Wine Analysis Blog

Don't be fooled - getting the same result twice with expensive equipment does not mean the answer is reliable!

Mike Miller — Mon, 02 Feb 2009 21:51:00 GMT

Don't be Fooled - Malic Acid Testing

I recently attended the Unified Wine & Grape Symposium in Sacramento. A few winemakers I spoke with asked about the precision of the Accuvin Malic Acid test kit. As has been reported, the accuracyof the Malic Acid test kit is +/- 10 mg/L at the 30 mg/L level, which is generally considered the end of MLF, and about +/- 20 mg/L at the 75 mg/L level.

These questions reminded me of the of the Wine Industry Interlaboratory Testing Program that has be going on since 1999.^1,2,3The 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.

(Data from the Wine Industry Interlaboratory Testing Program)
			Sample 1
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

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?
	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

Customers Pay if Winemakers Choose Expensive Tests

Mike Miller — Fri, 19 Dec 2008 23:31:00 GMT

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.

Adding Malic Acid: L-Malic Acid vs DL-Malic Acid

Mike Miller — Sun, 16 Nov 2008 00:59:00 GMT

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.