Defects in coffee. Where do they come from, what do they taste like?

Not every coffee bean looks the same. That's normal, because coffee is a natural product. We see different sizes and shapes, sometimes also differences in colors. But: There are more drastic deviations that can have a negative impact on roasting, preparation and taste. Then we're talking about defects.

On the one hand we distinguish between defects that can occur on the farm or during further processing and are thus inherent in the green coffee, and on the other hand between errors that occur during roasting and can diminish the sensory quality of the coffee.

Some may classify roasting defects as style, but fundamentally it's about whether the entire potential of the bean is realized at all.

Similarly, defects are distinguished by their impact on sensory quality. Sometimes it's enough for a single coffee bean with the phenol defect to end up in the ground coffee. This single bean can then mask the entire taste of a cup of coffee and make it undrinkable. Defects that have such a strong effect on sensory quality are assigned to the first category of defects.

Fungal infection. These beans are sorted out before export

Defects of the second category include beans that alone have no direct impact on a single cup of coffee. Often they are physically no longer intact beans, such as shells or ears, where roasting has a stronger influence and the roasting notes dominate.

What doesn't count as defects are beans that are not optically perfect but otherwise show no further irregularities. Or as a colleague once said:

"Ugly is not a defect."

I have judged the Roasting World Championships for several years. One subtask of the competition is sorting green coffee and classifying defects, which are usually manually mixed into a sample. Many competitors sorted the green coffees so precisely – or rather overly precisely – that many green coffees looked optically flawless afterwards, but the sorted-out beans had no defect at all, they just didn't conform to the standard.

Sorting at the Dry Mill

Sorting green coffees so that they are defect-free and largely uniform is the task of the dry mill – the place where coffee is made export-ready. Traders and roasteries can then choose from different qualities: Roasteries where taste is paramount usually buy coffees without category one defects. Brands that primarily have low prices in mind buy cheaper coffee – and thus coffees with defects.

Here coffee in parchment is sorted by hand on the farm

Some of these defects can also be recognized in roasted coffee. So let's look at the defects of the first and second categories and discuss how much their impact on taste is.

Coffee Defects and Their Impact on Taste

In the following, we distinguish where defects can occur and how much their impact on taste can be. Since some defects are recognizable in both green and roasted coffee, you can verify this at home and examine a bag of coffee for defects.

Phenol

You recognize phenol by a sharp, medicinal smell that often reminds of leather, bandage material or rubber. It develops mainly through uncontrolled fermentation or the ingress of unwanted microorganisms during processing. Traditionally, many roasteries in the Balkans and Eastern Europe use coffee beans with low to sometimes strong phenol content.

In many classic "Turkish coffees" phenol is part of the flavor profile. What is professionally classified as a defect, is for some a blemish, for others part of the overall impression. From Brazil, coffees with varying phenol intensities can be purchased: from the gentle, slightly unclean phenol cup to the strongly noticeable phenol defect that dominates the cup.

• Origin: uncontrolled fermentation, poor storage
• optically recognizable: no
• roasts differently: no
• negative sensory impact: minor to very strong

Quaker

An immature bean that appears as a quaker, a light bean, when roasted

Quakers are often lighter beans with low density or those that have high density but didn't receive all nutrients. Quakers appear faded in green coffee, then yellowish to light brown in roasted coffee. They stem from a lack of nutrients. In washing channels they can already be separated on the farm, as these floaters float to the top.

Later, shortly before export, the green coffee is hulled and classified in the dry mill. On shaking tables, the coffee is sorted again by density. Floaters, or quakers, can be separated here from denser beans. And yet a few quakers always make it into the final product.

Optically, the quakers are easy to recognize because they are noticeably lighter than intact roasted coffee beans. Sensorically, the impact on cup quality can vary: They can dilute the coffee taste, change it to an undefined nutty – often peanutty – flavor. Additionally, a high number of quakers can cause a dry mouthfeel.

• Origin: nutrient deficiency
• optically recognizable: yes
• roasts differently: yes
• negative sensory impact: barely to moderate

Insect infestation / Broca

Broca - from two punctures to total infestation

The coffee berry borer (broca beetle) infests the cherries while they are still on the tree. It burrows into the cherry, the entry point is usually the end opposite the stem. With its ovipositor, the insect lays up to three eggs per day over 20 days. After a rest period, the process starts again. Up to 120 eggs are laid this way by a single female coffee borer.

Infested beans are riddled with tunnels, become susceptible to uncontrolled fermentation, spore infestation and lose density. These beans are sorted out before export. The holes are visually recognizable in the bean and testify to the infestation. Taste-wise, few holes have no great impact, as long as the bean hasn't been infested by other microorganisms. From four holes per bean, one speaks of heavy infestation and according to the earlier Q-Grade standard of a severe defect that you could taste.

• Origin: coffee berry borer beetle
• optically recognizable: yes
• roasts differently: little
• negative sensory impact: from little to strong

Shells, Ears, Crushed Beans

The shells, ears, crushed or split beans originate from the depulping of the coffee cherry. If the machine calibration is poorly adjusted, the discs can crush or split apart the still very moist bean. These "split" beans are sorted out before export, but also always make it into the final product.

Since these beans are no longer intact and have significantly lower density, they roast quite differently. They burn faster and become charred to ashy.

• Origin: depulping of coffee cherries
• optically recognizable: yes
• roasts differently: yes
• negative sensory impact: moderate to strong

Little Uniformity

Coffee beans don't all look the same – because they're coffee beans. What helps during roasting are beans of as uniform screen sizes as possible.

When you roast very small and very large beans together, heat transfer takes place extremely differently: with the small bean the heat penetrates the core faster, while the large bean is barely warmed inside yet.

Beans of as uniform size as possible produce a uniform flavor profile because they behave more consistently in the roaster.

Broken Beans

Already dried green coffee beans can also break during transport – usually into smaller pieces because they are already dry and unlike shells don't contain moisture. The broken fragments roast quickly and become charred and ashy.

Defects That Occur During Roasting

Officially, the following "defects" are considered errors, some may categorize them as style. In any case, they can have a minor to strong impact on taste. Whether that contributes positively to the character of the coffee or not remains an individual decision.

Oily Beans

Very high end temperatures cause coffee to become increasingly darker. The cells burst and the coffee bean becomes more porous. The oil contained in the bean encounters less resistance during off-gassing and is pushed outward, so an oily layer deposits on the surface.

The problem with oil is that it eventually becomes rancid – the same with coffee oil. We avoid heavily oily coffees because they also unnecessarily soil grinders and super-automatic machines.

Tipping and Scorching

Two more consequences when roasting very hot – but this time in different ways:

Tipping:

Especially in industrial hot air roasters, where beans literally shoot through the hot air stream, often results in tip burn, the so-called tipping. At the point where the bean is weakest – at both ends, because the heated water is displaced there and escapes. This is clearly visible at a small black dot: the embryo. Fast, hot roasts can result in tipping. In very light roasts, tipping can be sensorially detected – perhaps less because of the burned embryo, but because of the very rapid temperature rise that prevents the development of other aromas.

Scorching:

Scorching occurs when a bean lies too long against the very hot drum wall. This can happen with slow drum speeds or full batches. Intensive scorching results in a roasty, ashy shimmer in the coffee cup.

Contextualization: Are We All Too Picky?

Coffee is a natural product and therefore optical differences in beans are a fact. There are coffees that are not uniform in appearance but convincing in taste. There are also coffees that are uniform but fall in quality. The optical assessment of defects helps especially to gain an assessment of the coffee's workmanship:

• Where could improvements be made?
• Which pest is currently spreading on the farm?
• And are the depulper and shaking table precisely calibrated?

One defect that will show even more strongly in the future due to changed weather patterns is the quaker bean: the light bean that often smells like peanuts. If this is not sorted out in green coffee, it can be separated after roasting, where it becomes optically much more visible. In recent years, roasteries have upgraded for this and installed so-called color sorters – sorting machines that selectively sort out beans by color or shape using an air jet.

These color sorters are helpful and reduce inequalities, but also volume. If I set the machine to remove three percent of all beans, and I roast, for example, 100 tons of coffee annually, that would be three tons of waste per year.

At a green coffee kilo price of 10 CHF, that would be 30,000 CHF spent on coffee that is then not used. So it's also an economic factor whether and how strongly sorting out occurs.

And then I see the tendency that sometimes too close attention is paid: There are roasteries that sort out every slightest color change. That's interesting in a boutique market, and those who want to pay more for flawless beans can do so. However, this shouldn't lead to an exaggerated expectation. Just because a bean doesn't look "perfect" doesn't mean it's bad. That reminds me of the time when I cupped coffees with hardened Q-graders who then picked apart a 79-point coffee because it wasn't top tier.

But there's a big world between waste material and boutique coffees. It helps sometimes to use common sense – or more importantly: trust your palate.

Reality Check on the Coffee Farm

Last but not least, there's the reality on the coffee farm. In the Adventure Filter Subscription we present special, rare coffees monthly – again and again also the smallest lots from individual producers. These coffees are usually never "perfectly" sorted because there isn't more coffee. If a producer grows 500 kg of coffee, she will hardly divide it into five different sizes and then sell five separate mini-lots of 100 kg each. There's simply no market for that. So she mixes all her coffee and maybe removes the smallest and largest five percent, which already gives a bit more uniformity.

And then it also depends on how a roastery handles these beans. I love roasting beautifully sorted beans. But when they're not available, we adjust the roasting profile a bit, roast a bit longer – and achieve the desired results.

So it's worth taking a closer look at all coffee defects.