Fish Meal On-Line Moisture Measurement with Near Infrared Transmitter

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Fish meal is usually made from “oily” fish such as mackerel, menhaden, pilchard or other local oily fish.  Very little fish meal is made from white fish (<2%) such as cod and it is referred to as “white fish meal” with an oil content of less than 6%.  Fish meal is also “fish” with only water and some oil extracted.  While historically used as a fertilizer additive fish meal’s protein and nutrients have shifted the market to animal feeds.

Fish meal is generally manufactured by the wet press method where the fish is cooked to coagulate the protein and then pressed to yield a solid phase (press cake, about 30% of feed) consisting of bones, protein and oil and a liquid phase of water, dissolved or suspended solids (press liquor, about 70% of feed).  The press is usually a dual screw rotating in opposing directions with increasing shaft size that decreases available volume in the press thereby increasing pressure and forcing the liquid component out through the perforations in the housing.  The press squeezes out as much liquid as possible to increase oil yield and quality while reducing moisture content that in turn reduces dryer energy consumption and increases dryer capacity.  The press reduces moisture from approximately 70% to about 50% and oil content to 4%.  The ground press cake is mixed with the processed stick water after the evaporation unit operations.

Most modern fish meal processing plants utilize either a direct fired two stage rotary dryer or an indirect steam dryer dependent on feed stock and required capacity.  The dryer exit moisture target is typically less than 11% to inhibit microbial growth and increase structural integrity of the fish meal cake or pellet.  The specific fish meal product being manufactured determines the appropriate moisture target.  The dryer temperature is usually less than 195?F (90?C) to avoid degradation of the available nutrients and protein within the cake.

Amazingly, most moisture measurements at dryer exit are conducted manually with a grab sample that is either “felt” for moisture content or measured off line with a di-electric, capacitance or microwave devices.  These techniques can be sensitive to density and temperature variations or simply the experienced operator out with the flu!

Moisture variations are due to changes in feedstock, density and throughput.  Typically air flow and dryer temperature are adjusted to maintain fish meal moisture set point at dryer exit.  Near infrared techniques allow for off line measurement of moisture, oil and in some cases protein.  Protein measurement off line usually requires a full spectrum NIR analyzer with chemometrics  calibration modeling programs such as PLS (Partial Least Squares) or PCR (Principle Component Regression).

On-line, Near Infrared photometers such as Process Sensors Corp. MCT460 NIR Transmitter allow for moisture and oil measurements.  Product variability in terms of bone and fish scales contribute to variation in sample handling and fish meal moisture measurement on-line with NIR transmitters.  Typical moisture range at dryer exit is 0-15% with reproducibility of 0.2% and accuracy of 0.2 to 0.3% fish meal moisture.

Fish oil is mainly made up of long chain fatty acids and due to absorption shifts in the Near IR has more variability than fish meal moisture measurement.  Fish meal composition changes in bone and fish scale can contribute to measurement variability.  Protein measurement is best made in the lab with Soxhlet Extraction or full spectrum NIR.  On-line photometers can trend protein at best, and are not suitable for on-line protein measurement.

The NIR moisture transmitter (photometer) is typically mounted 8 to 18 inches above a conveyor equipped with an air purge to keep the sight glass clean and requiring 3 to 10 psig air pressure.  The MCT460 Moisture Transmitter supplies a large 5.7 inch display of moisture percent and is also capable of displaying operator selected trend cycle simultaneously.  The Operator Interface (OI) supplies analog outputs and Ethernet communications.  Moisture and Oil Measurements from a single transmitter can be displayed on one Operator interface.

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Photo I MCT 460 Operator Interface

 

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Photo II MCT460 Moisture Transmitter

 

Illustrations of a typical Fish Meal Flow Chart and Process Outline are shown below.

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Illustration I Fish Meal Process Flow Chart

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Illustration II Fish Meal General Unit Operations

 

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Illustration III Fish Meal Press

 

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Graph I Fish Meal Trend Chart vs. MCT460 Moisture Measurement

 

Conclusion

NIR Moisture Transmitters can provide accurate and reliable fish meal moisture measurement at Dryer Exit to improve product quality, reduce energy costs, increase structural integrity and quality assurance with a quick Return on Investment (ROI) based on reduction of fuel costs and increase in product quality.