The definition of optimal maturity will vary depending upon the style of wine being made, the winemaker’s definition of quality, the variety, rootstock, site, seasonal factors and viticultural practices. If the grapes do not contain varietal aroma/flavour characteristics or mature tannins at harvest, these characteristics will not be evident in the wine.
Provided that the TA (titratable acidity) or pH levels are not so high that acidity adjustments are compromised, or sugar levels so high the resultant wine will be too alcoholic, than flavour intensity is usually the prime consideration when deciding on the harvest date. You can best assess optimal maturity by monitoring levels of grape flavourants. You can taste berries — however, tasters are often looking for the absence of herbaceousness rather than the presence of flavourants.
Due to the importance of fruit maturity in the ultimate wine quality, you must conduct field sampling of fruit in an objective and statistically acceptable manner. It is important that growers and wineries standardize grape sampling techniques, because without standardization it is impossible to compare results. Analysis of all pertinent quality factors may be prohibitive, both timewise and economically, and it is unlikely you will discover any single index of maturity that can be applied indiscriminately to all growing conditions and all varieties.
Because historical data for each area will be a critical factor in determining the optimal maturity of grapes, records should be kept (for each vintage) of maturity observations, sensory and chemical analysis, weather, and condition of the fruit at harvest.
Berry samples tend to show greater ripeness then the actual must composition in a winery. Sampling methods should provide samples representative of the fruit to be harvested.
Samples can be taken as berries or whole clusters, but be sure to obtain a sufficient quantity of fruit to give a representative sample. Depending on the duration of the bloom period and fruit set, the range in maturity among berries on a cluster and among clusters on a vine may vary by up to two weeks. Berries on the inside of clusters are less ripe than outer berries.
With cluster sampling, collect at least 20 clusters from throughout the vineyard. The main disadvantages of cluster sampling are possible depletion of crop from repeated sampling and the difficulty of processing large samples.
With berry sampling, take 200-400 berries from a large number of vines. Take samples weekly, beginning about three weeks before harvest. Sample more frequently as the anticipated harvest date becomes closer, particularly if there are changes in the weather that could affect ripening or fruit condition. Riper berries are found on the top and shoulders of the cluster and the least ripe are near the bottom tip.
You can use one of two techniques to select berries for sampling. No difference in accuracy has been noted between the techniques.
First is to select only berries from the middle portion of the cluster. These will be intermediate in their maturity. Second is to alternate between selecting a berry from the top, middle and bottom of the cluster. The key here is to avoid taking all the berries from the top and shoulders, since this may give a higher Brix reading than exists in the vineyard. Berry sampling in various locations on the cluster may be significant in the case of larger clusters, but berries from the middle of the bunch are acceptable in the case of varieties with small clusters, sampling alternate sides of the cluster.
The variance between the sample analysis and the winery analysis is smallest if a minimum number of berries are collected from a large number of vines. The maximum area for sampling size should be 2 ha.
When sampling grapes, the following should be assessed and noted:
Prepare berry samples for analysis by crushing without breaking the seeds (seed breakage can elevate pH readings 0.2-0.3 units). This can be done by hand, taking care to thoroughly crush each berry. Immediately separate the juice — free run and press combined — from the skins and seeds and then allow it to settle to remove suspended solids. Use this settled juice for analysis.
Red grape samples can be left on their skins for 1-2 hours at room temperature before pressing if desired. This will increase the pH of the must and may more closely resemble commercial operations.
The important factor is consistently measuring the juice the same way for each sampling and allowing the berries to reach ambient temperature before testing. Storing juice in a refrigerator for later analysis can precipitate potassium bitartrate and may affect the pH, TA and potassium readings. Using only free run juice for analysis gives higher sugar and TA and lower pH and potassium readings than a free run/press juice sample.
Sample preparation best approximates commercial processing at a yield of about 65 ml/100 g. This corresponds to about 600 l/short ton.
From a winemaking perspective, the closer the harvested fruit is to the target Brix and TA, the less juice manipulation has to be done in the tanks to produce a consistent product, and hence the less money spent and the more natural the product.
Sugars accumulate in ripening berries from veraison to harvest. The smallest berries attain the highest sugar concentrations while larger berries have a much lower sugar concentration.
There is an initial rapid phase of sugar accumulation, then the vine ceases transport of sugar to the fruit and further sugar increases are due to dehydration. Too much heat or drought during later stages of ripening can delay physiological maturity and cause excessive dehydration and shriveling, resulting in abnormal sugar increases due to a concentration effect rather than grape maturation. Raisined berries will give a low initial sugar reading due to incomplete sugar extraction. During pressing and fermentation, additional fermentable sugar is extracted.
In a "normal" year, it takes about one week for grapes to accumulate one degree of brix or 1% soluble solids. Natural sugar accumulation stops at somewhere around 25°Brix.
Sucrose is produced in the leaves by photosynthesis and is transported to the berries through the phloem. In the berries, sucrose is inverted to glucose and fructose, which are the primary sugars present at harvest. Glucose predominates at veraison, but by harvest, fructose and glucose are about equal in proportion. Fructose is the sweeter of the two and may exceed glucose in overripe berries.
The percentage of sugar in the juice is measured as soluble solids content and is commonly expressed as degrees Brix (or degrees Balling) or as grams of sucrose per 100 grams of juice (%). Conversion tables can be used to convert specific gravity to Brix.
Several studies have shown no relationship between sugar levels and the accumulation of grape flavour compounds. Most flavour compounds show most of their accumulation later in the ripening process when sugar accumulation has slowed, hence the importance of hang time.
A handheld Brix refractometer at 0-32% sugar (% soluble solids) with automatic temperature compensation (ATC) and ± 0.2 % accuracy is commonly used on a grape juice sample. Refractometers measure the refractive index of grape juice. The sample does not have to settle for measurement by refractometry and juice may be placed directly on the glass surface.
A scale reading in % sugar is commonly used, but w/w or brix or % soluble solids are other commonly used terms. For example, 1°Brix = 1 gram of sugar per 100 grams of solution or 1% sugar.
Temperature correction to 20°C is essential for an accurate reading if refractometers are not ATC. Even with an ATC refractometer, the temperature of your sample should be 17-23°C for the most accurate results. Calibrate the refractometer to 0% using 20°C distilled water.
Use a 20°Brix solution for calibration by weighing 20 g sucrose (table sugar) and dissolving it in 80 g of distilled water at 20°C. These must be weighed accurately to give true readings. These amounts are weight/weight — not weight/volume. You should not add an amount of sugar to a volumetric flask and make it up to the mark with water or you will get falsely high results with your grape samples.
Clean the prism of a refractometer occasionally with alcohol or methanol and use distilled water between juice readings, blotting dry with a clean, lint-free cloth between readings.
Hydrometers also give a measure of the specific gravity of a solution, which relates to the total soluble solids content of the grape juice. Specific gravity is the weight of 1 ml of solution divided by the weight of 1 ml of distilled water at 20°C.
Accurate, narrow range hydrometers (1-5° or 1-10°Brix units) subdivided to 0.1° units are the norm. Inexpensive hydrometers, often covering a wide range such as 0-30° and having scales such as "potential alcohol," are not very accurate. A hydrometer gives slightly higher readings than a refractometer if suspended solids are present, therefore it is important to use well-settled juice.
The potential alcohol content of the fermented juice can be estimated by multiplying the degrees Brix on red musts by 0.55 and on white musts by 0.60. Note that with raisined berries, the initial sugar reading is low due to incomplete sugar extraction. During pressing and fermentation, additional fermentable sugar is extracted.
Tartaric and malic acid represent about 90% of the acids in wine grapes. These acids are synthesized primarily in the grapes, as opposed to the vine itself. Malic acid is less stable, with the sharp tang of green apples, while tartaric acid is stronger and more stable.
Wine grapes are unusual in that few other fruits accumulate significant quantities of tartaric acid. Both tartaric acid and malic acid increase rapidly in the berries prior to veraison. From veraison to harvest, the tartaric acid content of the berries decreases due to the increase in berry size and then remains relatively constant during the later stages of ripening. Malic acid also decreases due to increasing berry size, however it is used as an energy source by the berries in respiration as well. The decrease in malic acid during the later stages of ripening is more rapid during periods of warmer weather.
The tartaric/malic ratio varies with grape variety, the degree of ripening and environmental conditions during ripening. The overall net decrease in acidity during the later stages of ripening is primarily due to the decrease in malic acid content. Acidity is an important measurement for flavour balance and wine style and has a greater impact on the way the juice tastes than does pH.
To determine the total acidity of a juice sample, recorded as titratable acidity (TA), measure the hydrogen ions released when a juice sample is titrated with a standardized base to a defined endpoint using sodium hydroxide.
Do this by accurately pipetting 5 ml of juice into 100 ml distilled water (at 8.2-8.4 pH) in a beaker. Then titrate this with standardized 0.1 N NaOH using a calibrated burette to a neutralized endpoint of 8.2-8.4 pH. The number of milliliters of NaOH used is multiplied by 1.5 to equal the TA in grams per litre of tartaric acid (divide by 10 to give % acidity or g/100 ml).
Typically, wine grapes range from TA 5.0-10.0 g/l of tartaric acid at harvest with winegrapes from warmer sites or warmer seasons having lower TA levels than wine grapes from cooler sites or cooler seasons. TA can also be simply expressed as the volume of NaOH required to neutralize a sample. In some countries, the percent acidity (g/100 ml) is expressed as sulfuric acid. This percent, multiplied by 1.5, approximately equals the percent acidity expressed as tartaric acid.
Tartaric and malic acid can also be measured separately by colorimetric, enzymatic and chromatographic methods.
Measurement of pH is one of the most important laboratory procedures. pH is responsible for microbiological and chemical stability, colour, SO2 equilibrium and effectiveness, plus other oenologically significant factors.
pH is a measure of the free hydrogen ion concentration and is an intensity aspect, as opposed to TA, which is a quantity aspect. It is a direct measure of the total hydrogen ion content in solution and is expressed on a scale of 0-14 units.
A solution with a pH of 7 is a neutral solution, having an equal number of acid and base ions. As the pH decreases below 7, the acidity increases. As the pH increases above 7, the acidity decreases and the solution becomes more basic.
A change of one pH unit represents a tenfold change in concentration of free hydrogen ions. Thus, a juice with a pH of 3.0 has 10 times the acid intensity of a juice with pH 4.0. The desired pH levels at harvest range from 3.10-3.50. Wine grapes from warmer sites or warmer seasons have higher pH values than wine grapes from cooler sites or cooler seasons.
To determine the pH on settled grape juice — without dilution, refrigeration or freezing — a pH meter with accuracy of a least ± 0.05 pH units is required.
Carefully calibrate the meter prior to each use with standard pH 4.00 and pH 7.00 buffer solutions. It is desirable for the pH meter to have ATC. Whether or not temperature control is available, samples tested, as well as buffers, should be close to 20°C. A temperature correction table is required for meters without ATC.
Because grape solids can clog the electrodes, thus impeding the flow of ions, it is critical to clean electrodes regularly and properly. The most common error is the use of worn or insensitive electrodes.
Changes in pH are not necessarily a function of berry maturity and relate to potassium and vigour. The amount of potassium increases during ripening, especially in the skins, and is by far the most abundant mineral in grapes.
Excessively light crops on high capacity vines and sites will produce higher pH at optimum sugar levels. As well, the pH of irrigation water can affect grape pH.
After veraison, the pH will rise if there is significant potassium uptake at the same time as acid respiration. pH will fall if there is faster production of malic and tartaric acid in the berry than potassium uptake, for instance after a rainfall or irrigation.
Overripe grapes or a warm climate can result in high pH and low TA, due to the respiration of organic acids. Immature grapes or a cool climate can give low pH and high TA due to the high level of non-exchanged organic acids. A very long ripening period in a cool climate results in both high pH and high TA. Grapes with high acidity and high pH will also have high potassium, high malic acid content, poor colour and probably excessively vegetative aromas.
Yeast requires nitrogen compounds for the production of cell biomass and the synthesis of proteins and enzymes necessary for fermentation. The readily fermentable nitrogen compounds in juice consist primarily of ammonia and alpha-amino acids, with the total of these two compounds called YAN or YANC (yeast-assimilable nitrogen content). Nitrogen can vary year-to-year in a vineyard depending on weather (rainfall, temperature, etc.), irrigation, fertilization, crop load, etc.
From veraison to harvest, the ammonia concentration in the pulp of the berry decreases at the same time the amino acid content significantly increases. In unripe fruit, the ammonia content may represent up to 50% of the pulp nitrogen. At full maturity, the amino acid content may represent up to 90% of the pulp nitrogen. The pulp of mature berries contains up to 20% of the total berry nitrogen, with the remainder distributed in the skins and seeds.
Fermentable nitrogen content affects the fermentation rate, and deficiencies may produce sulfide odours, contribute to slow or stuck fermentations and accelerate wine aging.
You can measure the ammonia content with a spectrophotometer at 340 nm using an enzymatic ammonia diagnostic kit or by using an ammonia ion selective electrode. The enzymatic test has the advantage of being fast, accurate and inexpensive (provided you have a UV spectrophotometer). You can also measure the alpha-amino acid content by spectrophotometric assay as well. This assay is called NOPA for "nitrogen by OPA" and is reliable and fast.
Add the ammonia and alpha-amino acid readings together to determine a YAN reading. Recommended levels of YAN needed for healthy fermentations are 200-300 mg N/l. Tthe higher the sugar content and the riper the grapes, the more nitrogen is required.
Variations in climate, soil, cultivation practices, soil moisture content and fertilization practices may have a significant impact on juice nutrition.
Excess nitrogen (from fertilization) on mature vines can lower the vines resistance to sun damage and fungal disease. Low nitrogen levels may cause fermentation and wine aging problems, and winemakers often add supplements to juice and fermenting wines to balance nutritional deficiencies. These supplements may include diammonium phosphate, yeast extracts and vitamins. Moderate nitrogen supplementation of deficient musts may also increase fruit aroma intensity and wine quality, particularly in white wines, while excessive additions may lead to increased formation of ethyl carbamate.
The following formulas have been used historically to determine grape maturity:
1) Brix x (pH)2
– optimal ripeness is 220-265 for table wine (late harvest over 270)
OR
2) Brix ÷ TA (g/100 ml)
– optimal ripeness is 30-32 for table wine (late harvest over 36)
However, the sugar-acidity ratio is quite variable across different varieties and growing conditions as well as year-to-year, therefore these universal rules may not predict the value for wine quality. As well, it is not clear whether the optimal sugar-acidity balance always coincides with optimal maturity of grape flavourants.
During grape ripening, normally sugars increase, pH increases and TA decreases to optimum maturity. Occasionally, the reverse will happen.
Reasons why sugars might decrease:
Reasons why pH might decrease or TA might increase
Seed colour is another useful and easily determined index of maturity. Wine grapes have an average of four seeds/berry, though two or three seeds are not uncommon.
During fruit maturation, seeds mature at a different rate than the accumulation of sugar. As seeds mature, they change colour from green to brown to dark brown. The brown colour is caused by oxidation of the tannins as they become fixed to the seed coat and thus less extractable during fermentation. This is particularly important in red wine production where tannins are extracted from the seeds and skins.
Tannins are highest in skins and almost equal in stems and seeds. There is no difference in tannin levels of seeds in white or red grapes. As grapes ripen, tannin levels of seeds may decrease. But later, as skins ripen, tannin levels may increase again.
Seed tannins make up over 60% of the total tannin concentration. Unripe seeds mean immature, readily extractable seed tannins that result in harsh or hard wines, whereas ripe seeds give soft and supple, less bitter or harsh tannins. Tannins that have bitterness attributes are derived from seeds, not skins.
Changes in seed tannin occur late season when it appears that no additional ripening can take place. Seed tannin astringency changes little with time. What does change is the extractability of these compounds into the wine. A balanced wine is achieved when only 15% of the available tannins are extracted.
Some winemakers taste seeds to assess grape maturity. However, seed bitterness may be overpowering, and many individuals may not be able to accurately discriminate levels of seed bitterness. Physical characteristics of the seeds – colour, texture and brittleness – may be more important indicators of seed maturity.
Colour in grapes is derived from increases in anthocyanins and biosynthesis during ripening, therefore grapes become more uniformly coloured with maturity.
Wine colour is predetermined by the levels of anthocyanin pigments and other, non-coloured phenolic components in the grapes and is little affected by winemaking practices. These anthocyanin pigments are present in the skins, and normally the pulp or juice is not coloured. When grapes are heated by fermentation (or heat extraction) these peripheral cells are killed, and the pigment is released into the pulp, aided by mixing, which facilitates pigment diffusion into the juice.
Unripe fruit has green juice because of high chlorophyll content and low pigment content in the skins. At full maturity, white grapes have light yellow to golden juice and red grapes have red to purple juice. It is easier to extract pigments from more mature fruit.
With some red varieties, such as Pinot Noir, if the grapes are pressed without skin contact, then a white wine is produced. Conversely, pink or light-red skinned varieties of so-called white grapes, such as Gewurztraminer or Pinot Gris, are allowed limited or no contact with skin to avoid pink-coloured wines.
Pigmentation of skins is greater in cooler temperatures and in areas with greater temperature contrasts between day and night. Excessive nitrogen decreases colour. Red wines will progress in colour development from light red to medium red to ruby to black-red. White wines will progress in colour development from dark green to light green to gold.
The physiological mechanism the vine uses to make sugar is not the same as the mechanism used to produce secondary metabolites such as aroma, flavour and phenolic compounds. You can have low sugar and a high concentration of varietal aroma and flavour or the opposite. In some poor sugar-developing seasons, such as cool, cloudy or rainy summers, flavours may reach their peak before sugar reaches the desired level.
Grape-derived aroma and flavour compounds present in the grape skins and pulp are the principle source of wine aroma, flavour and taste. They are the most important fruit attributes contributing to wine quality and should be considered a part of any grape maturity evaluation. If grapes are harvested when varietal character is lacking, then this character will be absent or reduced in the finished wine.
Phenols are present in grape skins, seeds and stems as well as barrels and tannin additions. Increased light incidence (longer daylight hours) on berries stimulates the production of phenol. Grape phenols have a significant influence on wine structure, including body, colour, tannin intensity, astringency, bitterness and dryness. As fruit matures, their phenolic compounds bind together or polymerize. This polymerization causes a sensory change from hard and bitter to astringent and finally to soft and supple.
Excessive herbaceousness results in a reduction in fruit intensity.
Herbaceousness is mainly derived from pyrazines. Methoxypyrazines are nitrogen-containing compounds present in green plant tissue, including grape berries. Concentrations range from 0-35 mg/l. The presence of methoxypyrazines increases the perception of tannin intensity, astringency and tannins, thus magnifying the sense of acidity.
Pyrazines decrease following version. This decrease is directly correlated to the decrease in malic acid. Malic acid decreases at a faster rate during warm night time temperatures, as do methoxypyrazines. High soil moisture delays fruit maturation, and the reduction of methoxypyrazines and over-irrigated/over-fertilized vines produce grapes lacking flavour and colour. Increased sun exposure increases the rate of grape maturation and the reduction in methoxypyrazines.
As well, canopy management can have a dramatic effect on aroma and flavour development.
Crop load (either too high or too low) also impacts these compounds, and excessive crop-to-leaf area can delay the rate of fruit development. On average, a leaf area of 8-14 sq. cm (single canopy) and 5-8 sq. cm (double canopy) per gram of fruit produced is required to adequately ripen a crop to specification.
Muscat varieties have the strongest vinifera aroma due to a distinctive mixture of volatile acids, alcohols, esters, terpenoids and other compounds.
The incidence of botrytis bunch rot and/or sour rot complex is important to distinguish. The fungus Botrytis cinerea causes botrytis bunch rot, and a complex of bacteria, along with fungi, causes sour, grey or vulgar rot.
The external development of rot is not an accurate indicator of internal damage to the berry. Transformations are already well advanced at the first sign of the characteristic brownish stains, which are clearly visible on white grapes and less so on very dark red grapes. The surface of sour rot infected grapes appears black, brown or green and less fuzzy than botrytis infected grapes. Also, grapes infected with sour rot give off a pungent, vinegar smell and tend to attract fruit flies.
Each has harvesting considerations. Botrytis cinerea or "noble rot" increases quality and is often desirable for "botrytis-affected wines." Sour rot decreases quality and 0% is preferred. Above 8% is unacceptable for reds and above 5% is unacceptable for whites. With sour, grey or vulgar rot the Brix will decrease, the TA will decrease and the pH will increase over unaffected grapes. With noble rot the Brix will increase, the TA will decrease and the pH will increase over unaffected grapes.
Botrytis bunch rot and sour rot infected grapes have reduced ammonia nitrogen available for yeast metabolism and require supplementation to avoid stuck fermentation and possible sulfide formation. As well, infected grapes form polysaccharides that create clarification problems for the winemaker and excess laccase that can cause oxidation. Tighter clustered varieties are more prone to infection then loose clusters.
A cool autumn slows down development, better balances can be achieved and more aroma and flavour constituents are accumulated. Cool climates generally produce the best quality table wines, and evidence suggests that it is the lower temperatures in the autumn that are of special significance.
In warm climates, ripening of grapes occurs early, when the weather is still warm or hot. These hot conditions cause rapid evelopment of sugars, rapid loss of acids and high pH. The juice is often unbalanced, and the grape appears to have had insufficient time to accumulate those many chemical compounds that add distinction to wine.
Further ripening potential is often weather dependent, and consideration should be given to extremes of weather that can delay or arrest maturation of the grapes.
