Coffee Bean Roasting | What Are Roasting Defects in Coffee | Which Defects Belong to Roasting

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Some flaws in coffee flavor come from defects in raw beans, some are produced during roasting, and others occur during brewing. Today, FrontStreet Coffee will first discuss what defects can occur during roasting!

The Definition of Roasting

Coffee roasting refers to the process of heating raw beans to promote a series of physical and chemical reactions inside and outside the coffee beans, generating various flavors such as acidity, bitterness, and sweetness, forming body and color tone, and transforming raw beans into dark brown beans.

Baking

This roasting defect appears no different from normal roasting; it can only be distinguished through tasting!

Coffee beans absorb very little heat in the roaster, and with less heat absorption, the total roasting time is extended. Coffee aroma is characterized by rapid volatility; with longer total roasting time, more aroma naturally dissipates. Coffee roasted this way has very weak aroma, with grainy and toasted bread flavors.

Imagine what a bread would taste like if baked in an oven at low heat for a long time?

Chipping

During the coffee roasting process, there are two cracking stages. The first crack occurs when there's moisture inside the raw coffee bean fibers that cannot escape. Due to heating, pressure builds up, and when the coffee bean cell wall can no longer withstand this pressure, the coffee beans crack with "popping" sounds. The second crack occurs because coffee beans themselves have a woody fiber structure. Due to combustion heating, coffee beans undergo thermal pyrolysis, producing sounds of woody structure breaking: "pop, pop, pop..."

During the first crack of coffee beans, the cell wall is broken by pressure.

Getting back to our topic, what we call chipping occurs during the second crack of coffee beans. Why does the surface of coffee beans chip? The reason is that coffee beans absorb less heat during the first crack, resulting in insufficient expansion rate. During the second crack, the surface absorbs more heat while the interior absorbs less, creating a significant difference in hardness between the outer and inner layers. During the second crack, the lower-density surface peels off. (Raw beans with lower density are also prone to this defect)

During the second crack of coffee beans, the woody structure breaks.

Top Tipping

This roasting defect actually has both good and bad aspects. The ends of coffee beans have the lowest density, and the lowest density absorbs more heat. For light roasts, more heat absorption at the ends might bring sweetness, but for dark roasts, excessive heat absorption at the ends can lead to carbonized flavors and smoky tastes during coffee extraction.

Scorching

Before discussing this roasting defect, let's first understand the three heat sources in coffee roasters: convection, conduction, and radiation. Machines with more convection are less likely to experience scorching because the hot air has good coverage, allowing beans to absorb heat more "comprehensively." Scorching defects are more likely to occur in machines with more conduction and radiation, as these two heat sources transfer heat quickly but with low efficiency, making it easy for a single "point" to absorb too much heat.

Among modern roasters, most are machines with more hot air, so this defect is less likely to occur. This defect typically occurs in hand-net roasting and roasters with more conduction heat.

Fast Roasting (Underdevelopment)

Fast roasting (underdevelopment) shows little difference from normal roasting to the naked eye and is unrelated to the degree of roast. Don't assume that light roasting means fast roasting. Fast roasting simply means the total roasting time is too short (this excludes variables from different roaster brands and is discussed under normal roasting conditions).

Important Notice :

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