Sorghum Production: Milling and Cooking

A great natural sweetener, sorghum is unique and delicious. Sorghum production begins with milling, and many cooking techniques can be used to make sorghum syrup. Sorghum production is practiced by farmers and supported by consumers who are looking for a natural self-sustaining sweetener.
November 6, 2012
Part of sweet sorghum syrup production is the day of milling and cooking the juice from the sorghum canes. A group effort, especially for the cooking and skimming process, makes producing this homegrown natural sweetener a fun, community event with a sweet end result.

Photo by Lara M. Ervin

Covering a modern homesteading tradition in his book, Sweet Sorghum (Lara M. Ervin, 1992), George Kuepper discusses ways to transform this natural sweetener from plant to plate.  Sorghum production is a practice used by many farmers to produce sorghum syrup. Sorghum is a natural sweetener that benefits health, body and food. This excerpt is from Chapter 12 and Chapter 13; it describes milling and cooking methods of sorghum production. Learn more about growing and harvesting sweet sorghum by reading Growing Sorghum: A Natural Sweetener. 

You can purchase this book through Kerr Center Publications: Sweet Sorghum.   

Sorghum Production: Mills

The three-roller mill has become the standard in extracting cane juice. It was invented by Pietro Speciale, the Prefect of Sicily, in 1449. The rollers were fastened vertically, and this type of mill supplied almost all of the world’s sugar for close to 350 years. The design is typified by horse- or mule-driven mills, commonly seen in very small sorghum operations and in traditional demonstrations at festivals and fairs. They are still easily found in the South and Midwest. Many have been adapted to gasoline engines or electric motors. Larger and more modern mills are of a horizontal design but still have three rollers. Power is usually supplied by stationary engines, electric motors, tractor pto, or hydraulics.

Most small- and medium-scale producers purchase and refurbish old mills, although a few have built new ones. The major problems with using old mills are metal fatigue and the difficulty in finding replacement parts. While most foundries have the capability of making them, the cost is often prohibitive. Like row binders, get a second mill for backup, or keep several old mills of the same model for parts. Maintaining a mill requires the same common sense applied to other equipment upkeep. Because bearings work at low speed, lubrication can guarantee a long life if not otherwise abused. Running the mill at a correct feeding rate and speed will make it last longer and decrease downtime. It is important to use only food-grade grease on any portion of the mill where the lubricant might come into contact with either stalks or juice. This includes all roller bearings. Food-grade grease can be bought from most apiary (beekeeper) supply houses and from suppliers to the food industry.

Milling Sorghum

Efficiency of juice extraction is the goal of milling. It is accomplished by proper maintenance of the roller surfaces, accurate spacing of the rollers, correct roller speed, and skilled feeding of the mill. Most rollers on three-roller mills are grooved. The surfaces are neither convex nor concave, so any adjustment is uniform along the length of the rollers' interface. After several seasons of use, the center of a set of rollers may become concave since cane is normally concentrated there during milling. Regrinding and regrooving are required for continued extraction performance. When rollers have been repeatedly ground and can no longer be properly adjusted, they can be sleeved with new metal and lathed to original size. Spacing of rollers on three-roller mills is standard; however, each unit should be checked for imprinted instructions to the contrary. A 3/8 inch gap is needed between feeder and top rollers, and a 1/16 inch gap between the expeller and top rollers. Adjustment bolts are positioned on each end of the smaller rollers and are used to set the gaps.

The current recommended speed for a three-roller mill with a top roller diameter of 12 inches is 7.5 rpm for the top roller. The speed of top rollers on smaller mills is closer to 9 rpm. This contrasts with earlier recommendations that called for top roller speeds of 9 to 11 rpm for large mills and 10 to 12 rpm for small mills (Freeman et a1., 1986). A study in Tennessee found top roller speeds ranging from 4.8 to 11.4 rpm, with most averaging close to 8 rpm (Wilhelm and McCarty, 1985). Trials carried out by farmers have shown that as much as 20 percent of the potential juice yield is lost by increasing mill speeds from 7.5 to 12 rpm (Wilhelm, 1987). One way to check a mill for extraction efficiency is to weigh 100 pounds of stalks, mill them, and weigh the extracted juice. An effective mill will squeeze about 45 to 50 pounds of juice per 100 pounds of cane.  Efficient feeding of the mill requires steady pushing of the stalks, butt end first. Capacity of mills will vary, but maximum capacity should be maintained as much as possible. A properly set and operated mill will expel stalks with joints breaking over as they leave the last roller. Cane waste should be dry to only slightly sticky.

Mounting sorghum mills for optimum performance is important and highly dependent on the situation. Most mills are stationary mounted. Sorghum cane is brought from the field to the mills for juice extraction. Increasingly, mills are placed on wagons or trailers, and crushing is done in the field. In all cases, no matter the power source, mills must be firmly anchored and set at a comfortable, safe height for hand feeding. Most mills intended for draft-animal power have rollers fixed vertically, although some horizontal ones were built. Traditional horse-driven mills are often supported by upright posts sunk into the earth. A long, wooden drive pole is laid across the mounting bar of the drive shaft. It is important that the end of the drive pole opposite the horse hitch is counterbalanced to reduce stress on the mill and mountings.

Other Methods of Sorghum Production and Processing

Where draft animals are used to operate the mill, a light lead pole is attached at 90 degrees to the drive pole, with a rope run back to the halter. This makes the animal walk in a circle. It is wise, especially with skittish stock, to use a lightweight linkage between the hitch and the drive pole to prevent unseating the mill in event of a runaway. The draft power needed at the end of the drive pole is actually quite low, so a light linkage can deter costly damage. In some instances, small-scale producers have substituted small tractors at the end of the drive pole to run their mills.

One southeastern Oklahoma farmer actually lets the tractor "run free." The tension of the drive pole causes the wheels to naturally angle and guide the tractor in a circle, without a driver. Since the potential for an accident is high, such arrangements are discouraged. Energy from electric motors or gas or diesel engines has also been successfully applied to vertical horse-driven mills. Some form of speed reduction must be used between the power source and mill. Truck and tractor transmissions have often been recycled to serve this purpose. Horizontal mills, designed for engines and motors, have also been modified to draft-animal power. Horse-driven treadmills are one example. Horizontal mills are usually made with gear reduction attachments, allowing direct hookup to the power source. Belt and pulley drives from farm tractors are common and easily adjusted.

Engine speeds usually range from 375 to 425 rpm (Walton et al., 1938). Adjustments in engine rpm should always be based on proper roller speed. Safety is a major consideration at any mill. Shielding of belts and gears and an easily accessible, rapid shutoff should be maintained.

Sorghum Production:  Cooking With Wood

As costs of fossil fuels rise, cooking with gas will become less economical and less suitable from a planetary viewpoint. Using firewood to heat evaporator pans will probably increase, especially among those considering a small-scale enterprise. A damper built into the chimney helps control heat during cooking. However, avoid closing the damper as much as possible. Restricted airflow results in the release of unburned hydrocarbons from the wood fire, adding to air pollution and reducing fuel efficiency. A garden hose with a jet nozzle provides added control, especially with an extremely hot fire. It is also handy for extinguishing the fire when cooking is finished for the day.

Other Methods of Cooking Sorghum

Several farmers use sawmill slabs for fuel. Depending on the size of the firebox, wood is cut into pieces three to five feet long. Old fence posts sawed in half also work well. Scrap construction wood is good, but check for any pieces with exposed nails. It takes little force to puncture the bottom of an evaporator pan if something sharp is carelessly thrown against it. While using a renewable fuel has its advantages, there are drawbacks. Controlling the fire in a consistent manner is difficult. More labor is needed for stoking and damping. Opportunities for contaminating syrup with particulate matter also increase. Burning wood to produce steam is a better option if affordable.

Cook Sorghum Using Steam

Steam supplies the ultimate control in cooking. Training and licensing is required to install, operate, and maintain a boiler. It is also expensive and better suited to a large-scale operation. However, smaller boilers are available and may be adapted to a smaller facility. Steam is highly desirable- from a production standpoint. Besides heat for cooking and hot water for cleaning, steam can be used to preheat the sorghum juice.

Preheating to a slow boil of 190 to 205 degrees F:

1. Allows mixing of alpha-amylase enzymes to break down starch before cooking.

2. Increases formation of coagulants, which are easily skimmed before cooking.

3. Permits filtration of juice.

4. Allows juice to be held overnight at a high temperature instead of refrigeration.

Preheating tanks can be of many types. The most efficient are rectangular with a skimming trough at one end. A wooden skimming pole is used to remove coagulants in one motion. Placement of heating elements is critical and should be well studied before new tanks are designed and built. Preheating and skimming allows the processor to filter juice. Cold, raw juice cannot be filtered, only coarsely strained. The abundance of fine, un-coagulated materials in raw juice clogs filters.

Preheating and skimming removes most of them. Diatomaceous earth filters are one type used in filtering sorghum. They are expensive and may not be appropriate to small-scale operations. Preheating can be a drawback when the processor holds the juice for an extended period of time, like overnight. Heat increases the formation of color and flavor compounds that may reduce the quality of the syrup.

Multiple-Pan Systems

Another benefit of steam is that it enables the use of multiple-pan cooking systems. It is common to find these systems among intermediate- and large-scale producers. 

Production and Processing

Typically, one pan is used to make semisyrup. One type of semisyrup pan is a modified Stubbs pan (Henrickson, 1958). Semisyrup leaves the pan at 38 to 45°Brix and is transferred to a finish pan. Finish pans can be a batch design, continuous-flow, or another modified Stubbs. Some larger setups also combine a vacuum kettle as a third cooking unit to finish the syrup. Final evaporation can be done at a lower temperature with a vacuum kettle. This reduces exposure to high heat, the threat of scorching, and development of color compounds.

Sorghum Production on a Large Scale

A diagram of a large-scale processing center is shown in Figure 18. It is based on an operation in Arkansas. Steam supplies the heat for all activities. Sorghum pummies, the cane waste, furnish a large portion of the fuel for the boiler.

A second type of large-scale processing facility is presented in Figure 19 and depicts an enterprise in eastern Tennessee. A batch cooking system is employed, and steam can instantly be turned on and off without the danger to the cooking pans as described in Chapter 7. Semisyrup or syrup can be completely emptied from a pan without chasing the liquids. Juice is converted to semisyrup in the three large pans as individual batches. Each batch, in turn, is routed to the single finishing pan, where it is cooked to syrup and removed. Batch cooking is not limited to large, steam operations. A description of a small-scale batch method is in the book Foxfire 3 (Wigginton, 1974).

This excerpt has been reprinted with permission from Sweet Sorghum by George Kuepper, published by Lara M. Ervin, 1992.