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Turbidity in Clear Juices

Turbidity in Clear Juices


Published with permission from Thermo Scientific

Introduction

To produce clear juice products, the clarification process is intended to modify clarity, color, flavor, and/or stability of juices. The Oakton® T-100 quickly and simply determines turbidity in clear juice samples. Because the light source is infrared, the measurement is independent of color.

References
E.I. Benitez and J.E. Lozano, Effect of Gelatin on Apple Juice Turbidity, Latin American Applied Research, 37:261- 266 (2007).

Result Statistics
See below

Recommended Equipment
Oakton® T-100 Turbidity Meter; Oakton Turbidity Vials.

Required Solutions
Oakton Turbidity Standards; turbidity-free water (TFW), e.g., by filtration through 0.1 um filter or equivalent water.

Solutions Preparation
None

Meter Setup
None

Meter Performance Check/Calibration Verification
Note: the Oakton Turbidity styrene divinylbenzene (SDVB) polymer standards never need mixing. Do not shake the standards as this will introduce bubbles and cause them to read inaccurately until the bubbles dissipate. Check meter accuracy by reading one or more turbidity standards (included with the meter) at the level of interest. For example, read the zero (0.02) and the 1 NTU standard. The zero should read <0.1 NTU and the 1 NTU standard should read within +/- 10%, e.g., 0.9 – 1.1 NTU.

If the meter performance check fails, take corrective actions as follows:
1) wipe the vial carefully with a lint-free wipe to remove all fingerprints and liquid drips from the exterior, handle the vial by the cap only, and remeasure;
2) tap the vial gently three times and let the vial sit for 60 seconds to allow for bubbles to release, then remeasure;
3) using a clean vial (which reads <0.1 NTU when filled with TFW), pour a fresh portion of turbidity standard into the clean vial, wipe carefully, and measure.

Sample Vial (cuvette) Storage, Soaking, and Rinsing
Store vials filled with TFW. Immediately after use, clean sample vials with laboratory detergent and rinse multiple times with TFW. Note: standards may be stored in supplied glass sample vials until the standard reading is no longer in specification. See Meter Performance Check section for corrective actions when a standard reads out of specification.

Sample Storage and Preparation
Allow the samples to warm to room temperature before measurement. Mix the sample well, but do not introduce bubbles by shaking the sample. Use a little of the sample to rinse a clean sample vial twice. Mix the sample again and fill the rinsed vial.

Calibration
The meter is shipped precalibrated. The meter performance is very stable and does not require frequent calibration. If a standard reading is not within criteria, take all necessary corrective actions (as described in the Meter Performance Check section) to improve meter readings. If corrective actions fail and recalibration is necessary, perform the recalibration only on the points that failed and do so with fresh portions of standard poured into clean vials. Ensure that all fingerprints and liquid drips have been removed from the exterior of the vial with a lintfree wipe before using. Handle vials by the cap only.

Analysis
Gently invert the filled sample vial a few times to mix well the sample without introducing bubbles. Wipe the sample vial to remove all traces of liquids and fingerprints, place into meter, and press the measure key. Record the reading. Press the measure key to take duplicate measurement(s). Continue until readings stabilize and results agree, for example, within 5% or +/- 0.02 NTU, whichever is higher.

Quality Control (QC)
Recommended QC procedures include: calibration verification, turbidity-free water analysis (optional), and sample duplicates.

Notes for Improved Accuracy of Low-Level Samples
If improved accuracy is desired, pay close attention to
1) the cleanliness of the sample vials;
2) the quality of the TFW;
3) the handling of the standards and samples;
4) use of matching vials;
5) storing clean vials filled with TFW;
6) use vials free of scratches or other imperfections.
For improved low-level accuracy, ensure that a clean vial filled with TFW reads <0.1 NTU before using that vial to test clear juice. If a clean vial does not read <0.1 NTU, discard it or set it aside for further cleaning. If no clean vials read <0.1 NTU, the TFW may need degassing or a cleaner source of TFW may be required. See ASTM D6855 Test Method for Test Method for Determination of Turbidity Below 5 NTU in Static Mode for more information about low level turbidity readings.

Results Statistics

Various clear juices and standards were tested for turbidity on different meters and the T-100 meter. Results are listed in the chart and are graphed below for comparison.

graphs


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