Dr. Monika Fekete on how to store fresh coffee beans and why correct packaging comes before freezing.
It was wonderful to see Melbourne streets animated again with people enjoying sit-down brunches, albeit shortly lived. However, as cafés re-open around the country, baristas and managers may find themselves evaluating how to maximise their roasted coffee bean supplies. Rapidly changing consumption volumes over the last couple of months has potentially meant that some coffee has been sitting on shelves longer than usual. But is it still fit to serve, and how can we keep coffee fresh for longer?
First, let’s look at what happens to coffee once it’s been roasted. Coffee roasting creates more than 800 compounds, nearly 300 of which are important aromatic compounds. These compounds are key indicators of freshness but are easily lost to diffusion.
Apart from this radical change in the chemical composition, physical changes inside the beans also accelerate staling. The build-up of hot gases during roasting breaks down barriers between cells, leading to increased porosity. Volatiles and carbon dioxide (CO2) are adsorbed inside the pores, making an important contribution to cup quality. However, increased porosity also leads to an accelerated loss of volatile compounds, as there is a larger surface exposed to the environment around the bean.
- Carbon dioxide: the amount of CO2 produced and retained inside the porous structure of the beans increases with roast temperature. However, retention begins to drop once roast temperature is above 220°C because cell walls lose their elasticity and cracks start to form, letting CO2 escape quicker.
- Volatile aromas: along with CO2, volatile Maillard reaction products (MRPs) are also lost to diffusion. Studies have shown that the majority of aroma loss is due to a few key compounds. Methanethiol has a sulphury smell we might find unpleasant on its own, however, in coffee, we recognise it as one of the most important indicators of freshness. Strecker-aldehydes and dicarbonyls are responsible for fruity, malty, flowery, honey and buttery aromas, all closely related to that beautiful, fresh coffee smell.
- Lipids: in roasted Arabica coffee, lipids account for about 15 per cent of the dry weight. When these coffee oils come into contact with air, they can oxidise and degrade, leading to a rancid smell we immediately associate with stale coffee.
The overall shelf life of coffee depends on environmental conditions. The biggest enemy of roasted coffee is exposure to oxygen, followed by moisture, temperature, and light.
- Oxygen: fresh coffee’s worst enemy
The best way to prolong the shelf life of roasted coffee is to keep it away from oxygen, which accelerates staling reactions in many ways. All freshness-indicating aromas are very susceptible to oxidation and can be lost quickly after roasting, while lipid oxidation causes rancidity.
Sensory studies have shown that even a very small amount of oxygen strongly affects coffee quality over time. Figure 1 shows just how powerful the impact of oxygen is on shelf life. In this example, shelf life was reduced by approximately 20 times when oxygen levels during storage increased from 0.5 per cent (a low-oxygen environment) to 21 per cent (in air).
Another point to note is the dramatic drop in shelf life between 0 and 5 per cent oxygen. Shelf life effectively halved when oxygen concentration increased from 0.1 to 1.1 per cent. Every additional 1 per cent increase in oxygen concentration accelerated degradation by another 10 per cent. While such low levels of oxygen are difficult to achieve in packaging, it shows just how crucial it is to keep coffee away from oxygen in order to preserve freshness.
- Moisture: the second biggest threat
Roasted coffee is a dehydrated product, with only approximately 0.75 per cent moisture on a dry basis. This makes coffee shelf-stable against enzymatic and microbial spoilage. However, it also means that coffee can absorb moisture from the air, which accelerates the loss of volatile compounds. Physically, the brittle and fragile structure of the bean gradually loses its characteristic crunchiness and crispiness due to moisture uptake.
Even though temperature gets probably the most attention when it comes to coffee storage, it is in fact the third most important environmental factor affecting coffee freshness. This means that we should turn our attention to storage temperature once barriers against oxygen and moisture have been established by selecting appropriate packaging.
The effect of temperature on CO2 and volatiles release can be described by the Arrhenius equation, which states that chemical reactions proceed at a faster rate at higher temperatures.
This has been demonstrated by Maria Cristina Nicoli’s coffee staling experiments. In this study, every 10°C increase (studied between 5°C to 45°C) resulted in an increased rate of degassing by 1.5-fold for whole beans and three-fold in the case of ground coffee. These losses are most dramatic within the first few days of coffee storage. Refrigerating or freezing coffee has been used as a method of preservation because staling reactions slow down at colder temperatures.
Light acts as a catalyst in many chemical reactions and can accelerate staling. Luckily, most types of packaging protect coffee from exposure to light. Packaging is the most important factor we can control to improve shelf life.
Let’s look at some standard options for coffee.
Air packaging is the most basic type of coffee packaging. It protects the coffee from light and moisture, but the presence of large amounts of oxygen means that staling reactions will proceed easily. This type of packaging can only be applied to coffee that had been previously degassed to avoid bursting from continued CO2 release. Adding a one-way valve can help solve this problem. It helps expel excess air, while CO2, due to its larger molecular weight, tends to sit lower, blanketing the beans. Some CO2 and volatiles will still escape, dulling cup taste.
Vacuum packaging is an old and widespread technique, applied to a variety of foods. By reducing the amount of available oxygen inside the package, it helps prevent oxidation reactions detrimental to shelf life. Its main disadvantage is that the vacuum effectively sucks the aroma out of the coffee: volatiles escape as soon as the package is opened.
Modified atmosphere packaging (MAP) is a highly effective way of excluding oxygen from product packaging and thus gain unprecedented improvement in shelf life. MAP is currently used by large international coffee companies to preserve coffee freshness for overseas export. Nitrogen flushing of coffee bags is also becoming popular with small specialty roasters, although sensory outcomes are debated.
Pressurised packaging is an interesting option. It allows CO2 and volatiles to build up until pressure pushes volatiles back effectively into the beans, where they dissolve in coffee oils or bind to melanoidins. The pressure helps spread the oil inside the call walls, forming an additional barrier to oxygen inside the beans. As a combined result, aging in pressurised containers can improve cup quality. The drawback is the higher cost of materials.
Active packaging is a system in which the product, package and the environment interact in a positive way to extend shelf life. The most widely used active packaging technology today are oxygen scavengers, mostly iron powder. The scavenger can be added to air or MAP packs in sachets or incorporated into the inner walls of the packaging. Table 1 shows the advantages and drawbacks of each type of packaging, and the shelf life it can help achieve.
To help coffee survive for longer, look at packaging options first, and storage temperature second. Once a packaging type is chosen, during storage, temperature becomes the key factor affecting primary shelf life. So, bring on the freezer-once you’ve got your bags sorted.
This article appears in the August 2020 edition of BeanScene. Subscribe HERE.
For more information, visit www.coffeesciencelab.com.au