Dr. Terry Bates - 8/27/01
Each year at veraison, we start monitoring fruit development in the Juice Quality project. Berry weight, juice soluble solids (brix), titratable acidity (TA), pH, and color will be measured weekly until the juice reaches 16 oBrix in the 11 different treatments. (To know more about the juice quality project, check out the web site).
Crop Load and Fruit Development
Since the processors pay for grapes based on tonnage with a sliding scale for sugar concentration, it is no surprise that growers are concerned with how they can manipulate yield and oBrix in the vineyard. Crop weight to pruning weight ratio, leaf area to fruit ratio, vine balance through pruning and/or thinning, etc. are all ways to describe crop load and crop load can have a big effect on the development rate of both the fruit and the wood.
Figure 1: 2000 sugar accumulation (A) and titratable acidity reduction (B) in Concord grapevines with high and low crop loads. Crop load affects oBrix but not TA within a given year. Sugar accumulation (C) and titratable acidity reduction (D) at one crop load in 1999 and 2000. Growing season temperature has a larger effect on TA than it does on brix.
Figure 1A shows that increasing crop load decreases the rate of sugar accumulation in the fruit (it takes longer for the fruit to hit 16 oBrix). A dense and inefficient canopy with a lot of shaded leaf area can further delay sugar accumulation and delay harvest. At veraison, Concord fruit is typically between 7 and 7.5 oBrix and veraison marks the point at which there is a rapid build up of sugars in the fruit. Photosynthesis provides sugar to the fruit, either directly from the leaves or indirectly from the translocation of starch in woody structures. In cases where us crazy researchers remove leaves during fruit ripening or in cases where early frost or severe disease diminish leaf photosynthesis late in the season, the fruit will continue to gain some sugar at the expense of vine capacity. It is also important to note that the curves in figure 1A are not linear. Sugar accumulation is rapid right after veraison and then slows down closer to harvest. There are a few things going on here. As the fruit increases in sugar concentration the sugar gradient between the fruit and the rest of the vine decreases which could slow down sugar loading in the berries. In other words, the vine goes from pumping sugar downhill at veraison to pumping sugar uphill near harvest. In addition, solar radiation and potential photosynthesis decreases in the weeks following veraison, which slows total sugar production (Figure 2).
Figure 1B shows that there is no effect of crop load on the reduction of juice titratable acidity (TA) in any given year. From other research done at the lab, vines with larger crops tend to have slightly lower TA but this is usually not statistically different, and certainly not supported by Figure 1B. TA decreases with time and (like sugar) the change is not linear with time - for a couple different reasons. Malate and tartrate are the major organic acids in grape juice. Both tartrate and malate are diluted as the berries pump up with water after veraison, causing some TA reduction. In addition to dilution, we talk about the acid 'burning up' in the grapes. The acid (primarily malate) is respired or broken down by enzymes in the fruit and this 'catabolism' is dependent on temperature. As we lose heat units later in the season (Figure 2), malate catabolism slows and causes the curve in Figure 1B to flatten out. Tartrate breakdown is slow, especially at typical Western, NY temperatures during fruit ripening, and contributes little to TA reduction before harvest.
Warm Seasons vs. Cool Seasons
In addition to the effect of crop load on sugar accumulation within any given year, there can be differences in fruit development from year to year within a given crop load. Figure 1C shows sugar accumulation from veraison to harvest in a cool (2000) and a warm (1999) year when the vine crop load is held constant (fixed node pruning). Note: the data is based on "days from veraison" not "day of the year." Veraison will be later in a cool year based on calendar date. At constant crop load there is little difference in sugar accumulation from year to year once veraison starts. On the other hand, the growing season heat accumulation can have a large effect on TA (Figure 1D). A cool year produces juice of higher acid because the berries enter veraison with higher acid concentrations. The literature indicates that temperatures during the fruit cell division phase and the fruit lag phase and not just temperature during fruit ripening can affect final fruit acidity. Theoretically, temperatures from veraison to harvest will determine the rate of acid reduction post-veraison. For example, unusually hot weather in late August and early September could increase the rate of acid reduction. However, the weather data from Figure 2 shows how we typically lose heat not gain heat during this period. Therefore, temperatures in June and July may be more important than temperatures in August and September in determining final TA in Lake Erie Concord fruit.
What can we look for this season?
With the low crop load in most vineyards and with the warm, dry summer, the fruit is developing quickly and will reach ‘maturity’ earlier than most years. Very preliminary data from the vineyard lab (Christine gave me a few numbers from the samples she was running in the lab) indicate higher sugar and lower acid when compared to the last few growing seasons. As a point of reference, the sugar is following the ‘low crop load’ curve of Figure 1A and the acid is well below the ‘warm 1999’ curve of Figure 1D. From these data we see that oBrix in the bin can be influenced by crop load and harvest date while TA is influenced more by growing season and harvest date. Both sugar accumulation and acid reduction are not linear with time, which further complicates the juice quality issue.
Why Be Concerned With Juice Quality?
In the Concord juice quality project, we are looking for 15.5-16.5 oBrix with 1.0-1.1 TA. In 1999, we were very close to those target values. In 2000, the TA was too high when the fruit reached the optimum oBrix and probably would have never made it to the optimum TA values no matter how long we let the fruit hang on the vine. In 2001, we will probably under shoot the TA target by the time the fruit hits 16 oBrix. From a grower perspective, the finer points of sugar-acid balance may not mean much. You are going to harvest your grapes according to the processor schedule. From a processor perspective, the closer the harvested fruit is to the target oBrix and TA, the less juice manipulation has to be done in the tanks to produce a consistent product, the less money has to be spent on juice manipulation, and the more natural the product will be.
Figure 2: Daily growing degree days (long term average) and daily solar radiation (1998) throughout the year. From veraison to harvest, Western New York looses heat and sunlight, which affects the pattern of fruit development in our region.