What is The Best Grape Harvest Maturity Index to Determine Harvest Time?

• Harvest maturity indices overlap significantly, but as a standard practice to fix harvest time for grapes meant for winemaking, more parameters are used than for fruits for fresh consumption.
• Total soluble solids, titrable acidity, taste, color, and phenols are critical aspects of the harvest maturity index.
• Non-destructive near-infrared spectroscopy-based techniques are increasingly used to estimate harvest maturity index.

Grapes are versatile, and 57% of global production is for winemaking, 36% for table, and 7% for dried grapes. The time of picking estimated by the harvest maturity index can significantly influence the yield amounts and quality of grapes slated for their end use. Find out what the crucial harvest index for grapes is in this article.

Grape Harvest Maturity

Grapes are non-climacteric fruits that cannot ripen postharvest or develop more color, flavor, and sugar. However, leaving it too long on the vines leads to quick rotting, berry shattering, or damage by insects and wildlife. Harvesting at optimal ripeness is a challenge with grapes, as berries within the same bunch can ripen at different times. So, harvest maturity indices are crucial to determine the correct time for picking.

Grapes reach physiological maturity immediately after verasion, or when fruit ripening begins, since seeds can germinate at this point. It is at verasion that berries accumulate sugars, expand, and soften while becoming less acidic, see Figure 1. Hence, grapes are harvested well after physiological maturity.

Grape ripeness depends on several external factors such as season, weather, and viticultural characteristics like variety, crop load, sun exposure, vine vigor, and health. So, grape harvest dates will vary from year to year in the same vineyard.

Figure 1. “Detailed schematic of berry development. Illustration by Jordan Koutroumanidis and provided by Don Neel: Practical Winery and Vineyard. (Image credits: https://www.awri.com.au/wp-content/uploads/4_phenology_berry_development_1_and_2.pdf)

Table Grapes Harvest Maturity Index

Table grapes are harvested when fully ripe or at horticultural maturity. Grapes harvested before fully ripe lack taste and color development and will be sour. While grapes harvested late will have a bland flavor, and quality falls rapidly. Grapes are monitored starting with veraison for firmness, size, color, total soluble solids (TSS), titratable acidity (TA), taste estimated by sugar-to-acid ratio (TSS/TA), and Chronological harvest time depending on the variety. For example, in India, early grapes are harvested in May; however, some grape varieties are available year-round.

For an accurate assessment of harvest maturity, growers are recommended to use multiple indicators. The choice and use of the harvest indices will depend on the grape variety.

• Cultivars with low acidity must be evaluated by visual aspects and TSS, such as the ‘Queen of the Vineyard.’
• Meanwhile, ‘Sultanina’ is harvested based on TSS and TA.
• In the years with more cloudy weather, color, sweetness, and taste development are less. Waiting for the correct development can delay the harvest and negatively affect yield.

Physical Harvest Maturity Indices

The physical parameters used as the harvest maturity index for table grapes are appearance and size.

Appearance: Grapes get a translucent bloom when ripe, and all berries must develop a uniform color and a hue characteristic of the variety.

• In California, a minimum color development is necessary for red and black grape varieties.
• In India, grapes are harvested after color change starts at the tip of the berries and development of sweetness.

Size: The berries in a cluster must all be developed entirely and uniform.

Chemical Harvest Maturity Indices

The chemical and internal parameters are increasingly used to determine grape harvest maturity. For grapes, these are TSS measured as Brix, TA, and taste.

Various regions have different sugar content requirements. Generally, quality grapes must have a minimum of 15%–20% soluble solids and taste or TSS/TA ratio of between 24 and 35.

• USA: In California, grapes are harvested when TSS is between 14 and 17.5%, depending on the variety, and taste harvest maturity index must have a TSS/TA ratio of 20 and above.
• EU: Seedless table grapes exported to the European Union (EU) must have a minimum of 14º Brix.

Sugranineteen table grapes juice from Italy must have a technological maturity, with an average of 17.9º Brix and a TA of 4.86 g/L.

• Brazil: In Brazil, ‘Sugraone’ and ‘Thompson Seedless’ were harvested when the TSS reached 15º Brix.
• India: The sweetness in India depended on variety. The TSS required was
  • 18º Brix for Thompson seedless
  • 12-14º Brix for Bangalore Blue
  • 14-16º Brix for Anab-e-shahi

Near-infrared spectroscopy-based tools can be used to estimate internal and external color, TSS, and TA in a non-destructive way.

Harvest Maturity Index for Wine Grapes

At maturity, the grape consists of 74% water, 25% sugars (fructose and glucose), 0.8% organic acids, 0.5% minerals, and 0.2% of phenols and aromatics.

For winemaking, the technological and phenolic maturity of white and red grapes is crucial, and it is done nowadays with non-destructive near-infrared spectroscopy.

• Technological maturity is based on sugar levels, titratable acidity, and pH. Sugar determines the alcohol content of wine. Titrable acidity influences wine structure, composition, color, and quality. The pH determines protein stability, phenolic reactions, and microbes present during fermentation and later in wines.
• Phenolic maturity measures the ripeness of grape skins, pulp, and seeds and depends on the accumulation of phenols- anthocyanins and tannins.

Technological Maturity Indices For Wine Grapes

The chemical parameters widely used to determine technological maturity and fix the harvest of wine grapes are TSS, TA, and pH levels.

Sugar Levels

Sugars are measured due to their high proportion in grapes. Sugar levels are measured by destructive sampling with a refractometer or non-destructive through NIR spectrometers. Generally, grapes are picked for winemaking when TSS concentrations are between 18 and 24 ° Brix, depending on the variety and wine style.

Titratable Acidity

Titratable acidity (TA) measures the total acidity in grapes. It can be measured through destructive sampling in laboratories by neutralizing with sodium hydroxide and non-destructively by NIR spectroscopy. Levels necessary are 0.6-0.8 grams per 100 mL.

The pH

A pH meter is used to measure this parameter to fix harvest time.

• White grapes must have a pH of 3.1 to 3.3
• Red grapes must have a pH of 3.3 to 3.5

Taste or sugar ratio to titrable acidity is also a ripening index. The optimal values for winemaking are 30-32 depending on variety and climate.

Table 1: Scorecard for the subjective evaluation of harvest time for wine grapes, Ohio State University Extension. (Credits: https://ohioline.osu.edu/factsheet/HYG-1436)

Attribute	Level of attribute	Points Awarded
Color	Green (lack of color)	0
	Color change: translucent	1
	Fully matured color	2
	Over-mature color	1
Ease of removal of berries from pedicels	High resistance	0
	Moderate resistance	1
	Little/no resistance	2
Texture upon touch	Firm	0
	Soft/elastic	1
	Shriveled; loss of shape	0
Texture—initial bite
Ease of skin collapse	High resistance	0
	Moderate resistance	1
	Low resistance	2
Mechanical features of the pulp	Thin; watery	0
	Viscous	2
	Jelly-like	1
Aroma	None	0
	Recognizable varietal aroma	2
Flavor upon chewing
Initial character (upon chewing)	Unripe; green; bland	0
	Some varietal character	1
	High varietal character	2
Release from skin	None	0
	Typical varietal character	1
After taste	None	0
	Bitter; astringent	0
	Typical varietal character	1
Maximum total		15

Subjective Evaluation of Phenolic Maturity

Optimum sugar levels do not always correlate with the necessary flavors and aromas for winemaking. While phenols and aromatics are crucial for winemaking, it is expensive to measure them quantitatively, so winemakers mainly estimate these attributes subjectively. However, NIR spectroscopy techniques to estimate phenols are also available since phenolic maturity is a crucial parameter in fixing the harvest time of wine grapes.

The subjective and cost-effective indices to determine phenolic maturity are skin and seed color and manual tasting of the grape juice used to estimate TSS, TA, and pH. The phenolic maturity can vary from year to year. Growers can use a scorecard to grade skin color, seed coat color, tannin content or ripeness, firmness, and vine and grape cluster physical condition, as shown in Table 1. 

Near Infrared Spectroscopy Evaluations

Near-infrared spectroscopy is ideal for measuring the chemical composition of grapes. State-of-the-art accurate instruments that give results in real-time are available on the market for vinegrowers. The F-750 Produce Quality Meter offered by Felix Instruments Applied Food Science is a tool for estimating the quality of fresh produce. The F-750 was tested to measure sugar and can theoretically also calculate the other necessary parameters for technological and phenolic maturity of grapes like TA, internal and external color, anthocyanins, and total tannins for table and wine grapes. The company provides tech support to customize the device for any fruit or new varieties, making it versatile.

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