I’ve been roasting my own coffee for a while now and learned how to control the roasting process. Like many beginner coffee roasters, I learned about the origins of beans and their characteristic flavours. I knew nothing about bean defects. I would just remove a few of the obviously dark beans and roast.
I took the beans I bought online at face value, taking the quality for granted. I’ve now learned to look closely and inspect for defects and sub-par bean quality.
I never really knew if the beans had defects from growing, harvesting, processing or storage. Most importantly, I knew nothing about how the coffee bean defects were changing my coffee taste in the cup.
I searched the internet and there was very little available. So, I did some research and decided to write about my results. I was surprised at just how much the most common growing and processing bean defects can affect taste in the cup. I have compiled and rated the bean defects below rated from 1 to 6, 6 having the most impact on taste.
The Defect Rating System
I’ve rated each defect on how it affects taste in the cup on a scale of 1 – 6 as follows:
| Defect Level | Impact on Taste in the Cup |
|---|---|
 |
LOW |
 |
LOW-MEDIUM |
 |
MEDIUM |
 |
MEDIUM-HIGH |
 |
HIGH |
 |
VERY HIGH |
Table of Contents
Click to jump to taste defect:
- Mouldy Beans – Taste defect 6
- Sour Beans – Taste defect 5
- Black Beans – Taste defect 4
- Insect Damaged Beans – Taste defect 4
- Frost Damaged Beans – Taste defect 4
- Old/Past Crop Beans – Taste defect 4
- Fungal Damage – Taste defect 4
- Chips/Pulper Nipper Beans – Taste defect 3
- Shells/Ears – Taste defect 3
- Floater Beans – Taste defect 3
- Immature Beans – Taste defect 3
- Mottled/Spotted Beans – Taste defect 3
- Withered Beans – Taste Defect 2
- Crystalized Beans – Taste defect 2
- Hulls/Husks – Taste defect 2
- Beans with Parchment – Taste defect 1
- Foreign Matter – Taste defect 1
Moldy Beans
VERY HIGH IMPACT ON TASTE
- Mouldy/off flavour
- Loss of all aromatic and taste characteristics
- Potential health risk
Agricultural Causes
- Can be caused by harvesting and processing of fallen coffee berries which have been in contact with the ground
Processing Causes
- Can be high humidity and/or temperature conditions during storage and transportation
High Impact on Taste
-
- “Stinker” cup in extreme cases
- Sour, winey or acetic acid characteristics
Agricultural Causes
- Picking over-ripe or fallen cherries
Processing Causes
- Water contamination during wet processing
- Over recycling water in wet processing
- Dirty fermentation tanks
- Over-fermentation from slow dryin
- Storage with too high moisture content
- Over-fermentation in the fruit still attached to trees during humid conditions
Medium-High Impact on Taste
- Ferment or stinker taste
- Dirty
- Mouldy
- Sour Phenolic taste
These can only be seen when the bean is unroasted. Just a few beans can contaminate a whole batch.
Agricultural Causes
- Bean has blackened from over fermentation
- Over-ripe coffee that has fallen to the ground
- Picking under-ripe coffee fruit with underveloped carbohydrate levels
Processing Causes
- Cased by unripe coffee that has been dried at temperatures that are too high
Medium-High Impact on Taste
- Ferment or stinker taste
- Dirty Mouldy taste
- Sour/ phenolic taste
Insects can invade the berry while on the tree or while stored and can kill the seed embryo and introducing musty and alkaline flavours in the cup. Should be removed at drying stage or affected beans can be removed before roasting.
Agricultural Causes
- Attack on the coffee fruit by the Berry Borer Beatle
Processing Causes
- Failure to remove of Berry Borer damaged beans in in the dry mill
- Weevel damage from poor storage practices
Medium-High Impact on Taste
- Minor frost damage leads to a loss of aroma and flavour, a decrease in mouthfeel intensity and overall quality
- Major frost damage leads to noticibly strong off-flavours in the cup
The change of taste results from the underdeveloped, stunted beans produced when the frost stops the development of the immature berry. The seed embryo can be damaged, resulting in an off flavour in extreme cases.
Agricultural Causes
- Frost damage to the coffee berry on the tree due to low temperatures
Medium-High Impact on Taste
- Reduction in acidity
- Increased astringency on surface of tongue
- Noticible woody taint to the flavour
- Baggy taste from rancid oils in coffee absorbing environmental contaminants
Agricultural Causes
- Bean has blackened from over fermentation
- Over-ripe coffee that has fallen to the ground
- Picking under-ripe coffee fruit with underveloped carbohydrate levels
Processing Causes
- Extended storage of coffee
- Unforseen delays in the transportation chain or extended transport routes to market
Medium-High Impact on Taste
- Fermented or stinker taste from rotten beans
- Dirty taste
- Noticible Mouldy taste
- Overly Sour taste Phenolic taste – plasticy, smokey, antiseptic taste
Agricultural Causes
- Left over beans beans in harvest sacks contamination newly picked berries
- Picking of fallen cherries which have been in contact with the ground
Processing Causes
- Poor cleaning of bean fermentation tanks between batches
- Delay in the drying process Storage of beans at high humidity and temperature levels
- Careless colour sorting in the dry mill
Medium Impact on Taste
- Chemical and ferment off-flavours
- Reduced aromatics
- Reduced acidity
- Scorching during roasting due to thin broken portions of the bean burning
Broken or chipped beans should be removed during sorting. many in a batch show carelessness in the production stage.
Agricultural Causes
- Can be caused by harvesting under-ripe, undersized beans
Processing Causes
- Too much mechanical pressure from pulping machines
- Processing under-ripe coffee through pulping machines
- Poor machinery callibration in the dry mill
Medium Impact on Taste
- Shells may easily become overroasted and char.
- A burnt or charred flavour in the cup
Shells are mostly a visual defect which affects the taste in the cup by becoming overroasted because the beans are hollowed out with thin walls. To avoid this remove them prior to roasting if possible.
Agricultural Causes
- This is a naturally occurring phenomena, caused by genetics
Medium Impact on Taste
- Results in noticibly reduced flavour and acidity in the cup
Floater beans are lightweight, underdeveloped beans. These spongy beans are distinctively light and faded usually with a mottled appearance.
Processing Causes
- Partially unknown cause, could be genetic
- Suspected causes are improper storage or uneven drying
Medium Impact on Taste
- Results in increased bitterness and astringency in the cup
- Creates a noticible straw-like or greenish flavours
Agricultural Causes
- Harvesting of under-ripe or young berries
- Cultivation in unsuitable coffee production areas
- Lack of fertilizer
- Coffee trees have been affected by drought and rust disease
Processing Causes
- Caused by the failure to remove unripe berries during sorting
Medium Impact on Taste
- Reduced acidity
- Reduced flavour and aroma
- Slight mouldy or sweaty/off flavour
The uneven drying creates inconsistencies in the roast.
Processing Causes
- Faulty drying
- Re-wetting during drying stage
- Broken bean parchment allowing mould to enter before drying
Low-Medium Impact on Taste
- Loss of acidity
- Reduced flavour characteristics Weed-like or straw-like
Agricultural Causes
- Trees stressed due to drought
- Underdeveloped fruit
- Poor fertilization
Processing Causes
- Bad bean density seperation during processing
Low-Medium Impact on Taste
- Beans will break during roasting
- Loss of acidityin the cup
- Reduced flavour characteristics
Processing Causes
- Excessive drying of the beans at temperatures over 50°C (120°F)
Low-Medium Impact on Taste
- Large amounts can lead to a range of off taints including dirty, earthy, fermented, mouldy or phenolic taste
- Also can be risk of fire in roasting
Processing Causes
- Washed Process Coffee: Poorly calibrated pulping equipment
- Natural Process Coffee: Poorly calibrated hulling equipment
Low Impact on Taste
- Reduced intensity of flavour
- Increased risk of chaff fire during roasting
Processing Causes
- Due to poorly calibrated milling equipment
Low Impact on Taste
- Reduced intensity of flavour
- Increased risk of chaff fire during roasting
Processing Causes
- Can be added to the bean mass throughout processing
- Sticks and stones are collected with beans during harvesting
- Foreign matter can be left on drying tables between batches
- Magnets, and de-stoners not operating properly in the dry mill
So What Does This All Mean?
Let’s be realistic. We’d be surprised if any coffee we bought was completely defect free. This list of defects gives you the ability to start to recognise the quality of the beans you are buying. You’ll also be able to tell if the defects are changing the taste of what you’re roasting and by how much.
If you find a few defect beans in the roast with parchment pieces will not be doing much to your taste. A few moldy beans however should throw up a red flag and you should be testing if they have contaminated the whole batch. Stones or metal fragments in the roast should be removed if you treasure your grinder.
Also after assessing the coffee you’re getting for defects, you’ll start to recognize if your supplier cares what he’s selling and if that inexpensive coffee is really a good deal.
Happy roasting (and sorting)
Just came across your page today. I bought a black light flashlight after reading some articles about how they aid in spotting defective green beans. I was suprised to find an El Salvador from Mill City Roaster had so many “weird” [as in illuminating under the light] versus a much less “weird” batch from Sweet Marias.
PS, I thought you did some really good work on this page.
Thanks, I’ve not used a blacklight but I’m going to give it a try. I usually just sift through by eye but anything that can simplify the process is worth a try.
I gained an extra knowledge from the notes
Thanks, good to know it helped you.
perfect
Thanks for the feedback!
Really nice overview and helpful. Thank you. Two more items are used for coffee grading. 1) the size of beans currently using sieves, and 2) a “points system,” for example, 1 black coffee equal to 1, 5 broken beans equal to 1, 10 beans in silverskin equal 1 point. the sum of the points and size makes the grade ( deepened on the standard)
We are implementing digital inspection for coffee, happy to share more information.
Thank you
Ron
Hi Ron, Thanks for the info about grading. We’ll try to incorporate that into upcoming articles.