8 Harvest & Quality Issues 8.1 Fruit Sampling and Maturity By: Lynn Bremmer, Mount Kobau Wine Services Obtain a sufficient quantity of fruit to give a rep- The definition of optimal maturity will vary de- resentative sample. With cluster sampling, col- pending upon the style of wine being made, the lect at least 20 clusters from throughout the vine- winemaker’s definition of quality, the variety, yard. The main disadvantages of cluster sampling rootstock, site, seasonal factors and viticultural are possible depletion of crop from repeated sam- practices. If the grapes do not contain varietal pling and the difficulty of processing large sam- aroma/flavour characteristics or mature tannins ples. With berry sampling take 200-400 berries at harvest these characteristics will not be evi- from a large number of vines. Take samples weekly dent in the wine. Due to the importance of beginning about three weeks before harvest. Sample fruit maturity in the ultimate wine quality, you more frequently as the anticipated harvest date must conduct field sampling of fruit in an objec- becomes closer, particularly if there are changes tive and statistically acceptable manner. It is im- in the weather that could affect ripening or fruit portant that growers and wineries standardize condition. The variance between the sample anal- grape sampling techniques because without ysis and the winery analysis is smallest if a minimum standardization it is impossible to compare re- number of berries are collected from a large num- sults. Analysis of all pertinent quality factors ber of vines. Berry sampling in various locations on may be prohibitive both time-wise and econom- the cluster may be significant in the case of larger ically; and it is unlikely you will discover any single clusters but berries from the middle of the bunch are index of maturity that can be indiscriminately ap- acceptable in the case of varieties with small clusters, plied to all growing conditions and all varieties. sampling alternate sides of the cluster. Maximum Because historical data for each area will be a criti- area for sampling size should be 5 acres (2 ha). cal factor in determining the optimal maturity of grapes, records should be kept for each vin- Sample Preparation tage of maturity observations, sensory and Prepare berry samples for analysis by crushing chemical analysis, weather and condition of the without breaking the seeds (seed breakage can fruit at harvest. elevate pH readings 0.2 to 0.3 units). This can be Grape Sampling done by hand, taking care to thoroughly crush each berry. The juice, free run and press combined, is Sampling methods should provide samples repre- separated immediately from the skins and seeds then sentative of the fruit to be harvested. Sample allowed to settle to remove suspended solids. Use distinct blocks within a vineyard separately. Collect this settled juice for analysis. Red grape samples proportional quantities of fruit from exposed and can be left on their skins for 1-2 hours at room shaded locations in different parts of the canopy, at temperature before pressing if desired. This will different heights on the vine and on opposite increase the pH of the must and may more closely sides of the row. Avoid sampling from vines at the resemble commercial operations. The important end of rows, irrigation rows and odd vines that are factor is consistently measuring the juice the same obviously different from the majority of vines in way for each sampling and allowing the berries to the block. Avoid sampling fruit with dew or wet reach ambient temperature before testing. Storing fruit after a rainfall. Include secondary clusters in juice in a refrigerator for later analysis can precipi- the sample only if they will be picked at harvest. tate potassium bitartrate and may affect the pH, TA Samples can be taken as berries or whole clusters. and potassium readings. 2010 Best Practices Guide for Grapes for British Columbia Growers 8-1 8 Harvest & Quality Issues Analysis of Total Soluble Hydrometers also give a measure of the specific gravi- ty of a solution, which relates to the total soluble Solids (Sugar) in Grape Must solids content of the grape juice. Specific gravity is Sugars accumulate in ripening berries from verai- the weight of 1 ml of solution divided by the o son to harvest. Sucrose is produced in the leaves weight of 1 ml of distilled water at 20 C. Accurate, o o by photosynthesis and is transported to the berries narrow range hydrometers (1-5 or 1-10 Brix units) o through the phloem. In the berries, sucrose is subdivided to 0.1 units are the norm. Inexpensive inverted to glucose and fructose, which are the hydrometers, often covering a wide range such as o primary sugars present at harvest. The percentage of 0-30 and having scales such as ‘potential alcohol’, sugar in the juice is measured as soluble solids are not very accurate. A hydrometer gives slightly content and is commonly expressed as degrees higher readings than a refractometer if suspended Brix (or degrees Balling) or as grams of sucrose per solids are present, therefore it is important to use 100 grams of juice (%). Conversion tables can be well-settled juice. used to convert specific gravity to Brix. Several The potential alcohol content of the fermented studies have shown no relationship between sugar juice can be estimated by multiplying the degrees levels and the accumulation of grape flavour com- Brix on red musts by 0.55 and white musts by pounds. 0.60. Note that with raisined berries the initial Most flavour compounds show most of their accu- sugar reading is low due to incomplete sugar ex- mulation later in the ripening process when sugar traction. During pressing and fermentation addi- accumulation has slowed, hence the importance of tional fermentable sugar is extracted. “hang time”. Analysis of Acidity in A hand-held Brix refractometer @ 0-32 % sugar (% Grape Must soluble solids) with automatic temperature compen- sation (ATC) and ± 0.2 % accuracy is commonly Tartaric and malic acid represent about 90% of used on a grape juice sample. Refractometers the acids in wine grapes. These acids are synthe- measure the refractive index of grape juice. The sized primarily in the grapes (as opposed to the sample does not have to settle for measurement by vine itself). Malic acid is less stable with the refractometry and juice may be placed directly on sharp tang of green apples while tartaric acid is the glass surface. A scale reading in % sugar is stronger and more stable. Wine grapes are unu- commonly used (w/w or brix or % soluble solids are sual in that few other fruits accumulate signifi- other commonly used terms). For example 1oBrix cant quantities of tartaric acid. Both tartaric = 1 gram of sugar per 100 grams of solution or 1% acid and malic acid increase rapidly in the ber- sugar. Temperature correction to 20oC is essential ries prior to veraison. From veraison to harvest for an accurate reading if refractometers are not the tartaric acid content of the berries decreases ATC (even with an ATC refractometer the tempera- due to the increase in berry size and then remains ture of your sample should be 17o to 23o Celsius for relatively constant during the later stages of ripen- the most accurate results). Calibrate the refractom- ing. Malic acid also decreases due to increasing o o eter to 0% using 20 C distilled water. Use a 20 berry size, however it is utilized as an energy Brix solution for calibration by weighing 20 grams source by the berries in respiration as well. The sucrose (table sugar) and dissolving in 80 grams of decrease in malic acid during the later stages of distilled water at 20o C. These must be weighed ripening is more rapid during periods of warmer accurately to give true readings. These amounts are weather. The tartaric/malic ratio varies with grape weight/weight NOT weight/volume. You should variety, the degree of ripening and environmental not add an amount of sugar to a volumetric flask conditions during ripening. The overall net de- and make it up to the mark with water or you will crease in acidity during the later stages of ripen- get falsely high results with your grape samples. ing is primarily due to the decrease in malic acid Clean the prism of a refractometer occasionally with content. Acidity is an important measurement for alcohol or methanol and use distilled water between flavour balance and wine style and has a greater juice readings, blotting dry with a clean, lint-free impact on the way the juice tastes than does pH. cloth between readings. 8-2 2010 Best Practices Guide for Grapes for British Columbia Growers 8 Harvest and Quality Issues To determine the total acidity of a juice sample, rec- To determine the pH on settled grape juice (without orded as titratable acidity (TA), measure the hydro- dilution, refrigeration or freezing) a pH meter with gen ions released when a juice sample is titrated accuracy of a least ± 0.05 pH units is required. with a standardized base to a defined endpoint Carefully calibrate the meter prior to each use with using sodium hydroxide. Do this by accurately standard pH 4.00 and pH 7.00 buffer solutions. It is pipetting 5 ml of juice into 100 ml distilled wa- desirable for the pH meter to have automatic tem- ter (at 8.2-8.4 pH) in a beaker. This is then titrat- perature control (ATC). Whether or not temperature ed with standardized 0.1 N NaOH using a cali- control is available, samples tested, as well as buffers, brated burette to a neutralized endpoint of 8.2 to should be close to 20o C.