Encapsulated Coffee Brewer Science

EPO Overview

This page gives additional information about some of the science that underpins the Encapsulated Pour-Over (EPO) coffee brewing method, used in the Simple Smart Coffee Brewer.

We have not repeated the detailed arguments and evidence that is available in the sources we quote.

Intention

The SSC Brewer, and therefore the EPO Method, was developed to simplify the brewing process for the brewer. We set out to get the process as simple as possible, while producing consistently good quality pour-over coffee. We achieved this.

An unintended consequence, and one we are still trying to fully understand, is that the EPO Method has a unique extraction process that impacts the taste of the coffee in subtle ways. We are hoping to engage the pour-over coffee community to jointly explore this.

This page provide exposition, and some explanation, of the EPO Method from the ‘User Process’ and ‘Coffee Extraction Process’ perspectives. There are also some detailed comments about several of the issues / points of interest.

Summary of EPO Method

The EPO Method is a modification to traditional pour-over (albeit a zero by-pass one).

Instead of pouring water onto a loose coffee bed, the water is poured onto a coffee bed that is encapsulated within a space bounded by the brewer body and filter plate. (Unlike a Phin filter, the filter plate fits snuggly inside the brewer body to help form a seal.) The EPO Method uses the fact that coffee grinds expand in hot water to form a perfect coffee bed that eradicates the possibility of channeling and uneven extraction. See below for more details.

User Perspective

This means that the user does not need to:

  • Do a pre-infusion (or ‘bloom’). The carbon dioxide harmlessly bubbles away, like a soda, and the column of water ensure uniform wetness through the relatively thin coffee bed.

  • Perform any specific pouring technique to get the coffee even wet, and evenly extracted without opening up channels. This typically involves a delicate circular pouring action, with a gooseneck kettle, or the use of a ‘shower’ or ‘dispersion’ screen.

  • Agitate the coffee in any way.

  • Time pours to control the flow rate.

  • Manage any issues associated with ‘fines migration’.

See User Guide Page for an illustration of the user process.

Extraction Perspective

The coffee extraction process is modified by:

  • Pure Percolation – there is no coffee slurry that sits above the coffee bed acting as a partial ‘immersion brew’. The water above the coffee bed is virtually clear, meaning that it has a high extraction gradient when it passes through the coffee bed. This generates faster extraction.

  • Percolation Dissociation from Filtering – this is not an essential feature of EPO, but it is the approach adopted in the SSC Brewer. The coffee bed performs a primary filtration as the water percolates through it. Secondary paper or metal filters are used to remove particles or oils from the final brew, at the user’s discretion, but they do not influence the extraction as this is already completed before the liquid reaches the filters.

  • Zero Agitation – the coffee bed sits undisturbed throughout the entire extraction process – aside from internal dynamics associated with the coffee swelling.

  • Accelerating Self-control Flow Profile – the flow profile is very different from that associated with a V60 type brew. We do not yet fully appreciate how this interacts with the Pure Percolation to impact extraction, especially later in the brewing process. (See Standard Method video on User Guide Page)

Detailed Comments

Coffee Expansion

Coffee grounds swell relatively quickly when immersed in hot water. After about 30 seconds they will have reach 60%-80% of their final unrestrained size.

In a 2020 research paper it was reported that the diameter of coffee grounds increases by about 15% (see Swelling properties of roasted coffee particles - Verena Bernadette Hargarten, Michael Kuhn, Heiko Briesen). As the volume is proportional to the cube of the diameter, this means that coffee will expand to about 150% of its original volume in hot water, if not restrained.

If the coffee is encapsulated within a space that is less than 1.5 times the volume of the dry coffee (as it is in the SSC Brewer) it will expand to effectively fill the entire space available. The expanding coffee will create a uniformly dense coffee bed.

This gives the ideal uniform and flat coffee bed for the most even coffee extraction (see ‘Benefits of Flat-bed Extraction’ below).

The encapsulation of the coffee removes the issues associated with channeling in a “loose bed of coffee” (see The Physics of Filter Coffee, p101 - referred to as ‘TPofC’ below). See ‘Filter Plate Design’ below, for more details.

Also any micro channels that may start to be opened by carbon dioxide escaping from the ground coffee are quickly closed off by the expanding coffee.

Coffee As the Primary Filter

As Jonathan Gagne has pointed out (TPofC, p105), “for gravity-driven brews, the bed of the coffee is our best filter to retain fines and produce a clean cup.”

The SSC Brewer can be thought of as having a two-stage filter process. The encapsulated coffee bed in the filter body serves as the primary filter, which controls flowrate and therefore the extraction of the coffee flavors. Users can then apply a secondary filter, if they wish to, using the included fine-mesh metal filter or a paper filter to modify the ‘texture’ and ‘cleanliness’ of the coffee to suit their preferences. The paper filter will remove many of the coffee oils that are extracted as an emulsion rather than being dissolved by the water.

This design removes any issues associated with ‘clogging’ of the filter, as if a secondary metal or paper filter is used it does not affect the  flowrate (unless the wrong type of paper filter is used - see FAQs).

A tertiary (or third) filter can be added by stacking two of the Adaptors with paper filters in them (or metal then paper). This is useful for dealing with ‘clogging’ or ‘choking’ of the paper filter if the ground coffee contains too many fines (or if you want an exceptionally ‘clean’ brew!).

Benefits of Flat-bed Extraction

In TPoC p103, Gagne highlights that the ideal coffee bed is a flat cylindrical one (as opposed to a cone shape) with no agitation and no water bypass. This is because it “produces an extraction that is more even … because the velocity of water remains constant across it”. This is exactly what the SSC Brewer delivers.

Filter Plate Design

The Filter Plate performs two roles.

Firstly, it is carefully designed to fit sufficiently snuggly into the Filter Body, so that the coffee is encapsulated. The plate is made of a lighter gauge steel than the body, to provide the necessary flexibility to seal the profile and so that it expands slightly faster than the filter body - due to thermal effects - enhancing the seal.

The filter plate alone does not form the seal. It is the plate interacting with the expanding coffee - particularly in the gap between the vertical wall of the plate and the brewer body. (For the SSC Brewer, we work with a maximum gap of 0.65 mm.)

Secondly, as the water is poured onto the plate - not directly onto the coffee - it removes the risk of the water digging “a hollow at the surface of a coffee bed” which can open channels in traditional pour-over methods. This mean we can use a shallow coffee bed, not needing the “3 centimeters in depth” attributed to Scott Rao (TPofC, p102).

With its shallow coffee bed - it will have a thickness of approximately 0.7 t0 1.0 centimeters (depending on coffee dose and type) - the SSC Brewer removes all the issues associated with unevenness of extraction through layers the coffee bed referred to in TPofC p102-3.

Pre-infusion Processes (The ‘Bloom’)

The pre-infusion - or ‘bloom’ - phase typical of most pour-over methods has two primary objectives:

  1. To get the coffee uniformly wet before the main volume of water passes through it. This is to avoid overextraction through channels where the coffee is wetter (with lower hydrostatic resistance) as water passes more easily through this than drier areas.

  2. To prevent carbon dioxide escaping from the coffee grounds opening micro-channels in the coffee bed. This again leads to localized over-extraction.

Neither of these are an issue for the SSC Brewer.

As the coffee bed is uniformly deep (and relatively shallow) and because the water is poured onto the filter plate - not the coffee directly - water is uniformly distributed across the entire coffee bed. The coffee is always uniformly wet within the first few seconds of the pour.

As the coffee is expanding during the brew (it gets to about 70% of its total expansion within 30 seconds and is fully expanded after 4 minutes) any micro-channels opened by escaping gases are always closed off.

There is no need for a pre-infusion phase with the SSC Brewer

Temperature Issues

The use of a Lid on the SSC Brewer has two functions. It enables the user to lift the filter when it is still hot. Secondly, it keeps the water at a more constant temperature while the brew is happening. Tests have shown that the water temperature is approximately 5°F higher at the end of the brew when the lid is used.

As the capillary action that extracts compounds from the ground coffee is more effective at higher temperatures, this might have a positive slight impact on the extraction, but this has not yet been thoroughly tested.

There is also no need to preheat the brewer as it is of modest mass and made from stainless steel which has a relatively low thermal capacity (which indicates how much energy is required to heat it up). Our tests have shown that if you pour water that has just boiled in a single pour (as recommended) the initial brewing