Samuel Gurel, founder of Sovda and Joel Smith, coffee processing expert and guest roaster, explain defects found in roasted coffee; including quakers, burnt beans and under-roasted beans.

At the Curate Coffee Collective in downtown Portland, the coffee specialists will describe how these defects are formed and cup them to explain how each one affects the coffee. 

This article will further expand on the science behind coffee defects and the research that has been done in this field. 

Quakers are coffee beans that don't turn dark brown when roasted. For coffee to be considered a specialty grade, the Speciality Coffee Association has required that it be free of quakers. 

Because Rabelo’s research indicates that quakers can still appear in coffee even after sorting at the farm and dry mill, we don’t fully agree with zero-tolerance as the green coffee standard. We believe that one quaker shouldn’t necessarily make the coffee–non-specialty green. Ultimately, it is the cup quality that should determine if a coffee is a specialty.  

However,  it does make sense to sort the defects out of your coffee after it’s been roasted. 

In order to understand how we get quakers, we’ll start by explaining why good coffee beans do turn dark brown. 

Inside every coffee bean there is an endosperm, which contains a honeycomb-like structure that stores different chemical compounds to nourish the embryo, such as sugars and amino acids. According to scientific research conducted by Mariane Helena Sances Rabelo, coffee beans have to go through seventeen different chemical changes to acquire these compounds. 

These chemical compounds are important because when they are exposed to heat they  go through Millard reaction and caramelize, thus creating the dark brown color of coffee beans. 

The caramel smell of a roasted bean is what tells our brain that the coffee is sweet although we have not added any sugar to the bean. Instead, heat contributes to converting sugars into volatile aromatic compounds (VOC.)  VOC have a large impact on how we perceive food (Shepherd, 2012). 

Unfortunately, this fragrance or taste can be ruined if anything unusual happens to the compounds inside a coffee bean’s endosperm. When a bean can’t caramelize because its sugars have been compromised, it is called a quaker.

Rabelo’s research noted that lighter quakers with a high agtron number (between 82-95) have a statistically significant impact on the flavor of coffee. Thus showing that the flavor of coffee is dependent on the availability of sugars to caramelize and darken the bean. 

Here are the four different ways a bean’s sugar can be compromised:

1. Immaturity

2. Inadequate Nutrients from tree

3. Insect attack

4. Microbial Attack

The Arabica Green Coffee Defect Handbook defines quakers as “immature beans, unripe.” However, more recent research has shown that quakers can also be caused by microbials or insects that compromise the chemical compounds of a coffee bean. 

Immature beans, or beans that received inadequate nutrients, generally result in “tame quakers.” These quakers will cloud the taste of your coffee and will result in a less than perfect cup but it’s not the worst flavor you can run into.

Some people claim to prefer tame quakers in their coffee because the over fermentation can produce positive fruity notes. It should be noted that there doesn’t seem to be any empirical evidence for thinking that quakers can have a positive effect on flavors. 

However, Sovda did some applied research and found that normal mature coffee beans, over fermenting and losing sugars to microorganisms, does not always have a terrible outcome. 

Sometimes, these microbes produce esters that are classified as VOC, containing strawberry and blueberry aromas.  

Yet overall, we found that the likelihood of quakers having a negative impact on the cup is significantly higher than the quakers having a positive impact on the aroma. When we ground and analyzed 100 quakers from Ethiopian Natural grade 2 coffee, we found less than 10 had positive aromas. Continuing, we found that by eliminating all quakers, naturally processed coffees can gain a clarity that brings out the less muddled fruity flavors.

Rabelo’s research highlighted that a few quakers in a cup of coffee can have an impact on the coffee’s cupping score, but four or more immature beans will have a statistically significant impact. According to her research, quakers affect the coffee flavor attributes of astringency and lack of sweetness. 

Insect attack and microbial attack result in what we call “stinky/defective quakers.” These quakers are responsible for some of the worst flavors in coffee. Insect attacks usually turn into microbial attacks because the insects have pierced through the many layers of the coffee bean and made the bean susceptible to disease. 

The kind of microbe that attacks the beans will have a diverse effect on the taste of the coffee. Some microbes like yeast can be tame in how they affect the taste but certain bacteria fungus will radically change the flavors. Bacteria, molds or fungus can produce really nasty compounds like orcratoxin, isovaleric acid and butyric acid which will create obnoxious flavors like rotting organic matter or throw up. We have seen that even one of these quakers can significantly impact a cup of coffee.  

Depending on the processing method that the coffee beans go through, different amounts of these quakers can be sorted out during primary processing (before the dry mill). Yet, even a modern advanced dry-mill might leave a number of quakers–significant enough to have a negative impact on flavor. 

 Natural and honey processed coffees tend to have more quakers than washed processed coffees because the washing process includes multiple steps designed to eliminate defects.  Floaters that could become quakers will rise to the top of the water in the flotation tank or classification channel.

Sometimes facilities will even have a pulper that separates the small, immature beans, before they are washed. But sometimes beans are also chipped or cut in the pulper, therefore damaging the bean and the sugars inside of the bean. 

The last sort of defective bean that will affect the taste of your coffee are over-roasted beans and under-roasted beans. Over-roasted beans will often result from beans accidentally going through the roaster twice or getting stuck against a face plate or another part of the roasting machinery that is hot.

Over-roasted beans will result in a smoky or bitter flavor.

Under-roasted beans are also often the result of beans getting stuck somewhere in the roaster but in parts of the machinery that are cold such as the hopper.

Under-roasted beans are very unpredictable and can taste like anything from cereal to extremely acidic. 

Producing a perfect batch of coffee without quakers can seem impossible. As stated previously, Rabelo’s research indicates that even after sorting has taken place at the farm and dry mill, quakers can still appear in coffee, after roasting. 

This is why Sovda exists, to give coffee people color-sorting technology that will remove the defects caused by quakers, under-roasted, and over-roasted.

These machines use technology that allows you to enter what kind of beans are/are not desired. The main criteria being the desired color spectrum for roasted beans. 

After entering a certain profile into the software, the optical sensor will recognize bad beans from good ones and eject bad beans through a targeted air blast. 

To avoid quakers in your coffee, talk to one of our experts about color-sorting and how it can be implemented into your production line.