A weighty matter

by Dana Holt
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Throughout the milling process,grain and its derivatives are continually being weighed. An accurate measurement at each step is crucial to assure a mill's efficiency, reliability and product quality.

A typical flour mill is divided into three main stages. In the first stage, grain is received, stored, cleaned, prepared and tempered. The second stage involves grinding, sifting and purifying. The final stage consists of flour storage, blending, mixing, bagging, palletizing and loading. Each of these stages involves several processes that require accurate weighing equipment.

After a wheat shipment arrives by rail, road or ship, the miller first seeks to verify that the amount of grain delivered matches the quantity of grain contracted. A certified hopper scale weighs the delivered grain and prints an intake protocol.

Each grain quality is stored in a specific silo or bin. To keep an exact bin record, each grain outlet is equipped with an in-line scale or a flow-rate controller.

The wheat is then transferred to the raw wheat bins. At this point, two or more wheat varieties may be blended according to a computer-controlled recipe, and then cleaned.

The next step in the process involves adding water to the wheat to achieve a pre-determined moisture content for optimum grinding characteristics. A computer calculates the amount of water that must be added, based on the quantity of wheat, to achieve the desired moisture content.

A scale is used to determine the exact quantity of grain while a state-of-the-art microwave technique measures the grain's moisture and a liquid flow-rate control unit calculates the quantity of water to be dispersed into the grain in the tri-rotor dampener.

After tempering, the wheat passes through a scourer for surface cleaning and then another dampener. A scale below the surge bin determines the exact quantity of raw product entering the grinding process. The wheat passes through a series of roller mills, plansifters and purifiers before flour, semolina and bran is transferred to the finished product section.

Near-infrared (NIR) in-line technology determines the flour's quality characteristics, such as protein, moisture and color (ash). For instance, if the protein level does not reach the predetermined quality standards, the computer instructs a micro-differential proportioning scale to add dry gluten to meet the protein standard.

Finished product scales are used between the grinding stage and the flour storage stage. Mill yield monitoring calculates the production yield between the input (raw wheat) scale and the output (finished product) scale. Only a high yield of a specific quality guarantees commercial success, and accurate recording scales are crucial.

PRECISION IN SPECIALTY FLOURS. Various base flours are stored in bins, but as more and more clients are ordering flour with individual specifications, mills are forced to produce "tailor-made flours."

The flour blending and mixing sections are typically equipped with three scales. The main scale, with a capacity from 1 to 4 tonnes, is placed below the flour bins.

A standard recipe contains a number of base flours plus mini-ingredients, such as gluten, which are proportioned and fed into the mixer by differential proportioning scales. The micro-ingredients such as vitamins and dough-improvers are proportioned into a 5-, 10-, or 20-kilogram scale. The hopper scales tip their contents into the batch mixer, which produces a uniform, finished flour.

The weighed and registered batch then passes through a sterilator, turbosifter, metal detector, magnet and sampler before it is conveyed to the bins above the main bag packer, the small parcel packer or the bulk load-out.

If the client orders a tailor-made flour, the complete order is placed in a bulk load-out bin, which loads the pre-weighed flour into the bulk tanker.

Accuracy and speed in bag packing is an important element in every flour mill because mistakes made here can cost a company money and reputation. As a result, new bagging machines have been developed. Differential bagging scales, a carousel packer, an automatic bag placer, an automatic bag closing line and a fully-automatic pallet-stacker allow capacities of up to 20 bags per minute in 25 kg units.

BAKING PLANTS. Scales also are used in bread or biscuit factories where raw materials are divided into several ingredient groups: bright flours, dark flours, crystallized sugar, powdered sugar, yeast, fats and liquids. Because each of these raw materials must be stored, conveyed, proportioned, weighed and mixed, scales are crucial. Two types of scales are used in this process.

Automatic hopper scales are used for centralized batch weighing. Several ingredients are weighed at one time, according to the recipe, in the scale's stainless steel hopper.

These sturdy scales are easy to install and can accommodate a wide range of capacities and bulk solids for use the grain milling industry and for use in many products in the food and non-food industries.

Differential proportioning scales are used to control a continuous stream of sugar or the proportioning of a "pre-blend." The rotary speed of the discharge and dosing screw can be controlled to produce a constant flow rate.

The dosing scale also is used for decentralized weighing of powdered sugar and micro-ingredients. The quantity of material specified by the product formula is proportioned with extreme precision out of the scale hopper.

Differential proportioning scales are used for small to medium quantities of granules, powders or liquids. Single ingredients are continuously dosed and weighed.

Dosing scales have a low overall height and can be used with grains, flours, germs, bran, gluten, vitamins, flavorings and salts.

For free-flowing material like grain, especially in the smaller capacity range, an automatic hopper with a tubular design works best.

The future of weighing and bagging lies in the application of differential dosing scales, which offer a continuous stream of material with a constant throughput.

The standard units of measurement — gram, ounce, kilogram, pound and ton — were established in 1875 by the International Convention of Paris. Precise proportioning and weighing are crucial elements of any quality management system in the food processing industry.

Research and development in the area of weighing and bagging will continue to be important in the year 2000 and in the coming century. Because the speed of the tubular discharge conveyor during the refill phase has to be exactly adjusted, the development of electronic differential dosing scales is not just a vision but is very close to reality.