Hop Freshness – Part 1: Hop Degradation and the Supply Chain

I’ve lost count of how many times I’ve been rummaging about in the freezer only to fish out a long forgotten bag of hops and wonder whether I should still use them. Being a frugal northener they invariably end up bittering something but it’s happened often enough to give me pause for thought. Curious about what others had to say, I started to read up on the subject and have since come to realise that it would be far trickier to get an answer than I expected.

The topic of hop freshness is problematic for a whole host of reasons. Hop chemistry is astonishingly complex and there are still many aspects of it that aren’t fully understood. There’s also a vast number of variables involved that can impact freshness, from variety, growing conditions and harvest date to how long a pack of pellets has been sat about.

As one of the main raw materials we use when brewing, I wanted to know a bit more about how hop properties can change through processing or over time and what the effects might be on my beers. Convinced this was worth exploring further and undeterred by decades of dauting research, I decided to think about what I really wanted to know in terms of some more manageable questions. These are:

  • How are hops degraded?
  • Which steps in hop production and supply impact freshness?
  • How important are storage conditions?
  • How can we measure freshness?
  • What are the consequences of brewing with aged/improperly stored hops?

My aim is to address these points from a scientific perspective and where possible, to include the most up-to-date research available. Due to the breadth of the subject matter I’ve divided my review into a series of more focused posts about hop freshness.

In this post I’m going to give an overview of the main factors that impact freshness and the role of the supply chain in preservering it.

Factors impacting hop degradation

There are a number of things that can affect the rate, type and extent of hop deterioration. The majority of detrimental factors are generic, meaning they’re applicable to a lot of organic matter on earth, including us. There’s also numerous hop specific variables that can influence their stability. This is a summary of the main elements to consider when we’re talking about hop degradation.

Exposure to air

Exposure to air really means exposure to oxygen, which can can cause oxidation. This is believed to be the main cause of deterioration in hops that leads to the loss of valuable brewing compounds. The mechanisms that underpin this process are still being actively researched.


Temperature is principally an indirect problem because it affects the rate of other reactions, an important one here being oxidation. Higher temperatures mean faster reactions, which in this context we generally want to avoid. We can of course use this to our advantage as at very low temperatures, reactions may proceed so slowly they can effectively be ignored. One complication with temperature is that certain hop constituents such as volatile hop oils can be lost through evaporation if exposed to high enough temperatures.


Light, both visible and UV can be responsible for a number of adverse effects. Absorption of sunlight can lead to an increase in temperature and can also cause discolouration of pigments. One particularly harmful consequence of light exposure is photodegradation. If both light and oxygen are present, a much more reactive form of oxygen can be generated, called singlet oxygen, that’s capable of accelerating oxidation reactions. No hop specific studies could be found regarding the effects of light but these negative light-induced changes have been well documeted in the food industry.1 I suspect the scarcity of research stems from the fact that light is largely a non-issue with hops. If they’re transported, stored and packaged correctly their exposure to light will be limited.


Sometimes overlooked, time simply allows any detrimental effects to proceed for longer. It’s also worth drawing attention to the fact that organic molecules are inherently unstable and will break down eventually despite our best efforts. How long it takes before we begin to see measurable degradation in hops seems to be, as we’ll see in the next article (see Storage stability of hop resins if you’re impatient), quite variable.


Moisture is predominantly an issue in the early stages of hop processing when hops have a high water content prior to drying. The consequences can be disastrous for the crop as moisture promotes both oxidation and encourages mould growth, both of which will quickly lead to spoilage. The same adverse effects are of course possible if hops are inadvertantly exposed to humid conditions or contact with water once they’ve been dried, as they’ll readily absorb water.


Most studies have shown that the storage stability of hops can be vastly different between varieties.2, 3 Why this is the case is still very much up for debate but as a fact of life it’s handy to know about. Professional brewers will have access to a hop lot analysis providing the hop storage index (HSI) for that lot (see Measuring changes to hop resins). For most homebrewers having a general idea about the storability of a hop is usually sufficient. There are lot’s of hop variety related resources out there but i’ve found both Hopslist and John I. Haas provide some useful details about storage stability.


By form I mean the type of hop product. Whole hops are clearly a very different animal from something like a concentrated tetrahydro-iso-alpha acid liquid CO2 extract and as you’d expect, different products have very different storage characteristics. I’ve generally focused here on whole hops and pellets because that’s what I use.

The hop supply chain

The supply chain plays a crucial role in determining both hop freshness and quality. In this section we’ll examine why this is the case and consider some of the most important steps involved.

Brewers, for the most part, want to preserve the desirable qualities of hops, present at the time of harvest, until they can be used in the brewery. It’s these qualities that constitute the brewing value of hops, such as bitterness and aroma.

Unless you have a source of green hops, most of the available hop products have already gone through various stages of processing and inevitably some form of storage and transportation before they make it into our hands. When trying to evaluate the quality of hops it’s useful to have an awareness of the supply chain and at which points quality loss might have occured. Forster describes the steps of hop production and distribution as the “quality chain”, defining quality in this context as the condition of hop components when added to wort.4

This brings us to the concept of hop freshness. I would argue that freshness is a quality criteria with a similar definition to Forsters. Where a decline in freshness generally means the loss of valuable components and/or an increase in ageing substances. Freshness is however, a property that by definition, can only decrease from the original raw state; once they’re picked they aren’t getting any fresher. There’s also a need to balance freshness against other requirements of the industry such as the need for stability during storage. Lets now look more closely at some of the important steps:

Harvest: Drying

After hops are picked they’re immediately dried to lower their water content so they can be safely stored without rotting. A moisture content of between 8-12% is a fairly common target. The drying process requires both the application of heat and the removal of moist air, so careful control of the temperature, humidity, speed of circulated air and bed depth are needed to prevent quality loss.5

In addition to the risk of degrading hop components faster through oxidation the danger with high temperatures at this stage is they can lead to over-drying. This can cause hops to become very brittle and makes the subsequent baling step hard to perform without damaging the delicate lupulin glands.

Drying is a combination of science and skill and if you’re using whole hops to brew with, it’s arguably the most critical step. A variety of equipment is employed for this task with varying levels of technological advancement. This can make the experience of the operator a crucial factor in achieving uniform drying and quality across many batches.

It’s worth remembering that the drying process is itself a compromise. On one hand hops must be dried to enhance their storage stability, with care taken to prevent degradation. On the other is the need to dry batches quickly enough to ensure hops can be harvested as close as possible to their optimal maturity.

Harvest: Baling

There’s an unavoidable delay between the initial harvesting of hops and their eventual processing into other products. To improve stability and prevent spoilage, whole dried hops are compacted into rectangular bales and wrapped. This helps limit both oxygen access and moisture absorption. Bales are also easier to store as they can be stacked and more convenient to move and transport. The process of hydraulic compression must be done carefully to avoid crushing the lupulin glands, which provide some protection from oxygen to the valuable components within.

Transport and Distribution

This is a stage in the supply chain that gets far less attention but is just as significant. There’s a lot of potential uncertainty about the temperatures that hops might be exposed to during transportation. This is compounded by seasonal variations and even the fluctuations between night and day. It can also be troublesome if transport involves regions such as the tropics, which can have elevated temperatures year-round. Cold storage is naturally the preferred option if there’s a risk of unacceptably high temperatures during transport but even when used, there can be periods such as loading/unloading that can be precarious. It’s at these times that there’s less monitoring of the temperature and the duration that containers may have been waiting.


There are many points in the supply chain during which some form of storage is required. One of the more critical times is after whole hops have been baled but have not yet been processed into other products or transported to whole hop customers. This is because the packaging of bales doesn’t exclude oxygen, unlike with products such as pellets and extracts, so the hops are exposed and at risk of oxidation. As a consquence, to delay the loss of freshness, cold storage (1-5°C) of bales in refrigerated warehouses is now much more prevalent.

Hop Product Processing

The production of hop pellets must be carefully controlled as the process not only ruptures the delicate lupulin glands but exposes the hops to elevated temperatures in the presence of oxygen.

The stages are generally milling, blending, pelleting, cooling and packing. From the point the whole hops are milled into a powder, speed becomes critical as the lupulin glands are exposed. Also, while the actual pelleting process is very fast, a lot of heat is generated during compression through friction and the formed pellets must be cooled quickly. Regulating the temperature of the pellet press and the cooling speed is vital to preserve pellet quality. Once the pellets are cool they’re then packed, this can be either using a vacuum or in a protective atmosphere of CO2 or N2 to exclude as much air as possible.

Final thoughts

Hopefully it should be clear that long before hops get anywhere near a kettle, there’s a considerable number of points where freshness can be lost. We’ve seen the integral role the supply chain plays in minimising the degradation of hop components while giving us products that can be stored and transported around the world. It’s obviously in the best interests of everyone involved to preserve freshness but in an imperfect world problems still occur and why, in a nutshell, quality control remains important.

Read Next:
Hop Freshness – Part 2: Hop Bitter Acids


  1. M Bekbolet. 1990. Light Effects on Food. J Food Prot 53:430–440. https://doi.org/10.4315/0362-028X-53.5.430
  2. Laws DRJ. 1983. Commercial brewing and storage trials with yeoman hops. J Inst Brew 89:92-94. https://doi.org/10.1002/j.2050-0416.1983.tb04154.x
  3. Foster II RT, Nickerson GB. 1985. Changes in hop oil content and hoppiness potential (sigma) during hop aging. J Am Soc Brew Chem 43:127-135. https://doi.org/10.1094/ASBCJ-43-0127
  4. Forster A. 2003. The quality chain from hops to hops products. Proc Eur Brew Conv Congr. Dublin, Fachverlag Hans Carl, Nürnberg, Germany, Contribution 17.
  5. Weber KA, Jangaard NO, Foster II RT. 1979. Effects of postharvest handling on quality and storage stability of cascade hops. J Am Soc Brew Chem 37:58-60. https://doi.org/10.1094/ASBCJ-37-0058

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