42 | Chemistry World | August 2008

Beer TONY MCCONNELL / SCIENCE PHOTO LIBRARY PHOTO SCIENCE / MCCONNELL TONY 42 | Chemistry World | August 2008 www.chemistryworld.org Trouble brewing Chemical reactions during storage can destroy a beer’s flavour. Henry Nicholls finds out how brewers are striving to stabilise beer’s chemistry Little beats a cool, crisp pint of beer avoid. The chemistry too is rather can pull out proteins important for on a hot summer’s day. But if left more interesting, with hydrogen In short a beer’s flavour, its foamy head or its out in the sun, ruinous chemical bonds forming between polyphenols The storage problems so-called mouthfeel, says Hughes. reactions would soon take their toll and polypeptides rich in the of beer have been known Adding enzymes to break on the taste, aroma and appearance amino acid proline. The insoluble about for centuries but down proteins is a popular way of the beer – be it lager or real ale. aggregates that result make a beer it is only in the 20th to penetrate the haze. In the past, Beer quality can deteriorate even cloudy and, although they initially century that consumers brewers have happily used cysteine under ideal storage conditions. break down when the beer warms up have started to care proteases such as papain from Preventing such reactions is crucial a little, after prolonged periods they about its quality papaya, bromelain from pineapple and brewers have honed their can form permanent complexes. The main bugbear for and ficin from figs to digest the chemistry in an effort to satisfy even It’s hardly surprising then that the brewers is the cloudy larger polypeptides. They have even the most exacting consumer. brewing industry has devised plenty haze that can form. been known to achieve this with of ways to disrupt this reaction. This can be removed by trypsin extracted from pig intestines, Blurred vision One strategy is to strip out targeting haze-forming says Hughes. For most brewers, the number one the polyphenols, without which proteins But such enzymes are not fussy bugbear is the cloudy haze that the haze-forming proteins have Light damages beer about the proteins they cleave. So, gradually forms over time and that nothing to bind to. The most and creates large as with tannic acid and silica gels, worsens with a stint in the chill common approach is to add numbers of off-putting enzymes can reduce the likelihood cabinet. ‘It’s very easy to appraise polyvinylpolypyrrolidone (PVPP), chemicals including one of haze formation at the expense of visual factors,’ says Paul Hughes, an insoluble synthetic and highly that smells like a skunk flavour, foam and mouthfeel. ‘There head of the International Centre cross-linked polymer, to mop up is good evidence to suggest that if for Brewing and Distilling at Heriot polyphenols through a combination you don’t kill off these proteases Watt University in Edinburgh, UK. of hydrogen bonding and π–π through pasteurisation, they can ‘You can hold your pint up to a friend interactions. The resulting bulky slowly work away and gradually and say “look at the state of that”.’ complexes can then be filtered out deteriorate foam stability,’ says This makes it easy to take back a easily. Hughes. Industry is keen to develop murky pint. ‘There’s no discussion,’ The alternative is to target the enzymes that selectively digest says Hughes. ‘Whereas with flavour, proteins in the haze-forming haze-forming proteins, leaving the if you’ve got an intractable barman, equation. Perhaps counter- ‘good’ proteins untouched. Dutch they might swear blind that it’s intuitively, one method is to add chemicals firm DSM thinks that it absolutely fine.’ more polyphenol to the beer in the has come up with the answer, with Haze forms through biological form of tannic acid. This precipitates an enzyme that goes under the and non-biological routes. A most of the haze-forming proteins, brand name Brewers Clarex. This is hygienic brewery can avoid infecting ready to be filtered off before added to the beer at the beginning the product with unwanted packaging. Or the beer can be passed of fermentation and cleaves microorganisms and filtration through a silica gel, with hydrogen polypeptides at the proline-rich can remove most of the bacteria bonds forming between silicon portions, thereby preventing haze and yeast that rapidly cause a beer hydroxide groups and hydrophilic formation, says Minh-Tam Nguyen, to cloud. Non-biological haze haze-forming proteins. Over-zealous from DSM Food Specialities’ beer formation is somewhat harder to use of these techniques, however, business. ‘The purity of the enzyme www.chemistryworld.org Chemistry World | August 2008 | 43 Beer selected for its low iron content, the concentration of Fe2+ in membrane- filtered beer was still slashed in half. If there’s a drawback of using membrane filters, it’s that they are much more expensive to replace than their kieselguhr equivalent, says Rik Schuurman, product manager at Norit Process Technology. So, in collaboration with Heineken, Norit has developed an enzyme-enhanced, hypochlorite- based cleaning agent that can efficiently dislodge carbohydrates and proteins fouled up in the membrane, he says. Even with all this intervention, oxidation products are inevitable. The most feared is the cardboard- like aldehyde (E)-2-nonenal. This is probably produced through several pathways, but the most important mechanism is lipid oxidation, says Freeman. The enzyme lipoxygenase, which is present in malt, catalyses the oxidation of fatty acids such as linoleic and linolenic acid, ultimately causing an increase in the concentration of (E)-2-nonenal. Browned off Despite pockets of knowledge, the flavour stability of beer is not ensures that no other hydrolysis can reduced to the peroxide anion Beer can turn hazy when yet satisfactorily understood, says 2– take place and so no other parameter (O2 ), which is readily protonated H-bonds link polyphenols Freeman. Yet our understanding of is affected,’ he says. to form hydrogen peroxide (H2O2). and polypeptides beer’s dynamic and complex system These ROS will react with all kinds improves all the time. For example, Fragile flavours of organic molecules, making a beer recent work on flavour deterioration Such methods can keep beer in taste more like soggy cardboard by Adriana Bravo and her colleagues storage for longer. But only at the than a carefully crafted alcoholic at Polar Breweries and Simón expense of flavour, with radical beverage. Bolívar University in Caracas, oxygen species (ROS) oxidising This explains one of the major Venezuela, suggests that a reaction alcohols, hop compounds and drawbacks of using diatomaceous that makes tasty flavour compounds polyphenols. ‘Beer is more oxygen- earth (DE) or ‘kieselguhr’ to filter in roasted or browned food may also sensitive than almost any other beer following brewing. This be affecting beer (see Chemistry product,’ says Gary Freeman, senior naturally occurring, soft, chalk-like World, July 2008, p4). The Maillard engineer at BRI, UK, an organisation rock has been used as a filtration aid reaction involves amino acids that provides technical information for decades. But it also leaches metal and reducing sugars and usually and research services to the brewing ions into the beer, increasing the requires high temperatures. Bravo industry. Oxidation is a particular abundance of ROS. discovered that a large number of problem for beers with a delicate Like other silica-based Maillard intermediates called α- flavour and many brewers will go to compounds, kieselguhr is extremely dicarbonyls are involved in flavour great lengths to exclude oxygen from dangerous to handle in the change at far lower temperatures. In the production process as early on powdered form and expensive to recent lab tests, they discovered that as is feasible. It’s certainly crucial dispose of safely. Consequently, says blocking α-dicarbonyl degradation that, by the time it is packaged, the Freeman, breweries are gradually reduced the number of ‘sensory- concentration of dissolved oxygen installing high-tech membrane active’ aldehydes that form in beer in the final product is less than 0.2 filtration systems to replace the during storage. Bravo now hopes to parts per million, says Freeman. dated DE approach. Most of these manipulate the Maillard reaction to ‘Otherwise you’re in trouble.’ membrane filters are made from prolong beer’s shelf-life. The presence of transition metal polyethersulfone. Norit Group, a ions, such as iron and copper, does Dutch firm, makes such a membrane Bad light not help. Oxygen will readily grab filter system. In a recent controlled ‘Beer is more Beer’s light sensitivity is better an electron from ferrous iron (Fe2+), experiment, the company compared understood. The main culprits are for example, to give the superoxide the oxidative potential and taste oxygen- isomerised α-acids, hop-derived – anion (O2 ). This can in turn be stability of batches of beer passed sensitive than molecules that give beer its bitter protonated to form the perhydroxyl through its membrane filtration flavour. When in an excited state, radical (OOH•). Alternatively, the system and a conventional DE filter. almost any photosensitive molecules such as superoxide anion may be further It found that even using kieselguhr other product’ flavins can cleave side-chains from 44 | Chemistry World | August 2008 www.chemistryworld.org Beer quickly, he says. One of the earliest documents containing advice on how to improve the drinkability of a brew is The English Huswife, a 1615 treatise penned by the English poet and writer Gervase Markham that claimed to spell out ‘the inward and outward virtues which ought to be in a complete woman’. This work contains the first practical ROSENFELD IMAGES LTD / SCIENCE PHOTO LIBRARY PHOTO SCIENCE / LTD IMAGES ROSENFELD instructions for brewing bottled beer and a process of ‘blinking’ or blanching the wort, says Hornsey.

Load more