Paper or Metal Filters?

Scroll down to content

I wanted to thank all of those who reached out with comments and feedback on my SCA talk! I love turning these posts into actual conversations and appreciate everyone who shared their thoughts. This week, we return to our regular format with an excellent question from Adrienne: “Why does my coffee taste better using a paper filter as opposed to using the reusable metal filter for the AeroPress? I assume that the paper absorbs some compound in the oil that, according to my tastes, is undesirable to my particular palette.” A great question and the perfect starting point for a new thread! Over the next few dispatches, let’s explore the universal versus the unique in roasting and brewing!

When brewing a cup of coffee, we make a series of choices. Each one impacts and shapes the final taste. We pick the roaster and origin, decide on a brew method, select a grind and extraction time, and we can even choose to add additional ingredients like milk or sugar. You could use a top-of-the-line La Marzocco and expertly grind your beans using an EK-43 with aligned and sharp burrs, but if your starting coffee is a generic and cheap dark roast, you will end up tasting the charred cardboard of past crop. Equally, a well-grown and well-processed coffee that is perfectly-developed will never show off its sweetness and beauty if brewed with a dull blade grinder, lukewarm water and then served in an acrid pot. Hence, each of these brew decisions garners a different result, the question is: what exactly changes?

In “Comparison of nine common coffee extraction methods: instrumental and sensory analysis”, the authors hint at an answer and break down the steps to tasting coffee. First, is visual impression extending from “the crema, the color and the volume”. Shine sunlight through a clear glass of coffee and it will appear bright and refreshing. But present a short espresso in a black cup and your mind jumps to strong and intense flavours. Next is aroma. While there are well over 1000 identified volatile organic compounds floating out of the cup, there are only about 20 that form our impression of a coffee. Taste combines with smell, as we bridge feedback from our taste-buds to the retro-nasal passageway. The key tastes for coffee being acidity, bitterness, and sweetness, which expand outward giving us flavours like citrus pith, stewed plum, or dark chocolate. Two last core factors: body and finish. The authors write, “the body, also called mouth-feeling or texture, is a further important sensory descriptor … related to the total solids and occasionally also linked to fat or fatty acid content”. And here we arrive at Adrienne’s question…

Selecting between metal or paper is but one of the numerous fork-in-the-road decisions in the adventure of coffee brewing. As you can see in the above diagram (taken from Scott Rao’s Everything But Espresso) a filter will change both the perception of body and the flavour clarity. This is because dissolved solids (those elements moved from grounds to water) create taste, volatile aromatics create scent, and a group of insoluble materials (including fines and oils) generate mouthfeel. So the more we increase oils and fines in the finished product, the more body we gain. But it comes at a price. Scott Rao writes, “the fines and oils in brewed coffee bind to form brew colloids” with “higher concentrations” we end up with “less flavour clarity“. So, Adrienne’s individual preference is likely for a well articulated cup over one with heavy body, thus using a paper filter with smaller pores and decreased fines is preferable. It is not necessarily taste from one individual oil but more about the overall concentrations. For the record: she is right.

If we consider the larger network here, we can see how brewing method perfectly maps over the visuals, tastes, smells, and feels of the cup and how each of these align the possibilities for an overall experience. Certain routes will enhance the ‘window into origin’. For example, using sharp burrs ensures an even extraction with less of the fines responsible for generic bitterness or the boulders that dilute and wash out the cup. We will have the unique pop and clarity of tasting where a coffee is from. Conversely, certain avenues will diminish the ability to explore terroir. As Adrienne suggest, a metal filter will leave the flavours muddy and opaque, often hiding the delicate notes and overwriting those more complex ones.

I would suggest that everyone consider what they value in a cup: taste (sweetness, acidity, bitterness), smell (preferred aromatics like fruit or chocolate), body (full or mild), and mouthfeel (juicy or thin). Depending on priority, it then becomes easy to work backwards and select origin, roast style, brew method, and even visual presentation! But again, the actual right answer is sweet, balanced coffee with a juicy mouthfeel and medium body served via filter/drip 😏.