A bio-engineered alternative has been found for hops, an essential ingredient in beer-making, says S.Ananthanarayanan.
Hops, the flowers of the hop plant, lend bitterness and the ‘hoppy’ flavor to beer. But hops are difficult to source and the cultivation has a high environment cost. It is not that hops were always used and the ale of early times used a group of herbs, called gruit, to impart the bitter taste. The practice of using hops started in Europe in the 13th Century and was resisted in England. In 1471, the city of Norwich banned its use and even half a century later it was described in England as a ‘wicked and pernicious weed’. But adding hops does something to beer that was not to be resisted and all beer is now made with hops.
World annual beer production is about 2 billion hectoliters, or 200 billion litres. With a world population of 8 billion, this puts production at 25 litres per capita. With a large part of the world population soon to reach the legal age of drinking, the figure is going to rise. And, as hops are needed for beer-making, the consumption of hops will also increase. Beer-making itself uses only grain and reasonable energy and water, but the production of hops needs resources, particularly, some 100 billion litres of water a year, at current levels, and adds to the cost. And what is more, the essential components of hops are variable from crop to crop and maintaining a steady quality of the beer produced becomes challenging.
As it is only hops that contain specific flavouring agents, no other natural products have been able to take the place of hops, which have commanded high prices since centuries. There is hence interest in finding an alternative to the hops plant, both to contain land and water use as well as to simplify the sourcing and blending of the right material in beer-making. The journal, Nature Communications carries good news in a report by Charles M. Denby, Rachel A. Li, Van T. Vu, Zak Costello, Weiyin Lin, Leanne Jade G. Chan, Joseph Williams, Bryan Donaldson, Charles W. Bamforth, Christopher J. Petzold, Henrik V. Scheller, Hector Garcia Martin and Jay D. Keasling, from laboratories in California, and in Denmark, that the cells of beer yeast have been engineered to do the work of the most important components of the hops flower!
The process of beer-making is that barley is first sprouted to free its starch content and an enzyme to convert the starch to sugar. When the grain has sprouted to the required degree, sprouting is stopped by roasting the grain. The grain is then crushed and steeped in warm water, when the starch changes to malt sugar. The liquid, called then wort, is boiled and hops are added to impart bitterness. The wort is then fermented by adding beer yeast, at room temperature, and the malt sugar is converted into alcohol. At this stage, hops are again added, to release ‘essential oils’ which give beer its ‘hoppy’ aroma and flavor.
The Nature Communications paper explains that hop flowers have a dense covering that secretes substances that accumulate in the outer skin. The secretion is rich in a substance called terpenes, which give beer its flavor. The composition of the secretion, however, differs from sample to sample and it is difficult to identify which components are active in giving flavor to beer. Nevertheless, two main ingredients have been isolated and these are the molecules known as linalool and geraniol. Hops are in fact the flowers of the plant, Humulus lupulu, a relative of cannabis indica, the shrub from which recreational drug is extracted. And this family of plants, which synthesise terpenes, even cholesterol and the active substance in cannabis, have an intermediary, called GPP, which leads to the products of synthesis. The paper says that the enzymes that help generate linalool and geraniol from GPP in hops have not been identified so far. What has been found, however, are the enzymes that promote the assembly of these molecules in some other plant species.
The group of researchers hence tried out six different candidate enzymes that could bring about the same action in yeast cells designed to have ample GPP. If the yeast cell, which is the agency of fermentation, were itself able to produce linalool and geraniol, this would eliminate the role of hops in imparting aroma and flavor, at any rate. When the results of trials were not good enough, however, the group checked on the conditions that made efficient creation of the molecules possible in the plant species. But they found that ways to bring about the same conditions in yeast were yet to be perfected. The group hence surveyed other sources and found that the enzyme from Lycopersicon esculentum, which is the common tomato, was promising. Once the enzyme was identified, the group used state of the art methods of gene assembly to engineer the yeast strains to create the terpenes required during fermentation.
The method, in fact, allows development of yeast strains with different sets of genetic design by varying the genetic changes in the yeast cells. What this amounts to is hence not just a method to get brewers’ yeast to produce flavor molecules that normally come from the addition of hops, but to control the mix of flavor molecules that would be produced. On the one hand, this is like getting hold of brace of hops strains and being able to fine tune flavor and aroma according to consumer’s preference. But then, the paper notes that “the full flavor imparted by traditional hopping is likely to rely on a more diverse bouquet of molecules”. But the method is a foundation, the paper says, “provide a foundation for generating more complex yeast-derived hop flavors, and broaden the possibilities of yeast-biosynthesized flavor molecules to those throughout the plant kingdom.”
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