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Recommended Health Practices for the Operation of Historic Grist Mills

Recommended Health Practices for the Operation of Historic Grist Mills
Theodore R. Hazen

Grain Source

1. The production of a clean flour and meal product starts with the quality and cleanliness of the grain purchased. The source of all grains should be from a reliable grain company or from a commercial miller.

2. All grains used in the mill should meet the current Food and Drug Administration standards for filth and extraneous matter. The use of grain from a local farm source, including that which may have been grown on a demonstration farm, is discouraged unless such grain has been cleaned by use of modern equipment normally used by commercial millers.

Storage Time

Grains used in the grinding process are subject to contamination from birds, rodents, stored product insects, and moisture. The degree of protection the stored grain has against these contaminants will determine the period of time for storage. The mill operator or mill manger of the grist mill should consider the following questions:

1. Are storage bins (or other facilities) of mouse and rat resistant construction?
2. Are bins (or other facilities) constructed to prevent the entrance of birds or the dropping of bird contaminants (feathers, feces, insects, or other contaminants such as grease and oil) into the storage area?
3. Will the temperature of the storage area be below 70 degrees most of the time during the storage period?
4. Is the storage facility free from overhead leakage?
5. Is the storage area free from flooding, excessive dampness or other moisture conditions that will cause the grain to mildew?

In addition to contamination that may occur during storage, grains generally contain the eggs of several kinds of insects. These eggs hatch during storage. The length of time required for this to occur varies depending on the temperature at which the grain is stored. Ambient storage temperatures below 60 degrees will extend the time required for the eggs to hatch to several months. Temperatures above 70 degrees will cause more rapid hatching of the eggs and shorten the acceptable storage time. When the temperature is between 70 degrees and 85 degrees, the hatching time is between 30 and 80 days.

Accordingly, grain storage periods during the colder months of the year may be longer than during the warmer months. Good management of the mill requires that length of storage be limited so as to afford maximum control of the stored product. The quality of grain milled during any day or week relates directly to the quantity of unprocessed grain required. Storage of the grain must be closely controlled.

Grain Storage Facility

Two types of storage may be practiced, bulk or bin. Either of these types of storage may be practiced, but the bulk storage of grain will usually depend on the availability of supplies and the amount of grain processed. In areas where a supplier is available within a reasonable distance, the bulk storage of grain is not recommended. Remember, the smaller the quantities of grain, the faster it moves and the less problem you will have with stored product insects.

Bin storage is the storage of cleaned grain in hoppers or bins that provide a gravity feed to the grinding millstones. The quantity of grain in these bins should be managed so storage time is kept to a minimum. Again, the time period during which grain should be stored is determined by the temperature during storage.

The bins or bulk storage facility should be constructed to exclude rodents, insects, and birds. It should also be constructed and located (especially bulk storage facilities) to maintain the grain in a dry condition. While cool temperatures will slow the hatching of insect eggs, storage in basements or other below ground facilities is to be discouraged because of the possibility of flooding or moisture seeping through exterior walls.

Storage bins and bulk storage facility should be cleanable. Cracks and crevices in in wooden bins make cleaning difficult or impossible. Grain or grain dust retained in these cracks and crevices for an extended storage period may contribute significantly to the presence of insects and insect fragments in the final product. Cracks and crevices should be filled with a caulking material which is non-toxic to humans.

Bulk grain storage facilities should be completely emptied and thoroughly cleaned after each lot of grain is used. New lots of grain should not be added to previous supplies or a problem with stored product insects will occur.

Grain storage areas should not be located in the basement and attic areas since these areas contain the two prime ingredients for insect infestation, heat and moisture.

Grain Elevators

Grain is usually delivered to the grist mill at ground level so it must be lifted to bins or hoppers above the grinding millstones. Elevators are usually used for this purpose. Their designs may vary but generally these elevators are movable buckets attached to a continuous cloth belt. Grains elevators at grist mills are usually used on a periodic basis to fill the bins or hoppers. Following each use, the elevator should be thoroughly cleaned. This cleaning can be best done with a bristled brush and a vacuum cleaner. Thorough cleaning requires the buckets and belt to be rotated so all buckets and conveyor to which they are attached can be cleaned. This also includes all chutes attached to the system.

The bottom of the elevator shafts (the "boot") should also be thoroughly cleaned with a bristle brush and vacuum. To do this will require the removal of a portion of the shaft enclosure. If the elevator shaft or boot does not have removable sections, the boot should be modified to permit this cleaning. Also the legs of the elevator should be modified with removable sections for better cleaning and attached with screws.

All inspection doors or windows in the elevator legs should also be cleaned since these areas will trap material.

Grain Cleaners

Even though cleaned grain has been purchased, it may be recommended that the grain be cleaned before it is placed in the bin or hopper above the millstones. This final cleaning of the grain before it is ground will help maintain a good sanitary product. Historical type cleaners with shakers or rotating screens, and fans can be effectively used when properly operated and maintained.

As with elevators, cleaners should be used only on a periodic basis. After each use, the cleaner screens and interior surfaces should be thoroughly cleaned. A bristle brush and vacuum cleaner should be used. If the cleaner does not have removable panels to permit cleaning of all interior surfaces and screens, modification should be made to allow such cleaning.

Also, the material which passes through the screens should be removed after each use of the cleaner and discarded to waster or hauled away from the mill. If immediate removal is not possible, storage of this waste material should be in metal containers with a tight fitting lid.


The area around the edges of the grinding stones should be cleaned by a vacuum cleaner along with the millstone covers after each days operation. Also clean the eye of the millstones including the driver and balance rynd. During the colder months, lift the millstones once a week for cleaning with a bristle brush and vacuum cleaner. During the warm months, clean between the millstones each day of operation.


When starting to grind, discard to waster the initial flour discharged from the millstones. The amount to be discarded will vary. Visually check the ground product and when it appears free of foreign material, the flour or meal may be discharged into the bin for later packaging. The bins should have a tight fitting lid.

Millstone Dressing

After the millstones have been dressed, they should be cleaned with a bristle brush and vacuum cleaner, and then purged with 50 to 75 pounds of grain to remove all stone chips. A brush and vacuum cleaner will not remove all the stone chips. This material should then be discarded as waste before product grinding can resume.

Flour Elevators

See section of elevators.

Flour Bolters

See section on grain cleaners.

Flour Bins

Clean the flour bins or meal boxes after each day's operation. These bins or boxes should be constructed so they are free of cracks and crevices. Cleaning should be accomplished with a bristle brush and a vacuum cleaner.

When these bins or boxes have ground product in them, and the product is not being bagged, their removable covers should be replaced. Visitors should not be allowed to handle the ground product.


Package containers (cloth bags), scoops, scales, and a clean work surface (table) are necessary. Containers used for packaging should be stored in a clean dry place protected from dust, cobwebs, dirt, moisture, insects and rodents. Scoops and paddles used to fill the package containers should be clean, cleaned after each days use, and stored in a location free of dust, dirt, insects, rodents, birds, and visitor contact. The scoops should be constructed of non-toxic material, the product contact surfaces should be smooth, and the handle should be plugged to prevent the accumulation of flour or meal in it. The work surface (table or counter top) should be constructed of a smooth cleanable surface preferably with a rounded edging. If the surface is constructed of wood, the wood should be sealed and free of cracks and crevices. If this surface is covered with a glass or plastic cover it should be removable for cleaning between its surface and where it makes contact with the table surface.

Storage of Meal or Flour

The insect eggs naturally occurring in grains will be carried through the entire process. Therefore, storage times and temperatures are as important for the finished product as they are with the grain. See section on grain storage for these guidelines.

To help control the hatching of the stored product insects, the management of the inventory of packaged flour and meal is important. The quantity should be limited to the amount that can be sold or used during a specific time period, and the inventory should be rotated on the "first in -first out" principle. Dating packages of the finished product may also aid in controlling the length of time any one package is kept. Mechanical refrigeration that keeps the temperature of the ground product below 60 degrees will extend the storage period. However, even under refrigerated conditions
the ground product at ambient temperatures should not exceed 10 days during the winter months and 7 days during the summer. This is why freezing the product immediately after packaging is most highly recommended.

The storage facility for the finished product should be located in a clean, dry place and the cabinet should be screened or have solid sides to keep rodents and insects out.

Flour and meal storage areas should not be located in the basement and attic areas since these areas contain the two prime ingredients for insect infestation, heat and moisture.

General Housekeeping

Floors, walls, ceiling, and windows should be kept clean and free of flour or grain dust and cobwebs. Vacuuming the floors, walls, and ceilings at established frequencies will maintain the mill in a clean condition. The frequency will have to be determined by each mill operator because the amount of flour and meal ground and the frequency it is ground will govern the general cleaning requirements. In any event, it is suggested that general cleaning be performed at least once a week.

Smoking in the mill should be prohibited.

No food, drink or other liquid or solid substances should be brought into the mill by visitors. Visitors should be required to wear shirts and have no bare feet. All visitors should be in a clean attire free from sweat, dirt and grease or any material which could contaminate the mill, grains or flour products. No pets should be allowed in the mill, with the exception of the mill's cat which should be kept for rodent control. This would include artists paints and thinners. Daily cleaning shall be conducted even when the mill is not operating.

Dustless cleaning methods should be used. Vacuum cleaning is the method of choice providing the dust bag is in good repair and is capable of retaining the flour dust. Flour dust is more explosive than gun powder and 35 times more explosive than coal dust. A sparkless vacuum motor is highly recommended which is capable of vacuuming up flour dust, and all sizes of grain. A built in vacuuming system is highly recommended which is hidden in a false bin and elevator or chute, but use traditional inspection doors as access ports.

Empty all vacuum cleaners daily, and remove all waste material from the mill at the end of each work day. No waste material should be stored in the mill overnight.

Prior to general cleaning it is suggested that chutes, millstones surfaces, and other enclosed area where vacuum cleaning is not practical be blown free of grain dust.

Window washing should be done using a mixture of water and ammonia or one of the commercially available window cleaners. Such cleaning should be done at least once a year.

When flooding occurs in the mill wet cleaning with a mixture of water and of commercial cleaners is necessary before grain grinding can resume, wash all surfaces and utensils in the mill that have been effected by flood waters, remove all contaminated grain and flour products immediately from the mill as waste.

Personal Hygiene

No persons working in the milling operation should have an illness that can be transmitted through the milled grains.

Persons working in the mill should wear clean outer garments and wash their hands before handling the grain or ground products.

Pest Control

The construction characteristics of grain storage facilities, storage bins or hoppers, flour bins, flooring, and work stations surfaces have been discussed in previous sections. The exterior of the mill building should be of such construction to resist the entrance of mice, rats, and birds. Where openings to the exterior exist, these should be stopped or plugged so pests don't have an easy access to the grain or finished products. A mill's cat should be kept for rodent control and be trained not to jump onto tables, bins millstone covers, etc.

Windows, doorways, and ventilation openings should be screened with standard window screen material. Preferably such screening should be on the exterior of the building to avoid a cleaning problem from flour dust. Also, the screening of windows that may be opened should be removable to permit cleaning of the glass area.

Operational doorways should be kept closed except when in use.

Chemicals to control pests should be used, if necessary, only when the exterior of the building has been improved to discourage entry by pests. Chemicals are at times the best, but only a temporary control measure.

When chemical control is found to be necessary, care should be exercised in what chemicals are used, the amount used. Avoiding contamination of the grain, process equipment, utensils, packaging materials, and the flour and meal products is a must. Pesticides should be used according to label instruction. Unless someone at the grist mill is specially trained in pest control and particularly knowledgeable of pesticides, it is suggested that chemical controls be purchased from a commercial pest control service or operator. If chemical pesticides are stored in the mill they should be stored in appropriate containers away from areas of grain and product storage.

When the pest problem is mouse or rat control, trapping is an effective and suitable
alternative to the use of chemicals or poisons. In addition, the staff at the grist mill would be able to set and maintain all traps.

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Copyright 2001 by T. R. Hazen