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Analysis of caffeine, pH, and acidity in coffee

AN-T-225

2021-09

zh

Fully automated determination including filtering, reagent addition, and sample pipetting using OMNIS

影响咖啡口味的许多关键因素都与可测量的化学特性相关。这些因素包括 pH 值、可滴定酸度、折射率和咖啡因。从历史上看，许多此类分析都需要使用耗时的液相色谱（LC）技术进行漫长的手动样品制备过程。 本应用说明介绍了使用单一滴定平台分析咖啡关键质量参数的更快、更简单的替代方法： OMNIS。

With the ever-increasing consumption of coffee due to the availability of small home espresso machines, shelf life and consistency of flavor are becoming more important for brand quality in a competitive market.

Many of the key factors that influence coffee taste correlate with chemical properties that can be measured. These include pH, titratable acidity, refractive index, and caffeine. Historically, many of these analyses have included long, manual sample preparation processes using the time-consuming, liquid chromatography (LC) technique.

This Application Note looks at a faster, simpler, alternative method for analysis of key quality parameters in coffee using a single titration platform: OMNIS. 

Coffee is primarily acidic, with most roast extracts displaying a pH of approximately five. When coffee is too acidic, it can taste sour and be harsh on the palette. When shifting to the alkaline end of the pH scale, the flavor then becomes bland and lifeless.

Different coffee beans require different amounts of roasting, depending on the bean origin and the level of acidity, to achieve the consistent flavor expected from the brand line. By analyzing the pH and acidity of coffee brewed under consistent conditions, it is possible to judge the final flavor of a roast. This is most beneficial to roasters of large volumes of coffee beans, or to those who supply products with expected flavor profiles (e.g. instant coffee pods).

The analysis of pH and acidity in brewed coffee is quite simple and very similar to the procedure used for juices and soft drinks. 

Table 1. Results of quality parameters in various coffee brews measured with OMNIS.
Analyte
 Sample 1 Sample 2
pH 5.37 6.41
Acidity (mg/15 g) * 9.9 7.1
Caffeine (mg/15 g) * 120 87

* A serving is considered to be approximately 15 g of ground coffee extracted on an espresso coffee machine producing a double shot of coffee. After cooling, OMNIS samples 40 mL of the coffee solution automatically for acidity. A 5 mL sample is used for the caffeine sample preparation and titration.

Unlike the simple sample preparation required for titratable acidity analysis, the analysis of caffeine in brewed coffee is a more intensive process that relies upon several manual preparation steps. Requiring reagent addition (iodine and sulfuric acid), filtration, and accurate sample volume transfer in specifically timed steps to provide consistent results, the conventional analysis is very time consuming when performed manually.

The flexibility of OMNIS allows for the automation of these steps, eliminating the variability introduced by manual liquid handling and manual timing. 

Figure 1. OMNIS Robot S with Discover and parallel analysis.

OMNIS automates the entire analysis process with:

  • Discover Capping system which keeps samples covered until the time of analysis
  •  Highly accurate dosing of iodine and acid
  • Consistent stirring time for reaction
  • Automatic inline filtration
  • Highly accurate dosing of sample to titration vessel
  • Automatic start of titration
  • Automatic cleaning of titration vessel and sample path 
Figure 2. Figure 2. Example titration curve for caffeine analysis with OMNIS.

Metrohm’s OMNIS titration platform provides the perfect blend of automation and intelligence for the analysis of coffee. Instead of time-consuming manual sample preparation and long analysis times with several different instruments, key coffee quality parameters can be measured accurately and reliably on a single system.

With OMNIS, you can enjoy your coffee without worrying about your analysis. 

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Metrohm AG

Ionenstrasse
9100 Herisau

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Internal reference: AW TI AU-001-072020