Don't be Fooled -
Malic Acid Testing
I recently
attended the Unified Wine & Grape Symposium in
These
questions reminded me of the of the Wine Industry Interlaboratory Testing
Program that has be going on since 1999.1,2,3 The program, set up by the ASEV and UC Davis,
is meant to help winery labs improve the quality of their results. In this program, two wine samples are sent
out a few times a year to the 40 – 55 participating winery labs. The samples are analyzed at least in
duplicate, and the results are sent back for statistical analysis. The most common method used by far was
enzymatic. A table of the results is
given below for wine samples with malic acid levels below 500 mg/L. From 10% to 23% of the results were found to
be unreliably inaccurate, and are not included in the table.
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(Data from the
Wine Industry Interlaboratory Testing Program) |
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Sample
1 |
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Year |
Season |
Sample |
mean |
sd |
95%
confidence |
cv |
actual
range |
samples |
|
|
|
|
No. |
|
|
interval |
|
reported |
discarded |
|
|
1999 |
Summer |
1 |
100 |
19 |
63
- 137 |
19.0 |
|
no. |
% |
|
2000 |
Summer |
3A |
440 |
54 |
334
- 546 |
12.3 |
|
|
|
|
2000 |
Summer |
3B |
210 |
47 |
118
- 302 |
22.5 |
|
|
|
|
2000 |
Fall |
4 |
180 |
54 |
74
- 286 |
29.8 |
|
|
|
|
2000 |
Winter |
5 |
160 |
55 |
52
- 268 |
34.2 |
|
|
|
|
2003 |
Fall |
15A |
96 |
24 |
49
- 143 |
25.0 |
35
- 160 |
4/37 |
11 |
|
2003 |
Fall |
15B |
103 |
34 |
36
- 170 |
33.0 |
35
- 175 |
4/37 |
11 |
|
2004 |
Summer |
17A |
165 |
44 |
79
- 251 |
26.7 |
66
- 277 |
4/43 |
9 |
|
2004 |
Summer |
17B |
118 |
42 |
36
- 200 |
35.6 |
15
- 225 |
4/43 |
9 |
|
2005 |
Spring |
19A |
112 |
43 |
28
- 196 |
38.4 |
25
- 230 |
5/47 |
11 |
|
2005 |
Spring |
19B |
135 |
41 |
55
- 215 |
30.4 |
50
- 240 |
5/47 |
11 |
|
2005 |
Fall |
21A |
165 |
56 |
55
- 275 |
33.9 |
11
- 300 |
5/49 |
10 |
|
2005 |
Fall |
21B |
163 |
54 |
57
- 269 |
33.1 |
17
- 300 |
5/49 |
10 |
|
2006 |
Summer |
23A |
275 |
34 |
208
- 342 |
12.4 |
210
- 351 |
6/44 |
14 |
|
2006 |
Summer |
23B |
176 |
34 |
109
- 243 |
19.3 |
120
- 260 |
6/44 |
14 |
|
2007 |
Summer |
26A |
218 |
76 |
69
- 367 |
34.9 |
125
- 450 |
8/51 |
16 |
|
2007 |
Summer |
26B |
110 |
62 |
0
- 232 |
56.4 |
30
- 300 |
8/51 |
16 |
|
2008 |
Summer |
29A |
98 |
41 |
18
- 178 |
41.8 |
8
- 210 |
13/57 |
23 |
|
2008 |
Summer |
29B |
99 |
41 |
19
- 179 |
41.4 |
3
- 219 |
13/57 |
23 |
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What do
these results show?
First, in
the nine years the quality improvement program has been going on, the results
have gotten progressively worse. The
coefficient of variation, a measure of lab to lab reproducibility, has doubled. In addition, while in the initial years 10%
of the reported results were so far off the mark that they had to be excluded,
the number of totally unusable results climbed to 23% in the latest report.
Second, the
results have deteriorated to the point where about half are flat out wrong
regarding what they tell the winemaker.
In 1999, a sample with a malic acid level of 100 mg/L was analyzed, with
the results showing a range of 63 – 137 mg/L.
That’s pretty broad. In 2007 a
wine with a malic acid level of 110 mg/L was analyzed, and the results ranged
from 30 – 200 mg/L. In 2008 a sample
with a malic acid level of 96 mg/L was analyzed, and the range for the results
had ballooned to 18 – 178 mg/L. Some
winemakers were told their wines had finished MLF (which they hadn’t), while
other winemakers thought they had a few weeks yet to go (which they didn’t). And remember, that huge range was the range
reported AFTER 23% of the results had been discarded for gross unreliability. (By the
way, if an Accuvin test strip had been used to analyze the above samples, you
could expect the results to be in the range of 75 – 125 mg/L. That’s considerably better!)
In 2002 Dr.
Christian Butzke of UC Davis wrote a critique of the first few years of the
program. Regarding malic acid he wrote: “Given the corresponding consequences of
uncertainty over incomplete or deliberately prevented malolactic fermentation,
in particular CO2 evolution in the bottle and changes in perceived acidity,
this performance requires major improvements.”1 What would he say
today?
Don’t be fooled!
Spending money on expensive equipment or excessive time running
laboratory tests does not ensure reliable results.
|
What are
typical results you can expect? |
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|
Actual
Value |
Expected
Range |
|
|
|
|
|
Accuvin |
98 |
73
- 123 |
|
per
the QC Survey |
98 |
18
- 178 |
|
|
|
|
|
Accuvin |
96 |
71
- 121 |
|
per
the QC Survey |
96 |
49
- 143 |
|
|
|
|
|
Accuvin |
112 |
87
- 137 |
|
per
the QC Survey |
112 |
26
- 196 |
|
|
|
|
|
Accuvin |
118 |
93
- 143 |
|
per
the QC Survey |
118 |
36
- 200 |
|
|
|
|
|
Accuvin |
103 |
78
- 128 |
|
per
the QC Survey |
103 |
36
- 170 |
|
|
|
|
|
Accuvin |
110 |
85
- 135 |
|
per
the QC Survey |
110 |
0
- 232 |
A final
thought: just getting the same answer
twice with a method performed unreliably does not make it the correct answer.
1
C. E. Butzke, Am. J. Enol. Vitic., 53 (20: 163 – 169 )
2002.
2
C. E. Butzke, Practical Winery & Vineyard, Jan. Feb. 2002
3
Collaborative Testing Services,
Inc., www.cts-interlab.com/wine/index.html,
reports, 2003 - 2008
Recently we
emailed several wineries regarding the Accuvin Quick Tests kit for Malic Acid
analysis. One response contained the
following statement: “I use Megazyme products and charge my
clients accordingly.”
Wow!
We live in
a very competitive world, and in very uncertain times. I’m sure the clients referred to above are
happy to pay extra for a wine because the winemaker has arbitrarily chosen to
use a more expensive analysis method.
The Accuvin
Malic Acid test strips for monitoring malolactic fermentation are less
expensive than the historic UV enzyme kits, like Megazyme’s. They don’t require a spectrophotometer. They don’t require sample pretreatment. They don’t require special laboratory
training. They only take 4 minutes from
sample to result. And since they don’t
require any equipment, they can be used right beside the barrel or tank that
needs testing. Because of the low cost,
each barrel can now be conveniently tested rather than using the method of
mixing samples from 5 barrels, and averaging the result. Our products fit right into your current
winemaking regime. And by the way,
Accuvin’s Malic Acid uses traditional enzyme analysis. It just offers the tests in a more convenient,
ready to use, easy to use format.
Accuvin
understands that some winemakers are interested in immediately adopting a new
technology they feel will provide them with better control of the winemaking
process, or provide a savings in time and/or cost. Other winemakers would like to evaluate a
product before adopting it. Accuvin is
happy to accommodate its customers, and does provide samples for comparative
studies.
I recently
received a question regarding the performance of the Accuvin Malic Acid test
kit. A customer had prepared a 75 m/L
malic acid standard from a quantity of repackaged malic acid he had purchased. He assumed the malic acid was the DL-malic acid
form, which is about 50% active in enzymatic tests like the Accuvin test
kit.
He tested
the sample using the Accuvin kit, and assumed the result would be about 30 mg/L
(actually 37.5 mg/L). The actual reading
was 75 mg/L.
What could
have happened? The malic acid he used
had been repackaged. It most likely was the
L-malic acid form, which is 100% active in the enzymatic test, and not the DL-malic
acid form he expected.
Chemically,
the two forms are identical. They differ
in some physical properties. For example, the melting point of the L-malic acid
form is 101C – 103C, while the melting point for the DL-malic acid form is 130C
– 133C.
The key difference is
biological. DL-malic acid, a mixture of
D-malic acid and L-malic acid, is the form most readily available commercially,
and is the most economical. In DL-malic acid, the D-malic acid form will affect
the acidity just like the L-malic acid form, but it will not later undergo
malolactic fermentation. Thus, only about half can later be converted to the
softer-tasting L-Lactic Acid; the other half is microbially stable. It should
be added that the D-malic acid form has caused a slowing of primary
fermentation by some strains of yeast. Also, especially when MLF is not
planned, with malic acid additions there is a risk of developing green acid
taste.
The bottom line: plan your chemical additions. If you want to make an acid adjust only
without malolactic fermentation, DL-malic acid is a viable option. If you want to adjust acid levels prior to
primary fermentation, and/or if you are planning MLF, it might be better to
stick to tartaric acid or use only the L-malic acid form.
Winegrape
growers and winemakers have long sought a method for selecting the optimum time
to harvest winegrapes. Measurements of
sugars (Brix, or soluble solids), pH, acidity, anthocyanins, berry weight,
glutathione, arginine, protein levels, and terpenes, among others, were made,
but they did not correlate very well with optimum wine quality. Some researchers looked for combinations of
factors, and found that ºBrix x pH, ºBrix
x pH2, and especially ºBrix/acidity
were much more useful in making harvest decisions than the individual factors
alone.1-6 These factors continue to be recommended by major
reference books.7-11 When using the ºBrix/acidity ratio, values in
the range of 30 – 32 have proven optimal, although best time to harvest varies
slightly between varieties and between general climate.
Some
additional techniques have been developed, such as gas chromatographic analysis
of aroma components, spectrophotometric measurement of anthocyanin levels, and
near infrared analysis of specific phenols.
For now, however, these techniques are at best research oriented, off
line methods.
There is
one method that has recently gained favor among some winemakers as a means of
judging time to harvest: sensory
analysis of grapes, otherwise known as tasting. A September 5, 2006, Wine Industry Symposium
on this technique was recently described in “Wine Business Monthly.”12
There are two schools of thought on this technique. First, at an earlier conference discussing
the technique of using taste to judge optimum time to harvest, Dr, Richard
Smart stated "Winemakers go into the vineyards and
they chew on some grapes, and they look at the seeds and the stems, and they
mumble a few things, and then they proclaim the harvest date. And curiously,
that date is always in the future, never in the past. I never heard a winemaker
taste the grapes and say, 'Darn, we got here too late’."13 Also,
in a recent study expert wine tasters showed considerable variation in their
assessments of wine quality.14
What do I
suggest? Perhaps we should follow the
advice of the late renowned French enology researcher Emile Peynaud:15 “Attempts have been made to express the state
of maturity of the grape using certain ratios between different ingredients. The sugar/acidity ratio is the simplest and
most meaningful. Since sugar levels rise
while acidity diminishes during ripening, the sugar/acidity ratio rises sharply
and develops more quickly than its two factors.
Although the accumulation of sugars and the combustion of acids not
being subject to the same factors, take place independently in the grape, this
index is a fairly accurate expression of the state of ripeness. Its value depends on the variety.” And if there has been a change in weather
conditions such as the very cool spring this year in Oregon, perhaps doing a
little extra canopy manipulation (e.g., leaf removal) will produce much better
grapes than simply letting them hang for an extended period of time.
What do you
think?
1. C.
S. Ough, V. L. Singleton, “Wine quality prediction from juice Brix/acid ratio,”
Am. J. Enol. Vitic., 19 (3): 129 -
138 1968.
2. C.
S. Ough, C. J. Alley, “Effect of Thompson seedless grape maturity on wine
composition and quality,” Am. J. Enol.
Vitic., 21 (2): 78 - 84 1970.
3. J.
Ribereau-Gayon, E. Peynaud, P. Ribereau-Gayon, P. Sudraud, Traite d’oenologie sciences et techniques du vin, vol. 2, Dunod,
Paris, 1975.
4. B.
G. Coombe, R. J. Dundon, A. S. Short, “Indices of sugar-acidity as ripeness
criteria for winegrapes,” J. Sci. Food
Agr., 31: 495 - 502 1980.
5. C.
S. DuPlessis, P. C. Van Rooyen, “Grape maturity and wine quality,” S. Afr. J. Enol. Vitic., 3 (2): 41 -45 1982.
6. P.
C. Van Rooyen, P. C. Ellis, C. S. Du Plessis, “Interactions between grape
maturity indices and quality for Pinotage and Cabernet sauvignon wines from
four localities,” S. Afr. J. Enol.
Vitic., 5 (1): 29 - 34 1984.
7. B.
W. Zoecklein, K. C. Fugelsang, B. H. Gump, F. S. Nury, “Wine Analysis and
Production,” Chapman and Hall, 1995.
8. R.
B. Boulton, V. L. Singleton, L. F. Bisson, R. E. Kunkee, “Principles and Practices of Winemaking,” Chapman and Hall, 1996.
9. P.
Iland, A. Ewart, J. Sitters, A. Markides, N. Bruer, “Techniques for Chemical Analysis and Quality Monitoring During
Winemaking,” Patrick Iland Wine Promotions, 2000.
10. J.
L. Jacobson, “Introduction to Wine
Laboratory Practices and Procedures,” Springer 2006.
11. P.
Ribereau-Gayon, D. Dubourdieu, B. Donneche, A. Lonvaud, “Handbook of Enology,” Vol. 1, 2006
12. M.
Greenspan, “Assessing Ripeness through Sensory Evaluation,” Wine Business
Monthly, Nov. 2006.
13. D.
Berger, “”Water into Wine: the smoking gun,” Wines & Vines. 86 (3):
52 – 56 2005.
14. R.
Gawel, P. W. Godden, “Evaluation of the consistency of wine quality assessments
from expert wine tasters,” Aust. J. Grape
& Wine Res., 14: 1 - 8 2008.
15. E.
Peynaud, “Knowing and Making Wine,”
Wiley-Interscience, 1982.
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