By Christian Butzke
Associate Specialist in Cooperative Education, UC Davis
Imagine a world where all winemakers can sleep soundly at night
and the winery lab staff feels appreciated. Also where consumers
can rest assured that all wines are stable and healthy and show
the finest qualities of fruit and style. Then wake up, smell the
barrels, and realize that only a small number of winery labs in
and outside California have lab quality systems in place that would
allow for such peace of mind.
That is why the Winery Laboratory Proficiency Testing Guidance Subcommittee
of the American Society of Enology & Viticulture (ASEV) established
a continuous interlaboratory proficiency testing program for the
American wine industry in late 1999. The subcommittees work,
under the guidance of ASEVs Technical Projects Committee,
represents the first practical cornerstone of a lab quality system
for American wineries.
The concept of proficiency testing was outlined by Sue Weeks in
her Winery Laboratory Quality column, Proficiency testing
Support groups for laboratories, (PWV May/June 2001).
The ASEV program is guided by the subcommittee and managed by Chris
Collaborative Testing Services, who prepares statistical summary
reports for the participants.
It is important to point out that participation in a proficiency
testing program alone does not automatically lead to better lab
performance. Below, we review the reasoning behind such a program,
discuss the current state of proficiency in winery labs, and contemplate
where we might go from here.
From a vintners viewpoint, interpretation of the test results
is up to each of us, as we aim to establish performance criteria
for basic wine analyses and, consequently, our winery labs.
Why should your winery participate?
Whether your winery bottles millions of cases and has a team of
enologists and lab technicians available, or you grow the grapes,
make the wine, and measure volatile acidity personally to make 100
cases, the criteria for sound winemaking practices are essentially
In addition, regulatory requirements and liabilities for selling
your wine will become more and more uniform for all marketplaces,
whether you export your wine to the United Kingdom or sell it out
of your own tasting room.
Adopting ISO standards
Euro-conspiracy, government plot, or a
Now that the U.S. delegation has pulled out of the Old World-biased
OIV (Office International de la Vigne et du Vin), it is even more
important that our wine industry closely observes international
competition and stays ahead of the curve when it comes to ensuring
the quality and integrity of American wines.
In a time of increasing wine imports, stagnant wine exports, flat
overall consumption, and a still looming worldwide economic recession,
it can be a costly mistake to ignore trends that bring global competitiveness
to a local level.
Quality systems, ISO accreditation, performance criteria
all this lingo sounds like some sort of paper-pushing regulators
idea of fun, generating more paperwork and less time available to
get the actual job done.
But in truth, these terms stand for processes that should lead you
and your entire winery staff to take a step back and remember the
most important task of your winery: making wine of the highest quality.
That is where the international standards for quality management
of analytical laboratories (ISO 17025) come in handy, as they outline
a ready-made lab quality system that all wineries can use to their
To make things even easier for you, ASEV, through its Technical
Projects Committee, has produced a Winery Quality Systems CD-ROM
that is available to all members
at cost. The multimedia CD highlights the principles of lab quality
management, including practical winery examples.
The CD can help staff at wineries of all sizes understand and implement
an internationally accepted quality system appropriate for their
own environment. The goal may range from implementing the first
basic steps to full ISO accreditation. The latter is already required
in some export markets and is being quickly adopted and implemented
by all our relevant government agencies, including BATF, FDA, USDA,
U.S. Customs Service, and others.
Five basic steps to a Quality System
Following are the essential steps required to initiate a lab quality
system at your winery:
- Preparation of a Quality Manual
that details the agreement between senior winery management, winemakers,
and the laboratory regarding the quality and performance goals
of the laboratory and the resources (equipment, staff, etc.) required
to meet those goals.
- Method validation constitutes
another important first step. Often, a winery laboratory inherits
methods or adopts them because they exist in a publication. ISO
emphasizes that a method is valid only after its performance has
been demonstrated to be adequate for your specific winery laboratory,
utilizing your staff and equipment. Furthermore, fully validated
methods make it possible to educate winemakers about the performance
limitations of individual methods of analysis. See Sue Weeks
column Method validation Making sure it works in
your laboratory, (PWV September/October 2000:76-77) for
- Documentation of lab procedures must
be done in a consisten format. This is necessary for uniform implementation
of individual methods of analysis, and is essential to the generation
of quality data.
- Training for new and existing staff
is available through courses at independent labs, community colleges,
or UC Davis extension classes. You can even do it within your
winery using your documented set of lab procedures.
- Proficiency testing is the
last piece of the quality puzzle. Internal proficiency testing
against standards and control samples verifies ongoing caliber
of performance, while external proficiency testing, such as ASEVs
program, measures the performance of the winery laboratory against
Advances in the Australian wine industry over the past 20 years were
not due to luck, but to enormous investments in winegrowing and winemaking
technology, wine research, and extension programs. One of Australias
successful techniques for optimizing wine quality while improving
cost and production efficiency was to establish the Interwinery Analysis
Group and lab proficiency testing scheme in 1983.
Almost 20 years behind its closest global competitor, the American
wine industry is only now on the right track to make up for this competitive
disadvantage. However, if the speed of Californias advances
in wine quality are any indication, it shouldnt take long for
our wineries to catch up.
Preharvest Wine and Grape Quality Air and
One of the Proficiency Testing Guidance Committees methods to
promote the concept of winery lab quality has been an annual statewide
road show, based on the Australian model and introduced by the Australian
Wine Research Institute and other extension groups.
In the first Preharvest Wine and Grape Quality Air and Road Show,
Gordon Burns of ETS Laboratories, Patricia Howe (teaching lab manager
at UCD Dept of Viticulture & Enology), Sue Weeks of Bibber International,
and I traveled to the North Coast, the Sierra Foothills, and the Central
Coast in August 2001.
The program provided an effective platform for conveying the general
concepts of a lab quality management system and included small group
discussions of particular analysis problems. In addition, the committee
organized Winery Interlab Group meetings to present and discuss test
results during the Unified Symposium and the 2001 ASEV Annual Meeting.
The Proficiency Testing Guidance Committee plans to continue the road
shows just before the 2002 crush, and PWV readers are encouraged to
suggest class locations
that are convenient to reach for most winery lab staff in their counties.
Current state of affairs
So far, there have been six rounds of testing in the ASEV Laboratory
Proficiency Testing Program for nine standard wine analyses. Proficiency
was tested for analysis of ethanol, total and free sulfur dioxide,
titratable acidity, volatile acidity, specific gravity, pH, residual
sugar, and malic acid.
Wineries were free to choose their methods of analysis, and the participants
remain anonymous. Between 32 and 55 wineries have participated in
each round, representing an estimated 90+% of U.S. wine production
Test rounds were conducted in February, May, August, and November
of each year to minimize interference with lab obligations during
crush. For each round, one 750 ml bottle each of two different commercial
wines of similar composition were distributed to participating wineries.
Each wine came from one bottling run at a large winery to ensure the
most consistent sample composition.
Since real wines were tested, the accurate, true values for each measurement
are known only to Mother Nature. Any comparison of measurements is
based on the averages of the winery participants as a group of peers.
With respect to individual analyses (Figure I), alcohol, specific
gravity, and titratable acidity measurements appeared to be precise
and were well-correlated among participating winery laboratories.
The increased availability of autotitrators may be one reason for
improved proficiency in TA.
The coefficients of variation for both sulfite measurements, especially
free SO2, pH, volatile acidity, residual sugar, and malic
acid were undesirably high. However, for some of the analyses, results
were biased toward the most popular methods of analysis such
as Ripper for total SO2, Cash still for VA, and enzymatic
assays for RS and malic acid all of which contributed proportionally
to the grand mean.
While some results may be suitable for internal production control
if reproducible, the range of results even without considering outliers
was extensive. This is a concern, as the program participants represent
the most advanced labs in the industry, and those that are most interested
in improving their performance.
The reasons for the unsatisfactory results need to be investigated
further, and all possible sources of error for each method of analysis
must be determined, documented, and subsequently minimized.
Below are some statistics and comments on individual measurements;
methods are listed in order of reported user frequency. The coefficient
of variation (CV) is the error for each analysis relative to the mean
of the whole group of winery labs.
(EtOH). Methods: 51% Ebulliometer, 31% gas chromatography,
8% near-infrared spectroscopy, 10% distillation/density. Proficiency:
Labs were most proficient in alcohol analysis compared to the other
analyses. They achieved CVs between 1.2% and 1.8%, despite use of
a wide range of equipment and analysis methods.
The application of ebulliometers, which were used by three out of
four wineries in 1998, declined in favor of automated assays. Results
confirm that non-standardized methodology, if validated, does not
compromise the ability to produce acceptable results.
However, the industry trend to make delicate alcohol adjustments to
wines for sensory or tax reasons would require an even greater proficiency
in order to make such decisions more precise and reproducible.
Total SO2 (TSO2).
Methods: 68% Ripper, 24% Aeration-Oxidation, 3% enzymatic assay. Proficiency:
CVs ranged from 5% to 16% between test rounds. The ratios between
concentrations of bound (total minus free) and free SO2
show a reasonable to generous use of sulfites (52 to 106 mg/L) throughout
the winemaking process, with insufficient amounts of free and consequently
molecular SO2 after bottling in three out of four commercial
Free SO2 (FSO2)
Methods: 48% Ripper, 48% Aeration-Oxidation, 4% colorimetric segmented
flow analysis. Proficiency: This possibly most important wine analysis
regarding microbial stability and general ageability was performed
with CVs between 12% and 29%.
The amounts of free SO2 in the tested wines were between
10 mg/L and 26 mg/L independent of pH. The consequences of additional
errors in pH analysis are discussed below. Molecular SO2
based on wine pH ranged from 0.14mg/L to 0.92 mg/L, suggesting a need
for improvements in proper SO2 adjustments.
Titratable acidity (TA) Methods:
54% manual titration, 46% automatic titration. Proficiency: The essential
analysis for a wines perceived tartness improved significantly
over the preliminary survey (8%) with CVs between 2% and 4%. This
coincided with increased use of autotitrators over manual titration
(13%:87% in 1998 to 46%:54% in 2000).
Volatile acidity (VA) Methods:
81% Cash still, 10% enzymatic assay, 3% capillary electrophoresis,
3% gas chromatography, 3% colorimetric segmented flow analysis. Proficiency:
The concentration of acetic acid and other volatile acids in wine
is a federally regulated wine quality criterion (limits: 0.14 g/100
ml for red, 0.12 g/100 ml for white table wines).
Concentrations of VA in the wines tested were between 0.02 and 0.08
and thus far below those legal limits, implying good winemaking practices.
However, the CVs ranged from 7% to 25%, which would make it difficult
to confirm compliance in some instances.
Specific gravity (SG) Methods:
59% hydrometer, 35% density meter, 6% pycnometer. Proficiency: CVs
were between 1.3% and 4% even if one compares the standard deviation
between labs to the maximum range in specific gravity that can be
expected in table wine. This assumes a range between 0.970 (14% v/v
ethanol, no RS) to 1.015 (9% v/v ethanol, 50 g/L RS) kg/L (20†C /
pH Methods: All labs used electronic
pH meters. Proficiency: Labs achieved CVs between 6% and 9%, given
the logarithmic nature of the pH measurement. This consideration increases
the relative errors but recognizes the critical consequences of small
inaccuracies in pH on the biological and chemical stability of the
wine, especially the amounts of molecular SO2 available.
The molecular SO2 concentration for a given free SO2
content at a particular pH [formula: molecular SO2 = free
SO2 / (1+10(pH-1.83))]
determines the microbial stability of a wine. The suggested values
have been between 0.6 to 0.8 mg/L (at 14%v/v and 12 %v/v ethanol,
respectively) for protection against lactic acid bacteria.
Concentrations below 0.6 mg/L may lead to a potential instability
in case of a failure or absence of a sterile filtration step at bottling,
especially in wines that have not undergone a complete malolactic
fermentation. An 0.1 underestimate of the pH represents a 26% overestimate
of actual hydrogen ion concentration, which results in a proportionally
higher requirement for free SO2.
Under commercial winemaking conditions, errors in the analysis of
pH and free SO2 add up because of the impact of the hydrogen ion concentration
on the equilibrium of the different ionic forms and the molecular
form of sulfur dioxide in an aqueous solution such as wine.
The combined CVs for both analyses were between 19% and 39% for all
labs and test rounds. If a wine is considered microbially stable because
it is dry and has completed ML, one can be less concerned about the
levels of molecular SO2. However, given the poor performance for both
RS and malic acid analyses (see below), no winemaker should rely on
those results alone.
Residual sugar (RS) Methods: 69% enzymatic/spectrophotometric
assay, 26% copper reduction assays (Gold Coast, Lane-Eynon, etc.),
5% HPLC. Proficiency: The analysis of residual fermentable sugar is
crucial to determine microbial stability of a table wine and to adjust
its sensory properties for the consumer.
The CVs improved significantly over the preliminary survey (140%),
ranging from 5% to 39%. Given the dual importance of the analysis,
this still appears to be unacceptable, even considering that some
labs from wineries that produce only dry table wines were not accustomed
to analyzing wines with relatively high amounts of RS.
The slight improvements may be attributed to an increased use of the
enzymatic assay over copper reduction assays (51%:46% to 69%:26%)
over the past three years. More important, wineries overcame problems
reporting the actual sugars measured and the magnitude of their RS
L-Malic Acid (MA) Methods: 97%
enzymatic assay, 3% capillary electrophoresis. Proficiency: Similar
to the enzymatic determinations for RS, the CVs of the assay for malic
acid varied in a wide range of 4% to 39%. Given the corresponding
consequences of uncertainty over incomplete or deliberately prevented
malolactic fermentations, in particular, CO2 evolution in the bottle,
and changes in perceived acidity, this performance requires major
The whole picture
Lab proficiency tests in 2000 and 2001 showed similar levels of proficiency
throughout the six test rounds. This was not a surprise, since a proficiency
testing program alone is not sufficient to improve a laboratorys
As mentioned above, a complete lab quality management system is required
to achieve this goal. However, it is positive to note that labs have
become more proficient in reporting the units of measurement correctly.
Statistical results of the proficiency testing program so far suggest
a continuing need for improved laboratory quality management systems.
With the proficiency testing program in place, the ASEV Technical
Projects Committee will be able to address remaining parts of the
winery lab quality management system, and proficiency surveys will
confirm anticipated improvements.
As a next step, wineries need to establish performance criteria for
the different analyses of wine that reflect the significance of their
accuracy and precision for the quality and stability of any wine.
So far, the proficiency testing program has judged lab performance
solely by statistical comparisons. Why? Because uniform performance
criteria for wine analyses have not yet been established. To make
proficiency testing most meaningful, it should be based on wine quality
and stability requirements that reflect the sensory, biological, and
chemical properties of the wine components analyzed.
As stated in the beginning, when it comes to performance criteria,
(winery) size doesnt matter. Every winery lab should be confident
in the data it creates, and every winemaker should be able to rely
on those data to make production decisions without nagging doubts.
We all need to agree on acceptable errors for each of our measurements
to assure safe, high quality winemaking practices.
Your input is invited!