Brewing is the final and most vital step in the coffee-making process – the one we are all a bit more familiar with.
While there are different ways to achieve the same end, the essential process involves soaking coffee grounds in hot water sufficient to allow the flavour-providing molecules to diffuse into the surrounding liquid.
A scientific approach to the chemistry of coffee brewing was pioneered by Massachusetts Institute of Technology professor Ernest Early Lockhart, who in the 1950s left an academic career in food technology to become research director of an organisation known as the Coffee Brewing Institute. He created the “Coffee Brewing Control Chart” for brewing the ideal coffee, balancing sweetness and acidity.
Lockhart’s basic formula depended on two basic factors. The first is the extraction or ‘solubles yield’, which is the amount of coffee chemicals extracted from the coffee grounds. This is affected by brewing method – grind, water temperature and time – and expressed as a percentage. The second is the strength or the concentration of solids dissolved in the brew, which is also expressed as a percentage.
Lockhart’s chart, developed through surveying coffee drinkers about their coffee preferences, recommended 18-22% bean extraction and 1.15-1.35% dissolved solids strength for the perfect brew. The ‘golden ratio’ was 17.42 units of water to 1 unit of coffee.
Considering how important temperature seems to play in the brewing process, this raises the question of the ‘cold brew’ and how it still manages to create a delicious and enjoyable coffee.
It all links back to the process of diffusion – which relies on temperature but also time. Lower temperatures require longer time: the molecules still move out of the ground coffee, but over a greater time period.
This can create an even more aromatic coffee by allowing other chemicals to form that normally would not during the hotbrewing process.