Accuracy and Precision
In the laboratory, the terms accuracy and precision each have very particular meanings.
Accuracy: Ever hear the old adage, "Even a stopped clock is right twice a day?" Well, this gets at the root of accuracy in the lab. The term accuracy refers to the agreement of a measurement to a true or correct value. Accuracy does not infer that the instrument, or the analysis, is good or even working; like a broken clock, an instrument or analysis can be "accidentally accurate." If one were to think of the results of any particular analysis as being like a game of darts, then the accuracy of that analysis is the average number of "throws" at the board. Since one averages analytical results, the scoring of this game is a little different. A throw that is three inches to the left of the center can be offset by a throw that three inches to the right of the center; a throw that is two inched above the center is offset by one that is two inches below the center, etc.
Precision: The term "precision" is used to describe the repeatability of a particular analysis. One can think of precision as the "scatter of the darts" of a number of repeated measurements. Thus a "tight pattern" is a more precise analysis. An analysis is considered less precise the further the results lie from each other. Perhaps the most difficult concept is that the precision of a result has nothing to do with the value of the result being close to the true or correct value. As we can see above, an analysis can be accurate, precise, or one would hope, both.
SO2 and other Routine Analyses
The analyses that a winery should perform most frequently on juice or must are sugar concentration (Brix), pH and titratable acidity (TA). A winery should check the Brix, pH, TA and volatile acidity (VA) most frequently for active fermentations. After fermentation, the analyses that a winery should perform most frequently on wine are pH, TA, VA, free sulfur dioxide (fSO2) and total sulfur dioxide (tSO2).
The "Chemistry and Measurement of SO2 Seminar" at this year's ASEV (American Society for Enology and Viticulture) annual meeting points to the overall importance of this almost ubiquitous antioxidant in wine. The analysis of sulfur dioxide in wine remains one of the most frequently conducted winery lab analyses. The traditional wet-chemistry analytical methods for determining SO2 -ripper and aeration/oxidation-while not particularly difficult, do require a fair amount of attention from the lab technician as the titration endpoints can be easy to overshoot. Automatic titration systems for SO2 analysis have been available for some time, but these systems have tended to be too expensive for most small to mid-sized winery laboratories. Both Hanna Instruments and Mettler Toledo have brought entry-level auto-analyzers onto the market recently.
The general trend toward higher pHs in wine over the past couple of decades means that wineries have less margin for error in determining what level of SO2 is appropriate for a given wine. The analyses of pH and TA are pretty straightforward and therefore are good candidates for automation. However, grape juice, must, and wine are not really simple solutions; they are complex "soups" that tend to degrade pH probe reliability over time. This often means that a winery lab needs to replace its pH probes every year. This, in turn, makes the pursuit of more robust and long-lasting pH probes something of a grail-quest for Brinkmann-Eppendorf, with their Metrohm Unitrode.
Rapid broad-spectrum auto-analyzers, like the FOSS WineScan series, are also increasing their presence in the wine industry. FOSS introduced three new WineScan models in May, all of which are almost complete labs in a box.
Sugar content, in the form of the various specific gravity-based scales like Brix, Baume, Oechese and the Klosterneuburg scale (KMV), is a crucial measurement during harvest. Historically, these measurements have been made using analog refractometry prior to fermentation and by hydrometry after fermentation starts. Analog refractometers cannot compensate for ethanol. Digital refractometers offer many advantages over analog units, including the ability to give results in a variety of scales, compensate for ethanol and even measure final wine ethanol. Hitherto, digital refractometers have been confined to the lab due to their significant expense. The Palm Abbe line from Misco is designed to have the advantages of digital refractometry at a significantly lower price than before.
Of course, total sugar content is only part of the story. The glucose-fructose ratio is an important indicator of a fermentation's tendency to proceed to dryness. Until about 25 years ago, the glucose-fructose ratio had been determined by thin-layer chromatography. Boehringer Mannheim then released enzymatic kits for glucose-fructose determination. These kits, like all Boehringer Mannheim enzymatic analysis kits, relied upon the use of a lab-grade spectrophotometer, the significant cost of which placed it out of reach of many smaller wineries. The recent trend in enzymatic kit design, as seen in the Megazyme kits below, has been to add a final step that allows the quantifiable results to be determined using a much lower cost colorimeter rather than a spectrophotometer.
Several of these new products are designed to make it easier for the winery to perform these routine analyses more quickly and with good analytical precision.
HI 84100 Sulfur Dioxide Mini-Titrator from Hanna Instruments
• Performs simple titrations in minutes
• Provides lab-grade sulphur dioxide measurements
• Small and affordable
The Hanna Instruments' HI 84100 is an easy-to-use, low-cost, automatic titrator that should make automated SO2 analysis available to a wide spectrum of wineries. The HI 84100 comes with a built-in algorithm to analyze the shape of the electrode response and to determine the reaction completion. This algorithm automates the analysis, makes all the necessary calculations, and assures a simple and effective interface for the user. By simply pressing the "Start/Stop" button, the instrument will automatically take the titration up to the equivalence point. The result is immediately displayed in convenient units; the instrument is then ready for another titration.
Each HI 84100 mini-titrator is supplied complete with reagents set for 20 titrations, two 50 ml beakers, two 20 ml beakers, scissors, tube set with cap, ORP probe, stir bar, power cable, one 30 ml bottle of refill solution, one 1 ml syringe, two sachets of cleaning solution for wine deposits, two sachets of cleaning solution for wine stains and an instruction manual.
DL22 Food and Beverage Analyzer from Mettler Toledo
• Preprogrammed for pH, TA and SO2 analyses
• "Entry-level" automatic titrator
• User-definable "soft keys" can be assigned to most frequently used methods
Mettler Toledo is probably best known for their scales; however, they also produce an extensive line of analytical instruments. The DL22 is one of Mettler Toledo's "entry-level" titrators.
The DL22 interface is very intuitive. Simply pressing the "Run" button twice repeats the method that was run most recently. This is very convenient for users who usually run the same method for several samples in succession. The titrator also offers three programmable "soft keys." This allows the user to assign three methods to the F1, F2 and F3 soft keys so that they are always easy to find and start. In addition, the titrator has pictographic keys (icons) for the most frequently used manual functions.
Available analyses: The DL22 can also be used for the three most common wine analyses: pH, TA and SO2. In addition, since the DL22 is designed for the general food and beverage industry, it comes preprogrammed with several analyses, like peroxide, vitamin C, salt and non-soluble acid determination, which are rarely if ever used on wine or must.
Connectivity: The titrator automatically calculates results with predefined units. Depending on the method, Mettler Toledo has predefined the most common calculations and corresponding units. For special cases, you can still multiply with a factor or subtract a constant. After the calculation, the results are stored and can be sent as a report to a Mettler Toledo RS-P42 printer or other commercially available printer (HP-compatible, PCL3 emulation).
L-Malic and Glucose/Fructose "MegaQuant" Test Kits From Megazyme
• Includes two different test kits
• No spectrophotometer is required (uses a colorimeter instead)
• Very simple and rapid
• MegaQuant™ is a specialized lab meter; the two new analysis kits are made to be used with the meter
Megazyme has brought several enzymatic bio-analysis test kits to the wine industry in recent years. These enzymatic bioassays can offer a typical winery laboratory several advantages over more traditional analyses:
D-Fructose and D-Glucose MegaQuant Format: For the measurement of D-fructose and D-glucose in grapes, grape juice and wine using the MegaQuant colorimeter (measurement at 505 nm). This analysis is particularly useful because yeast need to have access to glucose in order to consume fructose.
L-Malic Acid MegaQuant Format: For the measurement of L-malic acid in grapes, grape juice and wine using the MegaQuant colorimeter (measurement at 505 nm). Suitable for white and red wines at all stages of the winemaking process.
These assays are designed to provide a simple and accurate method for measuring either D-fructose and D-glucose or L-malic acid using an inexpensive colorimeter, costing about $700, rather than a significantly more expensive spectrophotometer, which can cost as much as $20,000. Although they use a different set of enzymatic reactions to get there, the final step for both analyses use a similar reaction: the reduction of iodonitrotetrazolium chloride (INT) to its corresponding INT-formazan to provide a quantifiable color change in the 480-520nm-wavelength range.
Updated Analyzers From FOSS
• Can analyze up to 20 main quality parameters in wine, such as ethanol, pH, sugars and organic acids
• Optional auto sampler allows the unattended analysis of 120 samples per hour
• Built-in clean cycle and zero-setting ensure that results are unaffected by previous analysis operations
FOSS recently announced the sales release of a new generation of WineScan™ analyzers for routine analysis of wine and grape must. They include: WineScan Grape for dedicated analysis of grape-must; WineScan Flex for analysis of both grape must and wine; and WineScan Auto for efficient, unattended analysis of wine
These new analyzers are successors to the FOSS WineScan FT120 and GrapeScan FT120 (see Wine Business Monthly, November 2002, December 2003 and October, 2004, for coverage of previous models), and boast a number of new features, including a new instrument software platform for simple operation and sensitive "outlier" detection for improved accuracy of results.
With the WineScan Grape, the filtration of grape must is automatic and integrated, allowing fast sample preparation during busy harvest periods. All of the WineScan family of auto-analyzers are also highly resistant to vibration and temperature fluctuations.
The WineScan auto-analyzers use Fourier Transform Infrared (FTIR) technology that allows up to 20 main parameters in wine to be analyzed in a single sample in 30 seconds. Maturity and soundness parameters of grapes can be analyzed in 90 seconds, including sample preparation. The 20 "main parameters" that the FOSS WineScan analyzes are: citric acid, CO2, density, ethanol, ethyl acetate, fructose, gluconic acid, glucose, glucose/fructose ratio, glycerol, lactic acid, malic acid, methanol, pH, polyphenol (as Folin C index), reducing sugars, sorbic acid, tartaric acid, total acidity (TA) and volatile acidity (VA).
The new WineScan solutions can work side by side with existing FOSS wine and grape analysis solutions. The original WineScan FT120 solutions will continue in the FOSS product portfolio in a basic configuration. Calibration databases developed for existing GrapeScan and WineScan solutions can be transferred to the new generation of analyzers.
New "Palm Abbe" Line of Digital Wine Refractometers from Misco Products Corporation
• Fills the gap between traditional (analog) hand-held refractometers and bench-top (digital) laboratory refractometers
• Programmable
• More versatile than analog refractometers
Misco Products Corporation has introduced a new line of hand-held digital refractometers engineered specifically for the international wine industry. The Palm Abbe digital wine refractometer provides an instant digital "field" determination of grape ripeness, grape must concentration, grape must density and sugar content, as well as the potential and actual alcohol content of the finished wine. Measurements are made with a precision comparable to mid-range bench-top refractometers.
Priced between $325 and $525, the cost of the Palm Abbe digital refractometers is potentially more accurate and versatile than traditional analog handheld refractometers. At the same time, the digital nature of the Palm Abbe allows certain models to display prompts and measurements in English, Spanish, French, German or Russian. Similarly, other models are available with scales for various international units of measure, including Baume, Brix, Oechsle, KMV, Babo, specific gravity and alcoholic strength. Users have the flexibility to mix and match up to five different scales on some Palm Abbe models, giving them the ability to create custom wine testing instruments.
Protection against inaccurate readings due to temperature differences-a major concern in refractive index measurement-is assured with non-linear temperature compensation specific to grape juices. Temperature compensation is automatic for fluids that read between 0 and 50°C (+32 to 122°F).
The stainless steel sample well requires only a couple of drops of fluid to take measurements. A simple, user-friendly interface consists of two buttons: one to take readings and the other to step through various menu options. A large, dual-line, multilingual LCD display is easily read, even in dim lighting.
The digital refractometer removes the subjectivity associated with analog refractometers that require users to interpret where a boundary line crosses tiny scale divisions. Calibration of the Palm Abbe is automatic and does not require special calibration solutions or tools, and they automatically calibrate themselves to water.
The following standard instruments and scales are available:
VINO1: Brix, Baume, Oechsle, KMV and sugar content (grams per liter)
VINO2: Brix and sugar content (g/l)
VINO3: Baume and sugar content (g/l)
VINO4: Oechsle and sugar content (g/l)
VINO5: Mass fraction of sucrose, probable alcohol, actual alcohol, alcohol-specific gravity
PA201: Brix only
Misco is headquartered in Cleveland, Ohio. Misco designs, manufactures and sells a variety of refractometers, including: digital bench-top laboratory refractometers inline process control refractometers, digital handheld refractometers, and traditional hand-held instruments.
Redesigned Metrohm "Unitrode" From Brinkmann-Eppendorf
Not all advances in winery lab equipment involve automation. Even the humble pH probe gets reinvented once in a while.
• Designed for improved reliability and durability
• Minimized cross-contamination
The new Metrohm Unitrode pH probe design aims to increase accuracy and durability. The combination glass electrode has been designed for a wide range of applications/industries, including: beverages (soft drinks, wine, juices, dairy); pharmaceutical; environmental; dyes and inks; electroplating; and emulsions
The Metrohm Unitrode offers an extremely reliable and precise measuring system. The Unitrode features a fixed-sleeve design that creates a sharp and well-defined interface between the sample and the sleeve; the possibility of sample carryover is greatly reduced, minimizing cross contamination and memory effects. The sleeve diaphragm allows a more defined electrolyte flow and minimizes any tendency toward blockage.
The fixed ground-sleeve diaphragm is resistant to dirt and particles, and has an annular design, which ensures that there is only a small difference in observed pH values between a stirred and an unstirred solution. This aspect is particularly important for pH measurements in more difficult samples since they usually have to be stirred during the analysis to maintain consistency.
This new electrode also uses the Metrohm "Long Life" reference system (with the Ag/AgCl cartridge) with its silver ion barrier to ensure an extended life and consistent measurements.