Effective Prevention of Insect Infestation in Flour

Departments - QA

December 11, 2014

Author’s Soapbox

In this article we use the terms “mesh size” and “micron.” What does each of these mean?

  • Mesh size. Figuring out mesh sizes should not be complicated: Simply count the number of openings in one inch of screen (in the U.S.); the number of openings is the mesh size. A 30-mesh screen means there are 30 openings across one linear inch of screen; a 60-mesh screen has 60 openings, and so on. As the number describing the mesh size increases, particle size that can get through the screen decreases. Although higher numbers equal a finer material, mesh size is not a precise measurement of particle size.
  • Micron. A micron is a precise measurement of the particle size. A micron is one/millionth of a meter or one/25 thousandth of an inch. Smaller micron numbers equate to finer material.

Confused flour beetles are discovered in 12-week-old flour without finding a source on the premises. GMPs seem to be in order. Receiving records indicate no presence of an insect problem. Where is the source? It could be in the flour itself. Flour-beetle eggs can pass through flour sifters and become adults in a few weeks—or more quickly or slowly depending on temperature. Establishing a 30-day use-by date is ideal, but what if flour is used in a dry bakery mix with a nine- to 12-month use-by date?

Sifting flour through a fine-mesh opening will separate out the insects, but tiny eggs can pass through with the flour. However, impact machines are effective in destroying all insect life stages. The use of sifting and impact machines as a final step before packaging and/or bulk loading will prevent infestation by grain-product insects. But what size sieve opening and screen type are necessary? What type of impact machine is needed? What is the best magnet type? Following are some practical solutions.

What is the best size opening?

The U.S. standard 30-mesh screen (about a 600-micron opening) commonly recommended for sifters in receiving and blending operations is not suitable for grain-product insect management in production applications. The filament diameter is so large that insect control is limited. An increased sieve surface would be required to maintain a comparable throughput rate with the smaller openings. Research at Kansas State University (KSU) has shown that insect eggs and other life stages are removed by a 180-micron opening or about 80-mesh screen. Although a 180-micron size is considered not feasible for production capacities required today, it does exist. A 60-mesh screen has 250-micron openings providing improved protection. A 250-micron opening will exclude insect adults and large in-star larvae, but insect eggs and small larvae can still pass through. When compared to an 80-mesh size, the 60-mesh is a good compromise for product purity and capacity. To prevent high numbers of insect fragments, you need to reduce the number of insects that reach the impact-machine stage.

What is the best screen type?

The screen can be nylon or wire (magnetic stainless steel). Nylon screen resists fatigue better than wire but can absorb moisture and lose screen tension. Frequent inspection of the sifter is necessary to verify lack of tears or openings in the screen. Another option is using magnetic stainless steel (400 series) wire, as metal fragments can be removed from the sifted material with a strong magnet; this is not possible with nylon. Wire is thinner than nylon allowing the screen to have higher sifting capacity. Installing screens using mechanical stretchers to a specified tension will keep the material well stratified and maximize throughput.

Why use an impact machine?

Due to the risk of grain-product insect eggs, impact machines are necessary to prevent insect infestation in finished flour and are widely used in the milling industry. Research at KSU has shown that all life stages of confused flour beetles were destroyed when infested flour was passed through an impact machine with 14 one-inch pegs with a smooth liner. The majority of insect fragments left after impact originate from adult insects, so it is important to remove as many insects as possible through sifting. The double-rotor impact machines with adequate diameter and tip speed handle large flour capacities, yet remain effective in destroying insect eggs.

What is the best magnet?

Several types of magnets are used in food manufacturing, such as plate, cartridge, and grate. The magnet size and strength must be carefully selected based on the product flow rate (density and velocity) and size of potential contaminates. Plate and cartridge magnets are effective when the material flow rate does not exceed the magnet capability; a problem in the food industry. An excellent magnet following a sifter and impact machine is the rare-earth grate-drawer type installed with an easy-removal design for timely inspection and cleaning. In either screen type, the magnets must be regularly inspected and cleaned to ensure effectiveness.


The use of sifting and impact machines followed by a magnet as the final steps before packaging and/or bulk loading will prevent infestation of flour products by grain product insects. This combination process is recommended for flour that exceeds a 30-day use-by parameter or is to be used as a dry mix with a longer shelf life. Extending shelf life to a longer parameter can be accomplished through cool storage; cool being defined as 60ºF—the temperature at which insect development stops. Producing clean insect-free flour is the result of intelligent effort.