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Sweet success: the latest options for on-line measurement of sugar solids.

When it comes to measuring sweetness, processors have more choices than a kid in a candy store.

Determining sugar solids in a syrup, beverage, blended product or any other kind of liquid or slurry is a vital aspect of quality control. But the significance of solids measurement goes far beyond QC. Accurate monitoring of solids on line, coupled with the right kind of control system, can keep product in spec during the highest-speed blending while eliminating sweetener giveaway.

Processors who need to keep track of sucrose solids have a wide array of options, from hand-held hydrometers to on-line refractometers and spectroscopes. Choosing one involves balancing considerations of cost, precision and versatility. Some systems can measure parameters other than sucrose solids, integrating those measurements into a total control system.

Although the major alternatives for measuring solids operate very differently, most of them have one thing in common: They work by extrapolation. What they actually measure is the liquid's density, from which the amount of sugar solids can be inferred.

The earliest - and still common - use of this principle was in the hydrometer. Basically, a hydrometer uses a discovery by Archimedes more than 21 centuries ago: It's possible to determine a liquid's density by floating and weighing a solid object in it.

In their earliest incarnations, hydrometers had the virtues and the vices of simplicity. "Essentially, it's a stick with a cork on it, with a bunch of little lines," says Kyd Brenner, vice president of the Corn Refiners Association, Washington, D.C. Those early devices depended heavily on operator know-how; there's an old joke about the reading depending on the height of the observer.

But the principles of hydrometry can be applied in some surprisingly sophisticated ways. The Florida Department of Citrus tests samples of fruit juice through a computerized system that involves suspending a nine-pound stainless steel plument in a gallon of the juice. A programmable logic controller from AEG Schneider Automation, North Andover, Mass., compares the weight of the plument in juice with its weight in water to determine its specific gravity, from which the controller then calculates the Brix measurement. (The system measures acid simultaneously, also by comparing weight. The juice is weighed, its pH is raised to 8.1 by alkali titration, it's weighed again, and the controller compares the results.)

No matter how sophisticated hydrometry gets, however, it essentially remains dependent on batch sampling: It's impossible to measure a moving stream by a method that depends on floating. But other methods can measure a liquid's density while it's flowing, making them suitable for on-line measurement and control.

Sweetness and light

Refractometers determine density by assessing how much a liquid bends light rays. The denser the liquid, the slower light travels through it, and the more refraction results.

Refractometers vary in sophistication even more than hydrometers. They range from hand-held models to on-line systems that can provide instant feedback to blenders, valves and other plant machinery.

On-line refractometers usually use a principle known as critical-angle refractometry. This involves shining light through a prism in contact with the surface of the liquid. If the angle between the light ray and the surface is too shallow, the light ray will reflect off the surface instead of being refracted into the liquid. The precise angle at which refraction ceases and reflection begins to occur is the critical angle, and it varies depending on the liquid's density. By determining the critical angle, a "smart" (i.e., processor-equipped) refractometer can figure out the liquid's density - and from that, its sugar-solids content.

From an operational standpoint, the biggest advantage of refractometry is its thoroughness. It measures density directly, rather than calculating it via flow rate. "It's a direct measurement as opposed to an inferred measurement," says Todd Hodges, sales and marketing director for Liquid Solids Control Inc., Upton, Mass., a manufacturer of in-line process refractometers. In practical terms, this means a refractometer won't be affected by entrained air or other substances that can throw off a system based on mass flow.

Another advantage of refractometers is their relatively small size. They can be mounted almost anywhere, on an eight- to 12-inch spool piece. They also are relatively inexpensive for online systems. A refractometer for a two-inch pipe would cost about $6,700, not including installation, Hodges says.

Perhaps the biggest problem with refractometers is keeping the crucial parts clean. If the prism or sensor heads get fouled by product, accuracy could be affected. Refractometers are available with self-cleaning options to accommodate especially aggressive process fluids.

Fixing the Brix

Pacific Coast Producers, an agricultural cooperative, had been using hand-held refractometers in its Oroville, Calif., plant to guide production of fruit cocktail. The goal was to blend water with just enough corn syrup, fruit juices and other sweeteners so that the total Brix of the final product - water, sweeteners and fruit collectively - would reach a minimum standard.

Both the testing and the blending were done by hand. "The system was put together as a manual system," says plant engineer Robin Dodson. "It was pretty archaic." Operators who blended the sweeteners and water found it difficult to get the mixture to within a few degrees Brix of the goal. To further complicate matters, the incoming fruit varied in Brix from day to day, requiring constant adjustment of the Brix target for the syrup-water mix.

"They might miss on the low side, which would affect quality, or they might miss on the high side, which would affect profit," Dodson says. The practice was to miss on the high side, but that meant Pacific Coast Producers was giving away syrup.

The company installed an in-line refractometer from Liquid Solids Control and tied it into a PLC running man-machine interface software from Intellution Inc., Norwood, Mass. Lab tests determine Brix from a sample of each day's fruit shipment, which leads to the Brix setpoint for the cocktail's fluid. This setpoint is programmed into the system, which then guides the blending process by operating the valves, solenoids and other devices that control the tank infeeds.

Going with the flow

Mass-flow measurement is another way to extrapolate sugar solids from a solution's density. Flow measurement, usually through coriolis flowmeters, gauges density by the speed with which a fluid travels though the device. A coriolis flowmeter uses the centrifugal force exerted by a diverted flow - as measured by vibrations in the diverting pipe - to determine the flow rate. These vibrations also indicate density: The denser the fluid, the less the pipe vibrates.

According to Simon Schoner, density measurement marketing director for Micro Motion Inc., Boulder, Colo., the coriolis mass flowmeter uses its density measurement to determine percent concentration. The accuracy of the percent concentration measurement is a function of the absolute density accuracy and the density differential between components in a two-component stream.

This technique is applied to sugar and corn-syrup solutions to determine Brix or percentage of HFCS. This is based on well-established data that characterize the effects of concentration and temperature on density. A "two-curve concentration method" can be used in cases where the effect of concentration and temperature on density is unknown. This method is effectively an inter-polative reference table based on an array of values obtained from laboratory testing.

"It is not always enough to know how much liquid has passed through a meter," Schoner says. "Measuring the flow and percent concentration simultaneously allows an operator to know - in real time - the exact amount of sugar that has passed through the meter. In the case of truck loading or automatic blend control, the total amount of mass may determine whether the vessel is full, but it is the net flow of sugar that determines how much sugar you have for your blend or purchase."

Flowmeters cost about the same as refractometers: Their price depends on the diameter of the pipe, but generally, microprocessor-equipped units are in the $6,000 range. Their big advantage is that they can measure flow rate and density at the same time. "You're really getting two measurements for the price of one," says Bob Hunsicker, vice president of engineering for ABB K-Flow Inc., Millville, N.J.

One big disadvantage of mass-flow measurement of sugar solids is that flowmeters can be thrown off by entrained air or other substances in the solution. This makes them less accurate than other methods in some applications.

Seeing red

One of the up-and-coming methods for on-line solids measurement is near-infrared (NIR) spectroscopy. Like refractometry, NIR spectroscopy uses light to determine the solids content of a solution. But NIR is unlike any of the other on-line methods in that it doesn't extrapolate solids content from density. Instead, it uses physical chemistry to measure sugar solids directly.

NIR shines light of 700 to 2,500 nanometers in wavelength, separated into discrete wavelengths, at the substance being measured. Various components of the substance will absorb light of different wavelengths. The difference between the light emitted and the light reflected back can then be read and interpreted by the spectrophotometer.

NIR is markedly faster and more accurate than any other on-line technique. Its margin for error is only 50 percent greater than wet-chemistry lab tests (the most accurate possible), and the elimination of operator error more than makes up for that margin, says Allen Bickel, food and agricultural marketing manager for the NIR systems division of Perstorp Analytical, Silver Spring, Md.

NIR has other advantages. It can be applied to high-solids liquids and slurries, even total solids like cheese. It can measure other parameters besides sugar solids, such as moisture, protein, fat and alcohol.

Its big disadvantage, at least in comparison with single-constituent sensors, is price. NIR systems cost between $50,000 and $85,000, not including installation. This puts them out of reach of low-volume processors; the ideal NIR user is a high-volume processor whose lines are dedicated to a small number of products. That's why an increasing number of corn syrup refiners are attracted to NIR, Brenner says.

"It's not something you buy in a box, stick it in your factory and let it run," Brenner says. "It takes sophisticated users."
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Author:Demetrakakes, Pan
Publication:Food Processing
Date:Sep 1, 1996
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