Small Scale Pelleting of Hops

Introduction

The availability on the market of small and medium-sized pelletizing equipment, along with the growing interest in locally produced "artisan" hops for brewing, has led to a surge of interest in pelleting hops at a smaller "farm scale." This article provides an overview of the basic information needed by a farmer interested in pelleting hops on the farm. Farm-scale pelleting has been a topic of interest for energy markets for some time, and many of the same technologies and skills can be applied to hop pelleting as well.

Why Pelletize Hops?

Hop pellets are a common commodity in the brewing industry and are often preferred to whole cone hops. There are several reasons why this is the case. First, hop pellets are easier to store and transport, due to their higher bulk density. Thus, it may be cheaper and easier to transport hops long distances after they have first been pelletized. Second, hop pellets can be easier to handle, due to both their density and their "flowability" relative to whole hops. This can make them preferable when used in industrial-scale automated processes. Third, pelleted hops have a tendency to have greater release of aroma compounds, presumably due to the way that the cells of the hop cone are crushed during the pelleting process.

Typical Hop Density Values

Whole Hops - ~50 g / litre

Ground Hops - ~100 g / litre

Pelleted Hops - ~500 g / litre

On the other side of the argument, pelleting can be an expensive proposition, and may not be all that beneficial for transportation if the hops are being grown close to their end use. Second, the pelleting process tends to heat the hops, which could vaporize valuable compounds and reduce the value of the hops. Ultimately, the decision to pelletize or not may depend on the specifics of your operation.

How Does a Pelletizer Work?

A pelletizer operates by passing a steel roller over a thick metal plate called a "die" and forcing material through holes in that die. This type of process is referred to as "extrusion", and is commonly used for a variety of food and non-food products such as animal feed and wood fuel pellets. Pelletizers tend to be designed in one of two configurations: a flat plate die with rollers in place above the die, or a ring-shaped die that rotates around rollers that are mounted inside the ring. This extrusion process causes three critical processes to occur to the material that is being extruded.

Densification

As the hops are being pushed through the die, resistance to flow causes the hops to be compressed in the longitudinal direction. Simultaneously, the tapered shape of the holes in the die causes the hops to also be compressed in the transverse direction. This two-directional compression causes the hop particles to squeeze and crimp together into a dense cylindrical shape. 

Heating

Compressing the hops into a tight compact shape creates a large amount of heat from friction, both from the particles rubbing against the die and from the particles sliding against one another. Both the die and the hop material are heated as a result. It is not uncommon for operating temperatures to approach the boiling point in a pelletizer.

Binding

As the hops are compressing and heating, compounds within the hops have a tendency to grow soft and sticky, especially if the hops are in the right moisture content range. This sticky material serves as a binder to hold the hop pieces together so that it retains its densified shape after it leaves the pelletizer. There are several compounds that can naturally serve as a binder in biomass, including starch and proteins (in feed pellets) and lignin (in fuel pellets). In the case of hops, oils and other secondary metabolites are the likely binding agents.

Practical Steps in the Pelleting Process

When making hop pellets with a small-scale pelleting machine, several steps must be followed to ensure successful operation. Keep in mind throughout that the specific optimum conditions and methods can vary from machine to machine, so it is important to carefully vary your inputs until the resulting pellets are of acceptable quality. In general, the steps involved in producing hop pellets consist of preconditioning, followed by pelleting and then cooling.

Preconditioning

Preconditioning consists of adjusting the properties of the hops feedstock until it is suitable for use in the pelletizer. The most common forms of preconditioning are moisture control and particle size control. Moisture control consists of increasing or decreasing the moisture content of the feedstock. The optimum moisture content can vary depending on technology being used. The optimum moisture content of hops for pelleting is usually in the 10-15% range. You can test the moisture content of your feedstock by selecting a small but representative subsample of the hops, measuring its mass, then placing it in a warm oven (recommended temperature 60°C) for 24 hours. Measure the mass again, and calculate the moisture content as follows:

MC = 100*(M_before – M_after) / M_before

Where MC = moisture content (percent), M_before = mass of sample before drying, M_after = mass of sample after drying

Size control is another form of preconditioning that is often used.  Grinders can be employed to reduce the hop cones to a heap of ground particulate material. Our experience has shown that whole hop cones tend to become ground up by the pelletizer without the need for a dedicated grinder. However, the resulting particle size may or may not be optimum for maximizing the extraction of compounds during brewing.

Pelleting

Pelleting is the main component in the process, where hops are fed into the pelletizer machine and are transformed into pellets. In general, the rollers should be tight to the die – every pelletizer is likely to have a different optimum setting, although in our small pelletizers, we find that "finger tight plus ¼ turn" works well.

The next important trick that we've learned is that it's sometimes helpful to start each run with a “premix” material that establishes a dynamic plug that can provide back pressure to the pelleting process, resulting in more reliable and high-quality pellets.  Our experience has been that a 70/30 mixture of switchgrass and Distillers Dried Grains works well for this task.

Some pelletizers seem to need to warm up in order to reliably option. We have found that our smallest equipment does not need any preheating – perhaps the smaller size allows for a quicker rise to acceptable temperatures and successful pelleting. 

Lastly, the manner in which the feedstock is added to the machine can have a large impact on its performance. Our experience with our open hopper device is that dumping all of the feedstock in at once results in better performance, perhaps because the thick layer of feedstock helps trap moisture in the machine, so that feedstock doesn't dry out before it enters the die.

Cooling

If all goes well, you should have a few premix pellets coming out of the machine, followed by green hop pellets. However, you will notice that the pellets are both warm and fragile. At this point, it is best to spread the pellets out on a drying/cooling rack until they have hardened and cooled to room temperature. A lumber frame with a window screen stretched over it can work well for this purpose.

Controlling the Pelleting Process

It is not easy to control the pelleting process, and successful operation can be as much an art as a science. However, the three main parameters that impact pellet quality are pressure, duration, and temperature.

Pressure

Longitudinal and transverse pressure within the die are highest at the point where feedstock enters the die, and decrease towards the exit. The magnitude of the pressure can be controlled by adjusting the die geometry (inlet taper, die hole diameter, die hole length), adjusting the pressure between the roller and the die (the "roller gap"), or by modifying the properties of the feedstock (particle size, particle shape, particle strength). The specific ways that these characteristics control pressure within the die are not well understood, nor are they easy to measure, and remain a research topic at present.

Duration

The amount of time that the feedstock spends under pressure can also impact the pelleting process, owing to viscoelasticity and other properties that occur over time during pelleting. Generally speaking, a longer duration will result in denser pellets. This can be controlled by adjusting the speed at which the pelletizer runs, or by selecting a die with a different thickness.

Temperature

The temperature at which the pelleting process occurs also impacts the pelleting process, whereby binders tend to be activated only at elevated temperatures, but the loss of aromatic volatile compounds also increases as temperature increases. Friction is often the only source of heat in pelleting, but it is typically sufficient to raise the temperature of feedstock to near or above 100°C.  This is much warmer than is ideal for hops, and a variety of measures are used keep temperatures as low as is practical.

Potential Problems

Several things can go wrong with the pelleting process, each with its unique issues and potential solutions.

Washthrough

If the pressure, temperature, and duration within the die are not high enough, the feedstock particles do not fuse together into a pellet. In the extreme, this results in a phenomenon called "washthrough," in which the feedstock flows through the die and exits in much the same condition as it entered. Conditions that are more prone to washthrough include low moisture content levels in the feedstock, larger die holes, and thinner dies.

Clogging

If the pressure, temperature, and duration within the die are too high, the feedstock particles can become lodged within the die holes, resulting in clogging. When the pelletizer clogs, often the only thing that can be done is to turn off the machine and manually

clear the die holes using either a drill or a hammer and punch. Conditions that are more prone to clogging include high moisture content of the feedstock, small die holes, thicker dies, and rollers that are not tight against the die.

Pellets too Loose

Loose pellets can be caused by a number of factors, including low operating temperatures, high springback ratio of the feedstock, large particle size relative to the size of the pellets, low operating pressure, or high moisture content of the material. This is a critical issue for biomass fuel pellets, but may not be as important for hops pelleting.

Pellets too Dense

Pellets that are too dense are an unusual problem when pelleting, but may occur nonetheless. They generally occur when the operating pressure is much higher than is necessary.

Loss of Volatiles

Loss of volatile aromatic compounds is the primary issue impacting the value of a hop pellet, and thus is of particular importance to the hop processor. The main tool available for reducing volatilization is by reducing the operating temperature of the pelletizer. This can be done by precooling the feedstock, actively cooling the pelletizer device, reducing the operating pressure (and thus reducing the heat generated by friction), or a combination of the three measures. Recommendations for hops drying equipment are that drying temperatures be kept below 60°C, and a similar temperature limit would likely be a reasonable goal for the pelleting process.

Oxidation

Oxidation of the hops is another important quality issue for hops, causing degradation of the valuable compounds within the hops and loss of value. Oxidation can be minimized by reducing the availability of oxygen (i.e., pelleting in an inert environment) and/or controlling temperature. 

The addition of liquid nitrogen to hops in the pelleting process has become a popular means of not only purging oxygen from the system but also reducing the operating pressure of the pelleting process. 

Tips for Operation

It is important to keep in mind that different pelleting machines have different operating characteristics. As a result, methods that work well for one machine may not be ideal for the next. While researchers continue to work to better understand the process, experience with your particular machine and feedstock remains an invaluable and unreplaceable asset when pelleting. That being said, work with small-scale pelletizers at Penn State has yielded a few pointers that may be of value.

First, as noted above, repeatable performance does not occur unless we use a "pre-mix" to establish an initial back pressure in the die from which the pelleting process can begin. This "dynamic plug" seems to be an important aspect of the pelleting process. Second, while industrial operations often use steam to add moisture to samples that are too dry, we have found that liquid water is not only easier to use at the small scale, but it also yields perfectly adequate results, provided that the water is uniformly mixed in the feedstock. Third, our experience has been that slowly adding the feedstock to the inlet does not work as well as dumping a large amount into the infeed hopper and keeping the rollers covered with feedstock. It may be that in the machines we utilize, keeping a greater amount of feedstock in the machine reduces the potential for the feedstock to dry out before it is pushed into the die holes.