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Solid serve to capsule study

Consumer interest and economic importance of single serve coffee products continues to grow, yet the sector remains a relatively untouched scientific field. GCR speaks to Professor Chahan Yeretzian on extracting a series of guidelines to achieve a stable extraction process.

In October 2020, Scientific Reports published a paper on the Extraction of single serve coffee capsules: linking properties of ground coffee to extraction dynamics and cup quality.

Authors André Eiermann, Samo Smrke, Loïc‑Marco Guélat, Marco Wellinger, Anja Rahn and Prof. Chahan Yeretzian set out to clarify the variables that govern coffee extraction from single serve coffee capsules.

Why? Because, according to Yeretzian, Professor for Analytical Chemistry, Bioanalytical Chemistry and Diagnostics at the Zurich University of Applied Sciences in Wädenswil, Switzerland, and Head of the Coffee Excellence Center, single serve is the fastest growing segment within the larger category of coffee.

The global coffee pod and capsule market combined is expected to grow from US$15 billion in 2017 to US$29 billion by 2025, according to Fior Markets, at a compound annular growth rate of 8.5 per cent.

The high-pressure extraction capsules segment alone, which was the focus of this study, currently leads the coffee pod and capsule market, and was valued at around US$9.61 billion in 2017 (63 per cent of the whole single serve coffee segment). The domestic consumption of single serve coffee is expected to increase due to globally imposed lockdowns during the coronavirus pandemic.

Chahan Yeretzian, Professor for Analytical Chemistry, Bioanalytical Chemistry and Diagnostics at the Zurich University of Applied Sciences in Wädenswil, Switzerland.

“One critical success factor is the proper adjustment of a range of input parameters like roast, grind, filling, water minerality, degassing, packaging material, pressure, and atmosphere inside capsules to achieve a consistent and stable extraction,” Yeretzian says.

“The main goal of this study was to reveal the key relationship between input parameters such as variety/blend, roast, grind, degassing and packaging, with the consistency of the extraction process and cup profile.”

Much research has been done on espresso and filter coffee, but when it comes to single serve, Yeretzian says it comes down to the fact that extraction is an intrinsically highly complex research subject, and because learnings gained from studying espresso extraction on a professional portafilter coffee machine do not apply directly to single serve coffee extraction.

The research group started with the hypothesis that grind and roast degree would be critical in finding general guidelines for a stable extraction. Upon completion, however, they realised that the particle size distribution, and in particular the share of fines (Q100 micrometres), defined as the volume share of particles smaller than 100 micrometres in size, was of particular important to understanding the extraction process.

Over a period of about six months, the study was conducted on 43 Nespresso and Nespresso-compatible capsules of the same geometry using the same machine.

In order for the study to be relevant to products actually available on the market, the research group chose to start with actual commercial products obtained from local supermarkets. The problem of working with such ‘real market samples,’ Yeretzian says, is the limited or complete lack of control over a range of important input parameters such as the varieties, blends, roast degree/profiles, and grind.

“Instead of controlling [every detail], some parameters remained either unknown, such as the variety of coffee, or were measured (particle size distribution). Hence, we had little control over the experimental plan,” he says.

“The uncertainly for some input parameters also created some ‘noise’, an added uncertainly.”

The internal pressure of each capsule was estimated, oxygen content inside the capsules measured, with each one opened and thoroughly emptied to measure the weight of coffee in each capsule.

The roast level and particle size were determined and analysed, using at least 10 grams each, and three measurements made per capsule type.

Each coffee was extracted from the capsule using a Krups Inissia XN1005 capsule machine. The aim was to reach 40 grams. The extractions were timed and manually stopped.

The concentration of coffee in the filtered brew was then analysed using a refractometer and calibrated with a soluble coffee sample. The extraction percentage was calculated based on the mean weight of the coffee in each capsule type, brew weight, and refractometric Total Dissolved Solid (TDS) measurement.

Brew analysis and sensory tests of the coffees was performed by three certified Q Arabica Graders from the Coffee Excellence Center. The sensory quality attributes that were determined were flavour, acidity, body, and balance on a scale from five to 10. The quality of each attributes was assessed by tasting three replicates of the brew by each of the Q-grader.

“Data from the sensory analysis is often subject to huge variability and large standard deviation, precluding a correlation with instrumental and input data. The most important requirement for this study was hence to achieve the highest possible reproducibility among repetitions of the sample capsule and differentiation among different samples. The lack of correlation of the sensory profile to technical parameters was not surprising since the most important driver of cup profile and quality was the green coffee quality (varieties and blend), which were yet unknown to us,” Yeretzian says.

Despite the wide variety of different capsule coffee formulations used for the production of capsules subjected to this study, the research team drew three general conclusions with respect to the extraction of coffee from capsules and espresso brew properties.

  1. The capsule extraction dynamics are highly influenced by the particle size distribution of coffee grounds. Only mean or median particle size information is insufficient to understand capsule extraction. The proportion of fines was highly correlated with increased extraction time.
  2. The best way to increase TDS is to use more coffee for extraction, or to grind the coffee more finely to increase extraction efficiency. Grinding finer to increase extraction comes with a technical problem – finer grind sizes tend to form more finer particles in the grounds, which cause a decreased permeability of the coffee bed. Yeretzian says this learning is in line with what is known from studies on espresso extracted on a professional portafilter machine.
  3. Cup sensory quality is not correlated to the technical variables of the ground coffee. It is suspected that cup quality is mainly driven by green coffee quality. While Yeretzian says this is probably a very general conclusion, it is important to recognise that technical extraction parameters do modulate cup quality.

With initial conclusions drawn, Yeretzian and his team of researchers are driven to further explore these main learnings in a more controlled experiment and to see whether they apply to espresso extracted on a professional portafilter machine.

As a result of these early findings, Yeretzian hopes they will help manufacturers develop coffee capsule ranges that achieve more consistent and constant extraction in terms of extraction flow and cup weight.

“[It would be great to see] more rational design of coffee capsules with improved consistency, and more professional customer support in case of reported inconsistencies in extraction performance,” he says.

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