Bitter sweet symphony: What causes bitterness in coffee and how to avoid it

bitterness in coffee

Mocopan Coffee’s Babin Gurung on what causes bitterness in coffee and how to avoid it.

As a barista, one of the most common complaints you get from customers is that their coffee tastes bitter or burnt. As happy as you are to remake the coffee, you need to know what’s causing the problem, otherwise you’ll be serving the same bad-tasting coffee. 

Babin Gurung
Babin Gurung is the New South Wales Barista Trainer of Suntory Coffee Australia.

Coffee is an acquired taste and some people seem to enjoy bitter flavours more than the others, but there are different kinds of bitterness, some that we want in our cup and some we don’t. The simplest way to tell a good coffee from a bad one is by analysing the aftertaste. A good coffee may start bitter, but will have a sweeter finish and get smoother on your back palate, whereas a bad coffee can have a lingering bitterness that leaves your mouth dry. 

As we learnt in the previous issue of BeanScene (August 2019), coffee beans have a complex mix of flavour compounds responsible for their taste and characteristics. Along with coffee variety and origin, roasting also has a large impact on the flavour of coffee, and if not careful, can give coffee excessive bitterness. 

Today, however, let’s talk about the three main factors that could be making your coffee bitter and how to control them. 

Coffee is quite resilient against bacteria and mould and thus, many people believe it is OK to drink an old batch of coffee. It may be so, but over the past few years we have learnt that although stale coffee may be safe to drink, it is not good for flavour. 

If you want the best out of your beans, you need to make sure your coffee is fresh. The recommended time for using your beans is between seven to 28 days after roasting. Your bags should be kept away from direct heat, sunlight, and moisture. Storing coffee in the fridge is not recommended as the dry, cold air can affect the integrity of the beans. Freezing coffee is fine, however, if done with the correct tools and know-how. 

As indicated in Figure 1 (top of page), a fresh batch of coffee will have its pores filled with moisture and CO2 gas, providing less surface area for water to absorb the solubles. So, despite running this coffee for standard extraction time, it will still be under-extracted in flavour as water can’t absorb all the good solubles. Fresh coffee will look gassy and bubbly with inconsistent crema. It would be recommended to allow this coffee more time to rest before consumed. 

At around the seven to 28 day-mark, some of the gases will escape making the coffee just right for absorbing all the good solubles, giving you a more consistent result. Past 28 days, the moisture level drops significantly, making the coffee highly porous. This increases the absorption rate, which causes over extraction of coffee. At this stage, the coffee starts to taste stale and will have sharp bitterness. It is not recommended to use beans after this date, but if you do, you will need to decrease brew ratio and allow less water to flow to minimise over-extraction.

But what about the coffee beans sitting in the hopper? 

The staling process is even more rapid in a hopper as the beans are in contact with oxygen, heat, moisture, and humidity. The beans lose their volatile oils through evaporation, which impacts the coffee’s colour, consistency of crema, and flavour. The best way to describe the taste of stale coffee is ashy, leathery, or cardboard-like, with sharp bitter notes. That’s why the recommended holding time for coffee beans in the hopper is two hours. 

The best practice is to keep the minimum amount of beans in the hopper and store the remaining coffee in an airtight container to prolong freshness. It is not recommended to pre-grind your coffee as the increased surface area will absorb more oxygen and moisture, causing it to lose its freshness almost immediately. If you do use pre-ground coffee, limit the time to just a few minutes and keep it sealed at all times.

Last edition we learnt how control of extraction time and dose can significantly alter the result in your cup. However, if calibration isn’t done frequently or correctly, it can lead your coffee quality to vary. Here are a few things that could further cause bitterness to your cup.

  • Over-dosing: Choosing the right dose will depend on a number of factors such as coffee variety, origin, roasting, and tasting notes. The other important variable is the basket size. Ground coffee expands as the water passes through it, so if you over-dose your basket, the puck will jam the screen and block water flow It also slows the extraction time, causing over-extracted bitter coffee.
  • Over-extraction: Extraction time helps you control the flavours you want in your cup. Sweeter and lighter flavours dissolve quicker than heavy and bitter flavours. The best way to find the ideal time is by tasting your cup at different extraction times. However, if you go past the ideal time, your coffee will start to over-extract. Because the coffee is exposed to water for a long time, it starts to absorb darker and heavier flavours. Over-extracted coffee can be best described as intense, bitter, burnt, or dry. Carefully selecting dose and adjusting grind size is the best way to control extraction time. 
  • Water temperature: Coffee needs hot water to release its solubles. The hotter the water, the quicker the coffee dissolves. Too hot, however, and it can damage the flavour notes of your coffee, leading to taste bitter. That’s why you need to make sure you use water that is 90 to 93°C. 

The bitterness you get from dirty or unsanitary equipment is much more acute – think pungent, medicinal, salty, or rubbery notes. This not only leaves your palate with an unpleasant taste but poses a potential health risk. Here are some of the key areas that need regular cleaning.

  • Machine and group handle: Your coffee machine and handles need to go through chemical cleaning every day that they’re used. Cleaning chemicals are designed to sanitise and dissolve coffee build up. If not done daily, the grind and oil will build up in the group head and handle, causing excessive bitterness. Rinse you group head and handle after every single use and backflush your group heads without chemicals at regular intervals. 
  • Hopper: The hopper needs to be kept clean as coffee oil can build up over time. In extreme cases, the oil can turn rancid, giving off a woody, rubbery, or metallic smell and taste. Run the hopper under hot water and wipe it dry. Avoid using chemicals, soap, or a dishwasher as it can create micro cracks in the plastic causing more build-up of oil. 

This article appears in FULL in the October 2019 edition of BeanScene. Subscribe HERE.

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