Processing Choosing the
Processing Choosing the Right Reference Material for Calibration, Safeguard Precise Sugar Dosage and Consistent Product Quality For soft drink production, dosing the amount of sugar precisely and accurately is critical. By doing so, soft drink producers can use raw materials more efficiently, ensure consistency between batches and production sites, and conform with bottling license regulations. To get accurate Brix results, though, analytical instruments need to stay calibrated and ready to go. BY BARBARA LAKY AND JULIA SATTLER Determining sugar concentration is one of the essential steps in quality control in any soft drink plant, and this is usually done with a density meter or refractometer. During an instrument’s operating life, it might be exposed to conditions that influence the quality of results, which makes regular calibrations essential. Calibrating a density meter, however, is easy. By calibrating the instrument, you know your measured results are as accurate as they were on the first day. When selecting a suitable reference liquid for calibration, there are a few things to keep in mind. Simply put: the better the certified reference material, the better the measurements. Why calibrate a density meter that’s used for Brix determination? Density measurement is the basis for determining the sugar content in syrup Figure 1: Anton Paar density standards. as well as in the finished product. Since it lets soft drink producers accurately control costly raw materials used, a precise Brix determination saves them a significant amount of money at the end of the day. Precise quality control lets them tighten accepted production tolerances, and it ensures consistency between single batches and between production sites. By doing so, producers can make sure consumers don’t taste a difference from a bottle produced at site A versus site B. Licensed bottlers also benefit from Brix determination by always having control over how they conform to strict regulations that are defined by a parent group. DIY calibration or calibration by an accredited lab – what’s best? Regular calibration with a density standard is the only way to ensure measurement results are as accurate as they were on the first day and are traceable to national standards. Globally accepted calibration certificates for an analytical instrument are obtained from a calibration lab that’s accredited according to ISO 17025. The calibration can either be done ex works or after a certain period of time (either in the lab or on-site at the beverage production plant). That said, regular calibrations for safeguarding a factory’s production quality should be conducted throughout the year as well. Criteria for selecting a suitable reference material for soft drink plants 1. Select a CRM with a density range that reflects your application You should verify the measuring performance of an instrument in the density range of typical samples (e.g., syrup or finished soft drink). Certified reference materials (CRMs) that are typically used for density meters being used in soft drink production are water (density: 998 kg/m 3 ), glucose/water 10 % (density: 1040 kg/m 3 ), and sodium bromide (density: 1250 kg/m 3 ). 2. Understand the difference between ISO 17025 and ISO 17034 conformity Most density standards only conform with ISO 17025, meaning that the reference values are valid at the day of determination but don’t consider any drift due to shipment or storage. Since the reference material isn’t immediately used after production, this is an element of uncertainty that, if possible, should be avoided. 24
Processing ISO 17025 standards ISO 17034 reference material Traceable to International System of Units (SI) Appropriate for ISO 17025 calibrations by an accredited lab Reference method fulfills stringent ISO 17025 requirements Yes Yes Yes Certified value is guaranteed … at the day of certification until the end of the density standard’s shelf life Yes Yes Yes Measurement uncertainty includes homogeneity, short-term stability due to shipment, and long-term stability Continuous stability measurements by manufacturer until end of shelf life No (not specified drift has to be considered by user) Table 1: Differences between density standards fulfilling ISO 17025 and ISO 17034. No Yes Yes For ISO 17034-certified reference materials, reference values are valid until the end of the standard’s lifetime, and they always conform with ISO 17025 as well. Producers must measure each batch of standards in regular intervals throughout the complete lifetime, so the specified reference values are always guaranteed. The stated “expanded uncertainty” comprises homogeneity, long-term stability, and short-term stability regarding shipment. 3. Look up the specified measuring uncertainty Ideally, a density meter is calibrated with a reference material 3x more accurate than the instrument’s specification (if available). When checking the measuring uncertainty, consider whether it refers to ISO 17025 or ISO 17034. At first glance, the specification might look appealing. But if the density standard is only certified according to ISO 17025, end customers have to estimate – imprecisely – the drift of the reference value between the production data and day of use. In order to get a feeling for specified uncertainties, the table below has an example from an Anton Paar density standard, which compares the uncertainty coming from the reference method and the expanded uncertainty. It also considers the long-term stability, potential influences from shipment, etc. (ISO 17034). 4. Check the reference method used for determination of specified values Another aspect of a trustworthy reference material is the reference method that’s used for determination of reference values. For several standards on the market, no information about the reference method is stated, and some refer only to pycnometer norms. Ideally, a reference material is measured with the primary method. For density measurement, this is hydrostatic weighing, which national metrology institutes (NMIs) around the world tend to use. 5. Verify the ampoule material and closure The ampoule material and closure both play a role ensuring the filled reference material doesn’t change its composition due to external influences or evaporation. For some density standards, the liquid from the ampoule can even be smelled because the plastic caps that are used are prone to exchange with ambience. What’s behind the “hydrostatic weighing” method? The hydrostatic weighing method is based on the Archimedes principle: A sphere with well-known volume and mass is immersed in the liquid under test. Its apparent loss of mass is equivalent to the mass of the displaced liquid. By knowing this mass and volume, along with weighing the apparent mass of the sphere, the density of the liquid can be determined. Example (specification of Anton Paar density standard) Expanded uncertainty* (ISO 17034) Expanded uncertainty* of reference method Water density standard (998 kg/m 3 ) 0.015 kg/m 3 ≤ 0.003 kg/m 3 *Based on a combined standard uncertainty multiplied by a coverage factor k=2, providing a level of confidence of approx. 95 %. Source: Anton Paar brochure I39IP001EN-B “Density Standards.” Figure 2: Hydrostatic balance – the primary method of density measurement. Since CRM flasks should be designed for one-time use, heat-sealed glass caps are ideal closures. drinkworld Technology + Marketing · June 2022 25
