Monday, 12 February 2018

California grapes: quantity versus quality

Grape production is a balancing act between quantity and quality — producing a greater quantity is usually assumed to result in a reduction in quality. Therefore, attempts by grape growers to increase grape quality are usually associated with trying to decrease quantity. The relationship works both ways.

It is therefore of interest to look at the big picture of this supposed relationship. This quality/quantity relationship is usually investigated only at the micro level — for example, individual growers might decide to increase their grape quality by thinning their crop. But what happens at the macro level, across all growers? This question seems rarely to have been asked.

One simple way to start looking at this topic is to compare the production area of particular grape types with the amount of fruit they produce. We might anticipate that the highest quality varieties produce less fruit than do lower quality varieties. This is a simplistic approach, of course, because there are many factors that affect fruit production, most notably the weather; but if we restrict ourselves to a particular viticultural area, then it might be a useful place to start.

So, I decided to look at the California grape data provided by the United States Department of Agriculture (USDA). The latest report is from April 2017, which shows the acreage of productive vines (in each US state), for both red varieties and white varieties. I then compared these data to the data for the 2016 California grape crush provided by the American Association of Wine Economists (AAWE), for the top reds and top whites.

The data are shown in the two graphs, one for each type of grape. Within each graph, each point represents a single grape variety in California, showing its bearing acreage horizontally and its grape crush vertically. The lines on the graphs are best-fit linear regressions, illustrating the "average" production expected from each variety based on its acreage. In both cases the lines fit the data quite well, explaining c. 85% of the variation in the data.

California red grapes by area and crush

The first graph shows the data for the red varieties, where Cabernet sauvignon is by far the most widely planted grape variety, as well as the one most highly esteemed by winemakers. I therefore calculated the regression line, as shown, without including this variety, so that the line is fitted only to the other varieties — this then tells us what production to "expect" from Cabernet, based on the observed data for the other varieties. We can see that, as anticipated for the top variety, Cabernet sauvignon produces a much smaller crop than do the other varieties.

Interestingly, both Zinfandel and Rubired (labeled on the graph) produce a larger grape quantity than we might expect from their acreage, whereas all of the other varieties are close to their expectation. This is notable because Zinfandel is the second most widely planted red grape, and it is usually considered to also be a premium variety. Other common premium varieties, such as Pinot Noir, Merlot and Syrah, produce crops at about their expected level in California.

California white grapes by area and crush

A similar pattern is seen when we look at the white grape varieties, as shown in the second graph. Indeed, the regression lines in both graphs have almost the same slope (and intercept), indicating that red and white production both have the same relationship to area.

Chardonnay is both the most widely planted white grape variety and the one most highly esteemed by winemakers. It is obvious from the graph that Chardonnay produces less quantity than is expected based on the other white varieties, as anticipated.

Interestingly, three of the next four most widely planted white varieties (labeled on the graph) produce a larger grape quantity than we might expect from their acreage, whereas all of the other varieties are close to their expectation. This matches the pattern observed for the red varieties, where only the top variety has a reduced crop.

Finally, the California Department of Food and Agriculture's California Grape Crush Report Preliminary 2017 allows us to look at the broad-scale economics of wine-grape production. The graph below shows the inflation-adjusted price per ton of wine grapes (vertically) versus the grape crush tonnage (horizontally). Each point represents the crop for one year from 1989 to 2016, inclusive.

Price per ton of California grapes through 20 years

As can be seen, for the white-wine grapes their price is unrelated to the crop size — prices do not go up or down when the crop is large. On the other hand, for the red-wine grapes the price has a tendency (ie. with a few exceptions) to rise when the crop is large (correlation = 33%). In both cases, price is not related to scarcity, which is the important point. This implies that voluntarily restricting crop size does not affect the overall economics — the reduction in crop is likely to be compensated by increased price.


So, Cabernet sauvignon and Chardonnay are the most widely planted and most esteemed red and white grape varieties, respectively, in California, and they both produce smaller crops than might be expected based on the production levels of other varieties. Furthermore, the situation differs for some of the other widely planted varieties, which produce larger crops than might be expected. This seems to match what is anticipated from the suggested relationship between quantity and quality — quantity is less when quality is at its very highest. For California grapes, less is more.


  1. It would be instructive see the graphs for Pinot Noir -- a grape that has rapidly risen in both quality and popularity -- "benchmarked" against Chardonnay and Cabernet Sauvignon.

    1. Bob

      I will have a think about how that might be done — I doubt that it would be straightforward with the data available.

  2. One comment on Zinfandel. Although Zinfandel is a red grape, about 2/3rds of the Zin crop is used to make the Rose termed White Zinfandel. Since the sugar level at harvest for grapes used for White Zinfandel don't need to be as high as for red Zinfandel (20-21 brix for White Zinfandel vs. 23-25 brix for red), the Zinfandel vineyards intended for White Zinfandel production can be cropped at a higher level. This inflates the yield for all Zinfandel.

    1. Jim, This is a good point, which I neglected to think about. I am not sure how to deal with it, though — the data are too coarse grained. It might be interesting to stick it into the comparison that Bob suggested.

  3. David: This is an interesting analysis but it cries out for disaggregation of the data. You write " if we restrict ourselves to a particular viticultural area, then it might be a useful place to start." But I'm not sure that treating all of California as a singular "viticultural area" is either useful or correct.

    Yield is determined by both humans and by nature in the form of environment. Managerial decisions such as pruning, irrigation and fertilization, trellis systems and whether or not to "drop crop" all effect vineyard yield. But so does the macro environment. Grapes grown in warm regions with plenty of sun and deep soils with ample water can, in the Southern San Joaquin valley (crush districts 12, 13 and 14) deliver 14-20 tons per acre. Grapes grown in cooler, coastal areas with less sun and heat and thinner soils (at least in the case of hillside vineyards) may average between 2 and 6 tons per acre.

    Grapes grown in cooler areas must fetch higher prices than the same variety grown in warm interior valleys--they coastal vineyards can't compete on yield. The cooler areas, such as the North Coast (districts 1, 2, 3, and 4) and Central Coast (districts 7 and 8) grown grapes with higher acidity, more phenolic compounds (pigments and tannins) and higher concentrations of aromatics and flavor.

    Cabernet and Chardonnay are grown in both coastal areas and in the warm Central valley (which delivers about 3/4 of the volume of all winegrapes). Taking an un-weighted average yield is thus misleading.

    Using the 2017 preliminary grape crush report and the 2016 bearing acreage by crush district, we can compare regions. Napa (District 4) crushed 66159 tons of Cabernet, which were grown on 19557 bearing acres, giving a yield of 3.38 tons per acre. In contrast, district 13 in the heart of California's San Joaquin valley, crushed 74243 tons grown on 5512 acres, yielding 13.46 tons per acre.

    I'm not sure that taking a statewide average is a particular useful way of comparing quality and quantity.

    1. Jim, Indeed, it would be interesting to look at the data in finer detail, as this would presumably reveal additional patterns. I may get the chance to do that at some stage. However, I have realized that compiling the official data in a usable format will not be easy. This would be a major research project, and probably best done by someone with access to the underlying computer files, not just printed reports.