Why does specialty coffee roast unevenly? Light beans, quakers and fermentation

Roasters and café owners often ask us the same questions: why does this specialty coffee roast unevenly, why are there so many light beans, and was it really picked 100% ripe? Some even suspect the harvest or sorting was poor.

Uneven colour after roasting is not necessarily a sign of low quality. In specialty coffee, especially naturally processed lots, it can be a sign of complexity in processing. Here is why it happens, and how to tell it apart from a real defect.

Roast colour is a visual cue, not a quality score

Uniform colour after roasting is usually read as a sign of good green coffee. Very light beans in a batch are often called quakers: beans from unripe cherries that lack sugar. Quakers roast lighter, taste astringent, grassy, bitter or woody, and lower the quality of the cup. Professional roasters often remove them with an optical colour sorter after roasting.

Yet even specialty lots made from fully ripe cherries, sometimes sorted three times, can still show lighter beans after roasting. The cause is usually not ripeness.

The real cause: fermentation and variation in natural processing

Fermentation decides how much sugar is left

In specialty processing, fermentation is a key stage. Put simply, it is the metabolism of microorganisms, which convert sugars into energy and other organic compounds. Measure the sugar in fresh cherries at harvest and again after fermentation, and you will see a clear difference that depends on how long and how the coffee was fermented.

With the equipment available in Vietnam, where specialty processing is still young, fermenting every cherry evenly is difficult, especially with the natural process (drying the whole cherry), which is the hardest method to control. After fermentation, cherries end up with different sugar levels.

Roasting and browning reactions

The browning that turns green coffee brown and creates roast flavour comes from two main reactions:

• Maillard reaction: sugars + amino acids, from around 120 °C
• Caramelisation: sugars broken down by heat, around 120–150 °C and above

Both reactions use sugar as their main fuel. Beans with different sugar levels after fermentation brown differently, so the batch roasts to uneven colours, even when every cherry was ripe.

Do lighter beans make the coffee worse?

Do not confuse fermentation-related light beans with quakers:

• Quakers come from unripe cherries and taste poor: bitter, woody or raw-bean.
• Lighter beans with less residual sugar still take part in other flavour reactions and can carry sweet, floral or distinctive notes, depending on fermentation and roast.

In a specialty lot, this variation can work like a natural blend, giving a layered, multi-dimensional cup.

Conclusion and recommendations

• Uneven roast colour and lighter beans can be a natural result of fermentation and roast chemistry, not necessarily a harvest or processing fault.
• In specialty coffee it can signal complexity: each bean contributes slightly different flavour compounds.
• To get a more uniform colour, look for lots with tighter fermentation control (time, temperature, microbial activity).
• If appearance matters for your product, use an optical colour sorter after roasting to remove the lightest beans.
• Always judge a lot in the cup: cup the light beans separately if in doubt.

At The Espresso we sample-roast and cup every lot, and we share lot details so you know what to expect from the roast before you order.