The Effect of Moisture and Stacking Height on
High-Alpha Variety Hop Pellets.
| Abstract | Procedures |
| Purpose | Research Report |
| Hypothesis | Results |
| Experiment Design | Conclusion |
| Materials | Bibliography |
Abstract
The purpose of my experiment was to see if moisture content or stacking height had an affect on heat generation in high-alpha variety hop pellets. I had hoped to learn if one of these conditions might be the cause of recent Yakima area hop warehouses spontaneously combusting. My hypothesis was that the stacking height would cause the temperature to rise, but that the added moisture would not. I expected the temperature to slowly rise over time. Some constants in my experiment were:
- The variety of hop pellets used in each simulation
- The year the pellets were harvested
- The environment the experiment was conducted in (15 degrees Celsius, standard pressure)
- The type of bag that the pellets were placed in
- The Styrofoam boxes the bags of pellets were contained in
The manipulated variables in my experiment were the amount of simulated stacking height added and the moisture content. The responding variable in the experiment was the change in temperature of the pellets over time. After the experiment I was able to conclude that neither stacking height nor the added moisture helped raise the temperature of the pellets. To gain further understanding, future tests might include reading temperatures for a longer period of time or increasing the simulated stacking height.
Purpose
The purpose of this experiment was to find out how moisture content and the method of stacking affect heat generation in Columbus variety hop pellets. I became interested in this experiment after hearing about several hop warehouses catching fire. The results from this experiment will benefit the owners of hop warehouses and the hop companies who own the hops because they lose a significant amount of money when hop warehouses catch fire.
Hypothesis
My hypothesis was that heat generation in high-alpha variety hop pellets is caused by the amount of pellets put in each bag. Also that added moisture would not cause heat generation in the hop pellets. I based my hypothesis on an article of a similar experiment that I read; "Moisture is not believed to influence the thermal course of events once the sample has reached ambient temperature." Also from the opinion of professionals of the industry; "The first reaction was that there was too much moisture in the hops, I think we've come to a conclusion now that that's not the case at all."
Experiment Design
The manipulated variables in the experiment were the moisture content and the pressure or stacking weight. There were 8 simulations for each variety of hop, all with different amounts of weight and 4 with a different moisture content.
The responding variable was the temperature change over time of the hop pellets. This was measured in degrees Celsius by use of a temperature probe.
In my experiment I had 7 experiment groups and 1 control group for each of the 2 varieties. The control group having no added moisture and the same amount of pressure per square cm as used in the industry.
The constants in my experiment were:
- The amount of hop pellets
- The variety of hop pellets
- The size of the hop pellets
- The crop the hop pellets came from
- The beginning temperature of the pellets
- The type of bags the pellets are put in
- The Styrofoam that the bags are put in
- The boxes the Styrofoam was put in
- The environment that the experiment was conducted in
- The type of temperature probe
- The amount of time the temperature was taken for
Materials
| Quantity | Item |
| 12 kilograms | High-alpha hop pellets variety 1 |
| 12 kilograms | High-alpha hop pellets variety 2 |
| 82 kilograms | Sand |
| 4 | 50.8 cm. length of 6 in. (15.24 cm.) pvc pipe |
| 4 | 25.4 cm. length of 6 in. (15.24 cm.) pvc pipe |
| 20 | Plastic bag |
| 4 | Styrofoam box |
| 4 | Cardboard box to fit Styrofoam box |
| 16 | Styrofoam cover |
| 15 meters | Duct tape |
| 1 | Scale |
| 1 | Measuring cup |
| 360 ml | Water |
| 1 | Temperature probe |
Procedures
Research Report
In this report hops, hop pellets, and spontaneous combustion will be explained. It will also explain the occurrence of hop fires and the losses caused by them.
HOPS
General
Hops are plants that are used in the process of brewing beer. Breweries use hops in their brews not only because it is illegal not to, but also because of several benefits it gives their product. Hops give the beer a bitter taste and aroma, they also have an antibacterial effect on the beer, making it last longer. Hop plants are grown in tall vines that produce hop cones. Later, when the plants have grown large enough, the cones are picked, dried, and stored for later use. Two varieties of hops with high-alpha content were chosen for this experiment.
Variety One
Variety One hop matures mid season to late and it has very good pickability. The rounded cones are anywhere from medium to large in size and are tightly packed. The aroma of this variety of hop is pleasant. It contains 14-16% alpha-acids and 4.5-5.5% beta-acids. This variety is becoming more and more popular with the brewers because of it's high-alpha content. It is mainly bred for it's alpha content, but is also popular for it's oil profile. Some sloppiness occurs while baling 200 lb. (90.718 kg) bales as a result of the dense cones.
Variety Two
Variety Two hop matures mid season and it has excellent pickability. The moderately long cone is heavy and tightly packed. This variety of hop contains 12-14% alpha-acids and 4-6% beta-acids. It's aroma is quite heavy and herbal. This variety is known for it's high alpha-acid content and it's good aroma profile. These hops dry normally.
HOP PELLETS
General
Hops are dried so that they can be put into bales and then stored for later use. But some are made into hop pellets and sold that way. Hop pellets are the hop bales broken apart and without the extraneous materials such as rocks and sticks. Then that is taken and milled in a hammer mill, the left over powder is taken and passed through a pellet mill. Pellets have several advantages over regular hops. Those are:
Pelletizing, in the simplest process, consists of breaking up the bales, removing a small amount of extraneous materials, milling in a hammermill, and passing through a pellet mill. Then they are packaged under a vacuum or in an inert atmosphere. These were the types of pellets that were used in the experiment.
Concentrated (Type 45)
Removing the fibrous portion of the hop cone makes this type of pellet. Thus producing pellets with double the alpha-acid content and half the original weight. However, some high-alpha varieties of regular hop pellets have the same alpha-acid content as the concentrated pellets.
Modified Stabilized
In this type of pellet, the hop powder is mixed with 2-3% magnesium oxide. Then it is made into hop pellets normally. By doing this, the alpha-acids are converted to their magnesium salts, providing better utilization than regular pellets. Magnesium salts isomerize more easily than regular alpha-acids. This type of pellet is mostly used to make isomerized pellets.
Modified Isomerized
For this type of pellets, modified pellets are warmed at about 50° C. for 7-14 days. This process converts 90-95% or more of the alpha-acids to iso-alpha-acids. These pellets are considered to be one of the most economic forms of bittering, utilizing 55-60% or more. Their storagablity is excellent with or without cold storage. However, if you wish to preserve aroma, cold storage is desired. You may add isomerized pellets into a brew at any stage of boiling while maintaining high utilization. A disadvantage to isomerized pellets is the uncertain affect it has upon flavor. There is a small amount of evidence that changes occur in certain components while the pellets are being heated.
SPONTANEOUS COMBUSTION
Spontaneous combustion is the ignition of a material that, in most cases, reacts with the oxygen in the air, resulting in the evolution of heat. This reaction is known as oxidation. Usually, this oxidation process is very slow, generating so small an amount of heat in the material that the change in its temperature cannot be measured. However many materials react vigorously with the oxygen, producing a significant amount of heat. Under these circumstances, the temperature increases until the rate at which the heat is generated, equals that at which the heat is being carried away from the material. When the material is well insulated, it prevents enough heat to dissipate, causing the temperature to quickly reach the ignition point of the material. In the rare case of hops, the middle of the bale is sufficiently insulated to prevent enough heat to dissipate, causing the hops to quickly heat up to it's ignition point. This same process can occur to hop pellets when they are stored in large superbags.
THE OCCURRENCE OF HOP FIRES
In the past 2 years there have been 3 warehouses containing hop bales that have spontaneously combusted. Each of these fires cost 2-3 million dollars in damage. The cause of the fires is being investigated; several possible reasons are the moisture content, stacking height (pressure), and the temperature at which the hops come from the farms. However, it is now commonly believed that the reason is not the moisture content, but perhaps the temperature at which the hops are baled and shipped from the farms. I interviewed Bob Mondor, Director of Hop Purchasing at a local hop company; he supports this statement. "The first reaction was that there was too much moisture in the hops, I think we've come to a conclusion now that that's not the case at all." In the future, more testing will be conducted to find the cause of hop spontaneous combustion and steps will be taken to prevent losses in the warehouses. Some of these steps are installing sprinkler systems to control the fire until fire fighters can arrive, and to have people feel the bales for condensation or heat.
ResultsIn this experiment I wanted to see if moisture content or stacking height affected heat generation in high-alpha variety hop pellets. I did this experiment because I wanted to see what the cause of the recent hop fires was. The results of my experiment showed that all the temperatures decreased to slightly below room temperature. However the temperature of the samples with added moisture decreased slightly less rapidly than the regular samples. Also the temperature of the samples with the lower amount of stacking pressure decreased at a slightly slower rate than the samples with more added stacking pressure. See the following charts and graphs.
Variety One
|
0 kg
|
2.268
|
6.804
|
13.608
|
0-m
|
2.268-m
|
6.804-m
|
13.608-m
|
|
|
initial
|
59.0
|
59.0
|
59.0
|
59.0
|
59.0
|
59.0
|
59.0
|
59.0
|
|
30min
|
58.7
|
61.4
|
61.2
|
60.7
|
60.2
|
61.0
|
61.1
|
59.3
|
|
2hr 30m
|
53.5
|
55.2
|
55.4
|
53.2
|
54.7
|
55.0
|
54.8
|
52.6
|
|
4hr 30m
|
47.5
|
48.5
|
50
|
46.9
|
49.4
|
49.2
|
49.1
|
47.2
|
|
6hr 30m
|
42.1
|
42.8
|
44.4
|
41.2
|
43.5
|
43.7
|
43.3
|
42
|
|
10hr 30m
|
33.0
|
33.6
|
34.8
|
32.4
|
35.2
|
35.0
|
34.6
|
34
|
|
16hr 30m
|
23.7
|
24.5
|
25.2
|
23.5
|
25.8
|
25.9
|
25.5
|
25.2
|
|
22hr 30m
|
18.2
|
18.8
|
19.5
|
18.1
|
20.0
|
20.2
|
19.7
|
19.5
|
|
28hr 30m
|
14.6
|
15.1
|
14.9
|
14.5
|
16.0
|
15.8
|
15.3
|
15.6
|
|
34hr 30m
|
12.0
|
12.7
|
12.9
|
12.3
|
13.6
|
13.7
|
13.5
|
13.2
|
|
40hr 30m
|
10.8
|
11.1
|
11.4
|
10.8
|
11.6
|
11.8
|
11.5
|
11.5
|
|
46hr 30m
|
9.9
|
9.8
|
10.2
|
9.9
|
10.7
|
10.5
|
10.2
|
10.1
|
|
52hr 30m
|
9.8
|
9.5
|
9.6
|
9.3
|
9.9
|
9.8
|
9.5
|
9.7
|
|
72hr 30m
|
9.3
|
9.3
|
9.2
|
9.2
|
9.3
|
9.2
|
9.2
|
9.3
|
Variety Two
|
0 kg
|
2.268
|
6.608
|
13.608
|
0-m
|
2.268-m
|
6.804-m
|
13.608-m
|
|
|
initial
|
54.8
|
54.8
|
54.8
|
54.8
|
54.8
|
54.8
|
54.8
|
54.8
|
|
30 min
|
56.2
|
57.7
|
56.1
|
54.7
|
53.0
|
53.8
|
54.3
|
47.6
|
|
2hr 30m
|
47.9
|
49.6
|
48.0
|
46.1
|
47.4
|
48.0
|
48.8
|
44.4
|
|
4hr 30m
|
41.2
|
43.1
|
41.5
|
39.7
|
41.9
|
42.3
|
42.9
|
41.4
|
|
6hr 30m
|
36.0
|
37.8
|
36.1
|
34.6
|
36.0
|
36.5
|
37.0
|
36.3
|
|
10hr 30m
|
27.7
|
29.1
|
27.6
|
26.7
|
27.3
|
28.0
|
28.6
|
28.4
|
|
16hr 30m
|
19.6
|
20.8
|
19.8
|
19.3
|
19.6
|
20.3
|
20.8
|
20.5
|
|
22hr 30m
|
14.9
|
15.9
|
15.1
|
14.9
|
15.3
|
16.1
|
16.9
|
16.1
|
|
28hr 30m
|
12.1
|
12.7
|
12.4
|
12.4
|
13.0
|
13.2
|
13.3
|
13.0
|
|
34hr 30m
|
10.8
|
11.2
|
10.7
|
10.7
|
11.4
|
11.4
|
11.7
|
11.3
|
|
40hr 30m
|
9.5
|
9.9
|
9.7
|
9.6
|
10.4
|
10.4
|
10.1
|
10.3
|
|
46hr 30m
|
9.1
|
9.1
|
9.0
|
8.9
|
9.5
|
9.5
|
9.3
|
9.3
|
|
52hr 30m
|
8.6
|
8.6
|
8.6
|
8.4
|
9.0
|
8.8
|
9.0
|
9.1
|
|
72hr 30m
|
9.1
|
9.0
|
9.1
|
8.6
|
9.3
|
9.0
|
9.3
|
9.0
|
From the experiment, the fact can be learned that within the parameters of my experiment, the temperature did not pose a threat of spontaneous combustion. The results of the experiment were halfway compliant with my hypothesis; the moisture content did not cause heat generation, however, neither did stacking height. My hypothesis was that the stacking height would cause heat generation in the hop pellets and that the added moisture would not. From the results of the experiment I can conclude that my hypothesis should be rejected and accepted at the same time. Because I hypothesized incorrectly when I said that the stacking height would cause heat generation and correctly when I said that the moisture content would not cause heat generation. The results of the experiment make me wonder if the temperature readings would have been different if I had a larger quantity of pellets, or if the results would have been different if I had simulated more stacking height on the pellets. Some systemic errors might be the surface area/volume ratio and the fact that in my experiment I couldn't use the same material in which the pellets are stored. If I were to do the experiment again I might try to simulate higher stacking by using 20 kg or more or I might insulate the pellets better so as to not let as much heat escape. Also I might try the experiment with more hop pellets per simulation, in which case I would also need to use more weight.Variety One 
Variety Two
Variety One
Variety Two
BibliographyHamilton, Jesse A., Warehouse Smolders; Cause Pondered, [Online] Available http://www.yakima-herald.com/cgi-bin/liveique.acgi$rec=15269?home, November 19, 2000
Jones, J. C. and Raj, S. C., "Self Heating
and Ignition of Hops" Journal of the Institute of Brewing 94 May-June 1988:
139-141
Machnicki, John, Spontaneous Combustion-Explained,
[Online] Available http://www.arson-codes.com/spontcom.htm, December 6,
2000
No Author, Hop Pellets, [Online] Available
http://www.john-i-haas.com/article.html#pellets, December 6, 2000
No Author, Hop Union, [Online] Available http://www.hopunion.com/hvcb.htm,
January 1, 2001
No Author, Technical Data Sheet, [Online] Available
http://www.john-i-haas.com/pellets.htm,
December 6, 2000
Roeder, Tom, Hop Warehouse Destroyed in Blaze, [Online] Available http://www.yakima-herald.com/cgi-bin/liveique.acgi$rec=7333?home, November 29, 2000
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