at room temperature but hardens in the freezer. A final example is ice magic which the label says contains vegetable oils. This product is liquid at room temperature but contains a fat with the required carbon chains and level of unsaturation to cause it to solidify at about 1-3 degrees, when it comes in contact with cold ice cream.

    The standard test for unsaturation used by organic chemists is decolourisation of bromine. This was illustrated with 2 demonstrations. In the first demonstration, canola oil was mixed with saturated bromine water. While the oil and water do not mix, on vigorous shaking, the bromine is decolourised. A more dramatic example of bromine addition is the "tomato juice rainbow" [1]. The characteristic red colour of tomato juice is caused by an unsaturated hydrocarbon called lycopene. 

    Lycopene has a molecular formula of C40H56 and contains 13 double bonds. Addition of increasing amounts of bromine gives rise to a spectacular rainbow spectrum of colours as the double bonds react with increasing amounts of bromine. Eventually when all the double bonds have reacted with bromine, or the molecule has been saturated with bromine, the solution becomes almost colourless.

    Fibre, Chips and Chewing Gum: Carbohydrate chemistry

    As well as fat in our diet, carbohydrates are extremely important and many foods are labelled as high in complex carbohydrates, low in fat etc. The name carbohydrate is derived from the fact that the simplest carbohydrates have a molecular formula of Cx(H2O)y or hydrated carbon. The simplest carbohydrates like glucose have a sweet taste and hence they are often simply referred to as sugars or saccharides. Biochemists and food scientists classify carbohydrates as reducing or non-reducing. A reducing sugar is one that is capable of reducing another species and in this process is itself oxidised. A simple demonstration that can be used to demonstrate this redox reaction is the giant silver mirror test [2] (Figure 2). A solution of glucose, when treated with Tollen's reagent, a silver diammine reagent, results in oxidation of glucose and reduction of Ag(I) to metallic silver. When this reaction is carried out in a freshly cleaned 1 litre round bottom flask, the flask is coated with a silver lining to give a beautiful mirror. 

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     While sugar is essential for our diets, there is a common perception by the public, which has been fuelled by advertising, that sugar is bad for you as it causes weight gain and tooth decay. As a result there are numerous artificial sweeteners on the market, the most widely used one being aspartame which is marketed as "NutraSweet".

    Firstly, what makes a compound sweet? Our tongue contains many tiny receptors and when certain compounds interact with, or bind to, these receptors in a non-covalent manner, this triggers a message to the brain that we associate with sweetness. By rubbing a lolly over the surface of your tongue it is easy to identify the tip of your tongue as containing the sweet receptors. There are also bitter, sour and salty zones on the tongue.

    NutraSweet or aspartame is in fact a dipeptide that has no structural similarity to sugars like sucrose and glucose (Figure 3). It was discovered by accident by a laboratory worker who was working with the compound for a different purpose and licked his fingers and noticed a very sweet taste! He was, of course, incredibly lucky that the compound was not toxic. NutraSweet may be purchased as a sugar substitute (marketed as "Equal") in a rather large jar that appears not very different in size to a 1kg packet of sugar. If you ask people to estimate how much NutraSweet is in the jar compared to the packet of sugar, estimates in general vary from 200-500g. There is in fact only 75 g in the jar! This is because NutraSweet is about 1500 times sweeter than sucrose and hence it is packaged for the consumer to resemble sugar in the sense that a teaspoon of the Equal can be used in place of a teaspoon of sugar. A disadvantage of NutraSweet is that it is unsuitable for cooking. This is because one of the key groups present in NutraSweet that gives its sweet flavour is the ester group. Esters are not very stable and in water with heating they are broken down to derivatives of aspartame that are not sweet.

    Carbohydrates are also a major component of chewing gums. Extra brand chewing gum is marketed as sugar free and is labelled as carbohydrate modified gum. Extra contains 43% sorbitol, a non-reducing sugar, that lacks the aldehyde functional group and hence would not give a positive silver mirror test. This sugar is not broken down as readily in the mouth as normal sugars and hence does not promote tooth decay. Reading the label advises consumers that "excess consumption may have a laxative effect". However, the package fails to define what excess means and individuals may interpret this as anywhere between 2-5 packs a day!

    Fibre is also very important in our diet and is also a carbohydrate. Fibre is a general name for polymers formed from many sugars connected together.

     OLESTRA: A fat replacer with all the taste of regular fat and no calories.

    One product which has a relatively high fat content that most people enjoy is potato chips. Analysis of a 50g packet of chips indicates that they contain about 15g fat and 25g carbohydrate. Olestra has recently been approved for use by the Food and Drug Administration Authority (FDA) for use in the USA as a fat replacer with all the taste but without the calories of regular chips. How does this work? Normal fats (see Figure 1) contain three long hydrocarbon chains which are responsible for the fatty flavour. Olestra (Figure 4) contains eight long hydrocarbon chains and in terms of flavour is indistinguishable from regular fat but there are side-effects associated with consumption. In the initial food trials with Olestra, bloating, wind and anal leakage were reported! In chemical terms, the olestra had a melting point that was too low so that at body temperature the product was more liquid that desirable! As discussed earlier, the degree of saturation and length of the carbon chains may be adjusted to give a product with a different melting point. The wind and diarrhoea remain side effects if excess olestra is consumed and if one considers what happens when you digest olestra then this is understandable. In contrast to normal fat, which is broken down by enzymes to generate essential fatty acids, our enzymes do not recognise olestra and hence it passes through the digestive tract largely unprocessed. In this process it can also deplete the body of certain vitamins and so food products containing olestra have vitamin supplements added to them. Lays brand chips produced with olestra are currently marketed in the USA and no doubt will be available in Australia in the near future.

    NAPPIES, ANTACID and NATURAL FIBRE : Polymer chemistry

    As discussed earlier, fibre is a naturally occurring carbo-hydrate polymer. Metamucil is a source of dietary fibre that contains psyllium-husk. Psyllium is composed mostly of hemicellulose and is not broken down in the small intestine but acts as a sponge in the intestinal tract, swelling as it absorbs water and waste material in the bowel. This remarkable natural polymer has the ability to swell to about 20 times its own volume and can be readily demonstrated by continually adding water to Metamucil and observing the result. For obvious reasons, people who are required to supplement their diets with additional fibre like Metamucil, are advised to drink a large amount of water at the same time.

    Highly absorbent materials are also required in products like disposable nappies. These are advertised as keeping baby dry at all times with no leakage, but what do they contain? In addition to cotton wool padding they contain about two tablespoons of a synthetic polymer, sodium polyacrylate. This polymer is capable of absorbing up to 300 times its own weight of tap water and about 100 times the volume of urine. This can be demonstrated by cutting open a disposable nappy and shaking the cotton filling in a plastic bag. The polymer beads collect in the bottom of the bag. Addition of water results in the formation of a soft gel-like substance that is dry to touch.

    Naturally occurring polymers are also used as food additives. Sodium alginate is used as a thickener in products like custards, ice-cream and sauces. It is a polysaccharide from seaweed with a molecular weight of around 240,000 g/mole. The polymer contains long chains with carboxylic acid sidechains present as their sodium salts. When a solution of sodium alginate is poured onto calcium chloride, the divalent calcium ions effectively crosslink the polymer chains and translucent worms are formed (3). This can also be shown with the antacid Gaviscon, which is a pink aniseed flavoured liquid containing the "alginate raft" to prevent gastric reflux and heartburn. If Gaviscon is poured onto a solution of calcium chloride, the sodium alginate present in the antacid results in formation of pink worms.

    COCA-COLA, CLEANERS AND JUICES: Acid-base chemistry

    Phosphoric acid is an important source of phosphate in the body. It is used as an industrial strength cleaner and is also an ingredient in Coca-Cola! Coke and many soft drinks are acidic with pH ~ 3.0. However, even if we drink fluids with a pH around 7.0 the stomach gastric juices are very acidic around pH 2.0. Other common acids and bases in the home include citric and tartaric acids (food spices), sodium bicarbonate (baking soda), sodium hydroxide (Drano) and ammonia (cloudy ammonia cleaner).

    If you were given an unknown compound and asked to determine whether it was an acid or base, or if you carry out an acid-base titration, the visual way to monitor the chemistry is to use a coloured indicator. One of the most commonly used indicators in the chemistry laboratory, phenolphthalein (Figure 5), is also present in laxettes. These are chocolate flavoured squares typically used to treat constipation in children. In the absence of the wrapping paper, laxettes look and smell like normal chocolate. How would you prove whether you had chocolate or a laxette (other than eating them and waiting to see if there were after-effects from the laxette). If you grate some of the "chocolate" and suspend it in ethanol to extract/dissolve the organic compound, phenolphthalein, then a colourless/milky solution is obtained in which most of the chocolate remains undissolved. Addition of some water to the supernatant followed by a few drops of cloudy ammonia gives a brilliant pink colour as the coloured form of the indicator is produced in a slightly basic solution. 

     Nature uses its own acid-base indicators that are called flavenols and anthocyanins. These compounds are responsible for the red colours in cabbage, roses and hibiscus. The pigments in red cabbage can be extracted into water by boiling the cabbage in water for 20-30 minutes. The pH of the coloured solution can be changed to give a beautiful range of colours due to the combination of the acid base chemistry that occurs with the two pigments. In the demonstration in the lecture, I added citric acid (red), nothing (purple), sodium bicarbonate (blue), cloudy ammonia (green), drain cleaner- solid€ NaOH (yellow).

    SUMMARY
    At the end of my lecture, I revisited my initial questions to assess whether students had modified their answers as a result of the lecture.
    • How many chemicals have you touched today?
    • Have you drunk any acids?
    • Have you eaten any polymers?
    • How many chemical reactions have you carried out today?

    Of course, everything we touch is a chemical, most drinks consumed are acidic, carbohydrates and food thickeners are polymers and tasting and eating food, as well as many other everyday process are indeed chemical reactions.

    REFERENCES
    1. Adapted from Journal of Chemical Education, 1986, 63, 1092.
    2. Adapted from Classic Chemistry Demonstrations, produced by The Royal Society of Chemistry UK.
    3. Journal of Chemical Education 1998, 75, 1430.

    ABOUT THE AUTHOR
    Margaret Harding is Associate Professor in the School of Chemistry at the University of Sydney. She completed her PhD in Organic Chemistry in 1987 and then spent 2 years in Strasbourg and 18 months at the University of Cambridge undertaking postdoctoral research. She took up an academic position at the University of Sydney in 1990 where she teaches organic chemistry. Her research interests are in the areas of medicinal chemistry and antitumour drug design as well as supramolecular chemistry and antifreeze proteins.
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