Making espresso





One of the most popular ways to drink coffee is as espresso, which many people like because it concentrates the flavour and caffeine kick into one small shot. It is made using a special machine which forces a small amount of hot water through the ground roasted coffee beans under pressure, to produce a drink with a thick foamy layer over the top.

The set up for the espresso machine has never been officially studied or standardised but the baristas seem to get it just right. The “best” conditions are handed down via training schemes or from barista to barista in coffee shops but are they necessarily the optimum conditions? A team of scientists in Italy, where else, has decided to find out.

Sauro Vittori and colleagues from the University of Camerino and the Centro Studi Assaggiatori (Taster Study Centre) in Brescia undertook a detailed study of the espresso coffee machine settings, with the aid of a panel of expert tasters. They measured 12 key aroma constituents, including those with positive and negative influences, and correlated them with the experiences of the tasters.

Coffee comparisons

Vittori tested two types of coffee and two brands of espresso machine. Robusta coffee (Coffea canephora and C. arabica at 95:5%) and pure Arabica coffee were used on Aurelia Competizione (AC) and Leva Victoria Arduino (LVA) machines. The AC machine was fitted with a pump and a heat exchanger whereas LVA had a water boiler and increased pressure with a spring.

The different machine constructions meant that they had different pressure and temperature characteristics. With the LVA, the conditions are automatic and cannot be set. The pressure rises to 8 bar over 8 seconds then falls away and the temperature behaves in a similar fashion, rising to 98°C within 3 seconds then falling away to about 87.5°C by 25 seconds.

For the AC operating under typical conditions, the pressure and temperature rise to 9 bar and 94°C over 5 seconds and remain at those values for the remaining 20 seconds of a typical 25-second coffee making period. However, unlike the LVA, the setup can be altered, so the team made coffees over 7, 9 and 11 bar and 88, 92 and 98°C, a total of 9 combinations.

The coffee was analysed at intervals over 40 seconds by headspace SPME-GC/MS for the 12 key volatiles, which gave positive, negative or no overall contribution to the overall taste and odour. They comprised six aldehydes, two furans, two pyrazines, one thiol and guaiacol, which eluted from the GC column in different time windows. They were detected by electron ionisation and measured by selected ion monitoring. The sensory panel also evaluated the espresso coffees.

Barista expertise confirmed

The variations in the abundances of the total positive or total negative key odorants for the Arabica coffee measured by GC/MS followed the same trends with changing temperature and pressure. The intensity of the aroma maximised at 9 bar and the greatest difference between the positive and negative contributions occurred at 92°C. This combination of temperature and pressure was also declared the best by the tasters, who thought that it had the highest value for the positive odorants and the lowest for the negative contributors.

When the two types of coffee bean were compared using the ratio of the total abundances of the positive to negative components, they followed different trends over 40 seconds. However, when the volume reached 25 mL, the aroma intensity and the level of the positive odorants both had high values for both coffees.

Coffees prepared using two espresso machines were compared by fixing the temperature and pressure of the AC machine at 92°C and 9 bar. With the LVA machine, the ratio of positive to negative odorants was particularly high for the fractions collected over the first 25 seconds. This was attributed to the fact that the pressure rise is delayed a few seconds, allowing the hot water to extract more volatiles. In the AC machine the ratio increased over the first 25 seconds at which time the volume of coffee reached 25 mL.

So, it turns out that the best operating conditions for the espresso machines are 9 bar and 92°C, which are very close to those used regularly in Italian coffee shops for drinks of 25 mL. The baristas had it right all along.

Related Links Food Chemistry 2012, 135, 1127-1133: "Optimization of espresso machine parameters through the analysis of coffee odorants by HS-SPME–GC/MS" Article by Steve Down Read more about Steve and our other columnists >>> The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Author(s)

Steve Down