The Phenomenon of the Swollen Pellet Foils

Every now and then – luckily very rarely – one can see big aluminum foils filled with hop pellets that are so swollen that one expects the foils to burst at any second. And that could actually happen with the result that the foil pulls in air and the oxygen will spoil the hops. How does this happen?


The main culprit can be quickly identified: heat! It is known that hops don’t like heat (at least post drying and packaging in bales after the harvest). They should always be transported and stored cold. If this doesn’t happen, the consequences can be a swollen foil. High temperatures dominate for example in the summer when shipped in non-cooled trucks or shipping containers. Even when the containers are loaded under deck, temperatures of 30°C or even 40°C, sometimes over many weeks, is not rare.


Now one has – this is simply physics – the problem, that the inert gas in the foils (most of the time a mixture of CO2 and N2) expands by heating. However, the expansion of the already present CO2 and N2 doesn’t explain the complete increase in volume. In a trial, this increase corresponded to almost 7 liters in a 20 kg foil!

Left side: Hop pellets in foil – cold storage: Right side: Hop pellets in foil – warm storage

Publications from Dr. Forster (1996 in the Swiss “Brauerei- und Getränke-Rundschau”, 2001 at the IHGC Congress in Caterbury as well as in 2002 in “Brauwelt International”) already explained that a continuous cold supply chain for hops is important and warm periods have clearly negative impacts for the quality of hops. Degradation of alpha-acids as well as damage to the hop oils are the keywords here (degradation products of the alpha-acids and various volatile substances). Some of these products were already identified including CO2, acetone, isoprene and DMS. What conclusively can’t be explained: which of these gases contributes to what extent to the volume expansion? An important insight of this study can be noted that a microbiological contamination, at that time and also today, can be excluded as a cause for the gas formation.


An anomaly with this topic: not all foils that are exposed to long warm periods show these distinct volume increases. Certainly the inert gas warms up and expands, but only some foils are swollen just short of bursting.


To obtain further insights into this phenomenon, we have started new trials and many different parameters that could play a role were evaluated:

  • Microbiological load of the whole cone and pellet hops
  • External temperature during pellet processing
  • Whole cone hop preparation during processing
  • Hop/pellet temperature during processing
  • Equipment for pelletization, machinery and quality of the foils
  • Quality of the inert gas
  • Moisture, moistness of the pellets
  • Temperature during container shipping
  • Duration of container shipping
  • Reversibility or irreversibility of the swollen foils
  • Quantified volume expansion
  • Change in climate


In conclusion, there were no significant deviations found between swollen and “normal” foils, with the exception of shipping temperature and duration. This was expected. Foils were only swollen when they were exposed to a certain time and high temperatures. Why some foils at the same temperature and time were swollen and others were not, can’t be explained. At least all of the foils of one processing batch showed similar behavior.


Even a comprehensive set of trials and analyses at the Technische Universität München –Weihenstephan could not conclusively explain the problem. However, it was found that mainly the formation of CO2 was responsible for the volume expansion. Also, a range of other gases were formed like acetone, isoprene, methyl esters, Strecker degradation products and sulfur compounds that were however only a small proportion of the complete gas makeup. CO2 is by far the most important component that is created during warm storage. What is the source or which reactions occur can only be left to speculation. A plausible explanation would be an enzymatic reaction that only occurs at temperatures between 30-40°C, for example. This would also explain the irregular occurrence of this phenomenon. Hops are a product of nature that reacts, during the vegetation period, intensively to weather conditions. Is there more and less precipitation, sunlight or heat, the enzyme capacity of hops can vary widely and consequently influence the possible reactions during a warm period. Also, the fact that the swollen foils in the last few years has somewhat increased supports this theory. The weather conditions were somewhat extreme with long dry and hot periods in hop growing areas. Combined with some longer transportation times, this makes sense (shipping companies want to save gasoline by reducing the speed of container ships). Higher temperatures over a longer period of time accelerate natural and thermal induced enzymatic reactions like the alpha-acid degradation.


Additionally, a possible handling problem of such swollen foils (ex. during the unloading of a container) there is a risk that the foils rupture or holes are created. This will cause the induction of air and the hops will spoil (oxidation!) Furthermore, it is beyond dispute that warm periods are generally detrimental for the quality of hops (keywords alpha and oil). The additional costs for a reefer (cooled) container are very strongly dependent on the targeted port (whether, for example, the higher demand for reefer containers in the opposite direction does or doesn’t exist) and of course to calculate exactly this. However, calculating the decrease in quality, for simplicity only the degradation of alpha-acids, would possibly make sense monetarily. As a bonus, one would receive fresh hops delivered! That is the simple reason, why nowadays most all sea shipments take place in reefer containers!

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