Motorized Mash Mixing
By Donald A. Put
Republished from BrewingTechniques' November/December 1994.
Tired of hand-mixing your mashes? A home-built motorized mash mixer can free up your time for other preparations and improve the consistency of your mashing routines.
Mechanical mixing of the mash has been a part of large-scale brewing protocols for some time. No matter what mashing system or schedule is used - straight infusion, temperature controlled, or decoction - the grains and water must be thoroughly mixed. Although numerous techniques have been used to ensure a good mix (such as premixers for the grist and water), most of them translate poorly to home brewing batch sizes.
One technique, however, fits very well within the constraints of the home brewing setup: a motorized mash mixer. Such a device can be used continuously during mashing or intermittently during temperature boosts, thereby freeing up the brewer's time for other aspects of the brewing process while increasing the efficiency of the mix.
The idea of scaling a large-scale mixer down to the home brewing level came to me as I was standing in front of my tun, trying to add a decoction back to the rest mash slowly and carefully, while stirring with my hand-held paddle. This is not an easy task when you are brewing a 10-gal batch alone, and it becomes even more tedious as the batch size increases. A motorized mash mixer, however, makes easy work of complete and consistent mash mixing.
PADDLE DESIGNWhat I attempted to do with this setup was to provide a folding action that eliminates temperature gradients without introducing hot-side aeration (HSA). Because the paddle is well below the surface of the mash,* the only disturbance of the air/mash interface comes from the entry point of the stainless steel shaft on which the paddle is mounted.
The ideal movement of the mash seems to be of a toroidal nature, a nice top-to-bottom or bottom-to-top mix. Although a slight amount of whirlpool action is desirable, it can become a problem if the mash just swirls around the tun without actually homogenizing. Also, if the mash spins too fast, it can create a vortex that can increase the aeration of the mash with possible HSA problems appearing in the finished beer.
FABRICATIONFabrication of the mixer is relatively straightforward, but it does require a bit of mechanical ability. The mounting bracket for the motor-paddle assemblies is made of 1/8 in. X 1 in. X 1 in. angle iron (Figure 1), and even though I welded mine together, it could just as easily be bolted together. Because the angle iron never contacts the mash (it should be painted to prevent corrosion), leaching iron into your mash will not be a problem. If you have access to stainless steel for the bracket, by all means use it, but it is not a necessity. You could even use a sturdy wooden base to support the motor and paddle.
I purchased my motor from American Science and Surplus (Skokie, Illinois; item #23378 in catalog 81); it is a continuous-duty GE Minagear (General Electric Co.) that delivers 30 in./lb of torque at 153 rpm. I used a 11/2-in. (pitch diameter 1.35 in.) pulley on the motor and a 6-in. (pitch diameter 5.85 in.) pulley on the paddle assembly. The effective rotational speed of the paddle with this arrangement is 35 rpm, and the mechanical advantage of the larger pulley increases the torque to 90 in./lb. A waterproof switch mounted on the motor's body gives easy access for starting and stopping; the switch is plugged into a ground fault interrupter (GFI) socket to ensure safety. One extra safety feature - not shown in the schematics - is the addition of a pulley shroud to prevent accidental contact with the belt and spinning pulleys.
The flange-type pillow blocks are mounted flange to flange on the top and bottom of the angle iron support, effectively sandwiching the support between them (Figure 2). They have phosphor bronze bushings and wick-type lubrication, which prevents any of the lubricant from finding its way down the shaft and into the mash. This arrangement gives the shaft-paddle more than enough stability. You could use sealed ball bearings here, but it would really be overkill for the load and rpm requirements.
The pillow blocks, pulleys, and V-belt can be purchased from any industrial supply or well-stocked hardware store. Although I chose to use a V-belt drive for my prototype, any type of power transmission can be used (direct, timing/blower belt, or chain, for example). The important thing is to keep the rpms low, within the published figures I have seen that range from 5 to 50, with a concentration around 30.
The paddle configuration was the hardest part of the fabrication to deal with. Toroidal movement demanded something other than the normal fan-blade or agitator geometry (Figure 3), although these might provide a basis from which to build because they can be easily bent to try different blade angles. I found some specialized drum and pail mixers, but they were too small in diameter and too expensive. Because I was trying to keep the cost of the materials to a minimum, I decided to make my own paddle out of some 20-gauge stainless countertops that a friend had recently removed while remodeling his kitchen.
The paddle is shaped to sweep very close (1/8 in.) to the bottom of the tun (to prevent scorching during temperature boosts) while pulling the mash down at the center and pushing it out at the bottom and up along the sides. I realize that the illustration legends that describe the bends in the paddle are not the most scientific method, but they are nevertheless the most easily understood. Imagine that the paddle is freed from the page and is floating on a horizontal plane, print side up, in mid-air. The black areas of the paddle would be bent down toward the floor at approximately a 45° angle; the medium gray areas would be bent up toward the ceiling at the same angle. Remember, this assumes a counter-clockwise rotation. If your motor setup ran clockwise, you would have to reverse the bends. This shape results in a nice, slow mix of the mash. The paddle shape that I use, however, is not the only one that will work, and I encourage experimentation in this aspect of the design.
The largest grain bill I have used with this paddle configuration is 25 lb, and it seems to be the limit for the toroidal movement without raising the paddle. My paddle is designed to clear the thermometer probe. As a result, I am limited to raising the paddle only about 1 in. Because of this, I added a small, two-bladed helper paddle (Figures 4 and 5) that is mounted on a stainless steel split collar, which allows it to be positioned anywhere on the shaft and below the surface of the mash, to assist the normal paddle when larger grain bills are used (though I rarely brew anything stronger than a Belgian Dubbel or Trippel, and 25 lb is the maximum I need for these styles).
The "helper" paddle is very easily fabricated from the same thickness stainless steel as the main paddle. Two grooves are cut into the side of the collar at a 45° angle with a hack saw and the blades are attached using silver solder. Each paddle blade is 2 in. wide by 5 in. long. After they are soldered in place, they are bent off the horizontal into a somewhat relaxed V with a 140° angle. The paddle is then mounted to the shaft with the opening of the V pointed toward the bottom of the tun; it is also offset from vertical alignment with the main paddle by 90°, thus the trailing edge of the auxiliary paddle forces the grain into the leading edge of the main paddle. The two-bladed paddle works so well that I thought about making three of them and using them in place of the main paddle. You could add or subtract the necessary number of paddles to accommodate different grain bill sizes.
The shaft that supports the paddle is an old 3/4-in. down-tube from a Sankey keg that was no longer usable because of a few small pinholes along its length. This kind of tubing is available from the larger industrial supply houses, but any tubing or solid mash-resistant material could be used as long as it provides the necessary support. I cut a slot in the end of the tubing that the paddle fits into. To make the tubing more solid when the paddle is bolted into it, I cut a piece of 5/8-in. hardwood doweling in half lengthwise and installed it into the end of the tubing. This prevents the tubing from distorting when the paddle mounting bolts are tightened. If you use a solid shaft, this would not be a consideration.
The complete mixer assembly (Figure 6) is mounted to the keg with one 1/4-in. bolt, which passes through the angle iron bracket and the keg handle, and one 1/4-in. J-bolt (actually a U-bolt cut to resemble a "J"), which clamps onto the other handle. The J-bolt gives some side-to-side adjustability. It takes only a few minutes to mount/dismount the motorized mixer.
THE MIXER IN ACTIONIn practice, the mixer works very well, so well that I never plan to return to the old "hand-jive" method of stirring. During the protein rest, the mash moves very slowly around the circumference of the tun while coming up the sides and down the center. If you keep your eye on a specific piece of grain, you can see it make its way from the edge of the tun to the middle in less time than it takes to travel about one-third of the circumference. As the mash thins out, the whirlpool movement becomes more pronounced, but it still takes only about one-half to three-quarters of the tun circumference for a piece of grain to circulate.
The mixer works well with any mash thickness, but I have found that a normal water to grist ratio (1.25-1.33 qt/lb) allows for freer movement. With regard to an increase in tannin extraction as a result of motorized mixing, which theoretically may cause some husk damage, I have noticed no perceivable difference in the beers brewed with an intermittent or a continuous paddle schedule versus those brewed using hand mixing. As long as the pH is within the proper limits, and the grain has been milled correctly, I would expect no extra tannin extraction or sparging problems resulting from husk damage; I have not had these problems with my system.
Advantages: Without a doubt, a constant, mechanical mixing action, which introduces no air into the mash, will create a more uniform temperature and grist mix in the mash than you can possibly hope to obtain using a hand-held paddle. Advantages include the following:
Disadvantages: As in life, there are no panaceas in brewing. Disadvantages of this system include the following:
HERE'S TO INNOVATIONAlthough this system is not for everyone, those who pursue it will be rewarded, not necessarily with better beer, but by having one less aspect of the brewing process that needs to be "hands on." All in all, the mixer cost me about 8 h of my time and $50. To me, it is a worthy addition to my home brewery. As with any design, improvements can be made, and I encourage you to use this information to build upon.
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