Scientists from Glasgow University in the UK have tested for the amount of caffeine in espresso coffees across different retailers, revealing discrepancies that could be putting consumers at risk of unexpected toxicity. The team analysed single-shot cups of espressos from 20 different outlets across the city, and reported a potential risk to pregnant women, those taking oral contraceptives and people with liver disease due to the large variations in levels of caffeine.

The study, to be published in the journal Food & Function [Crozier et al. Food Funct. (2012) DOI: 10.1039/C1FO10240K], involved high-performance liquid chromatography analysis of 20 different commercial espresso coffees, demonstrating six-fold differences in caffeine levels, a seventeen-fold range of caffeoylquinic acid contents and four-fold differences in the caffeoylquinic acid/caffeine ratio.
The team found that the most caffeine in a single espresso was 322mg, while three others contained over 200 mg, exceeding the maximum limit of 200mg per day recommended for pregnant women by the UK Food Standards Agency. The FSA has warned that too much caffeine can result in a miscarriage or a baby having a lower birth weight. The study suggests that the general assumption that a cup of espresso coffee purchased from one of the main high street shops contains only about 50 mg of caffeine could be a misleading one.
Alan Crozier, who led the project, said: 'The variations in the amount of caffeine reflect differences in the batch-to-batch bean composition, roasting procedures, grinding conditions and the many variables in the coffee-making process such as water temperature and the ratio of water to coffee.'
The research also stated that more than £730 million was spent on coffee last year in the UK, amounting to an average of 500g of coffee consumed per person, but that despite their only testing espressos, other types of coffee drinks, such as lattes and cappuccinos, could also be of concern as they are made using single or double shots of espresso.
Professor Crozier also pointed out, “The problem comes with people who should have a limited caffeine intake. You metabolise caffeine much more slowly when you're pregnant and people with liver disease do not have the enzymes to metabolise caffeine.”
The report argued that, although there are an increasing number of coffee shops to choose from, they provide little in the way of information regarding the amount of caffeine in the different types of coffee they sell. It called for further research to provide more data to offer improved labelling of caffeine levels, with details such as bean variety, and preparation or barista method, so that customers can make a more informed choice.