Considerations when Using NIR in a Snack Food Production Environment

Near infrared instrumentation is widely known to provide quick and accurate measurements for moisture and oil, but did you know that it can also provide instantaneous, precise percentages of flavorings and seasonings for snack foods?  

Flavorings are applied to snack foods either in slurry form, flavored powder or flavored oil. The flavoring itself might be a cheese, onion, garlic or some other type of flavoring. Many malfunctions such as clogged sprayers or jets and pump failures can lead to incorrect flavoring loads. Consistent application and distribution of concentrated flavorings is critical to producing a consistent final product.  

Measurement would traditionally be done by weighing discrete samples within a known volume container before and after coating.  

% Coating = 100 - (Mass of uncoated Product / Mass of coated product) x 100

If the solids ratio is kept constant, flavoring levels will increase/decrease proportionately with the coating weight.

The NIR gauge measures the oil content, and provided the solids ratio is kept constant, the percentage of flavorings can be inferred.

Flavorings are pumped as slurry and sprayed or dripped into a seasoning tumbler. The rotary action of the tumbler distributes the flavoring throughout the product. Measurement is made on the product exiting the tumbler or a couple of feet downstream of it. A minimum of 2” product depth is essential on the conveyor/vibro-conveyor for stable readings. Measurement on the bucket conveyor is not recommended.

Measurement Considerations

NIR is a surface-biased measurement. Light penetration is frequently just a few millimeters so it is important that the surface is representative of the bulk. For this reason, measurement locations are often chosen to be at or after a point at which the product has been rotated or turned over.

The larger and more un-homogeneous the product, the more variable the reading will be. The reason for this is that product orientation affects the manner in which radiation is backscattered. This “presentation noise” can be smoothed out by increasing the damping.

A sufficient bed depth of product should cover the transport mechanism so that all of the backscattered signal can be attributed to product and not the conveyor belt. Discontinuous flow of product requires some gating device to freeze the gauge output when product is not within view.

Furthermore, chips and other product immediately exiting an oven or fryer will often “steam,” resulting in a scattering of NIR radiation and unstable readings if an air purge diffuser is not used to disperse the steam.

Ambient temperatures may exceed 50ºC at the exit of driers and fryers. These temperatures will adversely affect the performance of the electronic components so air cooling or water cooling might be required.

The gauge should be installed in a location where it is unaffected by plant equipment vibration.

Factors Affecting Measurement Accuracy

In addition to those factors mentioned under “Measurement Considerations,” how well the gauge was first calibrated can strongly influence the measurement accuracy.

In calibrating a gauge, it is extremely important that the gauge “sees” the sample in the same form it will when it makes the measurement. In some powder formulations, the product appears similar on line to how it appears off line. However, with fried products and baked products, there is often a greater degree of translucency when the material is hot. For this reason, the product is best calibrated on line. Typical locations where calibrations are best carried out on line are at the exit of the fryer and at the exit of an oven. When carrying out calibrations on line, it is important that a moisture or oil range can be generated; this may result in some wastage.

As the range will be small, it is recommended that a minimum of seven samples be measured/collected for lab analysis. Additional samples will provide greater confidence in the span and zero. If calibration work cannot be carried out on line, then an off line calibration should be installed in the gauge using samples covering a range of +/- 2% of target moisture or oil; then a zero correction should be made on line when the line is producing “good” product.

Oil and moisture measurements are most useful when made at the exit of a fryer or oven, as they can be used in automatic or manual control to correct the process before out-of-spec product is produced. In these hot locations, it is important that the gauge is cooled, and the window is air purged. With respect to the product, the gauge should be positioned a minimum of 1 meter downstream from the exit of the fryer/dryer and 6–10” above the chips. Flavor and moisture measurements are made at the exit of the seasoning drum. The former can be used for process control purposes, and the latter as a final quality control check prior to packaging.