Madeline Wimmer, Extension Educator and Soon Li Teh, Extension Specialist
Grapes are a fruit crop with distinct, measurable ways to determine fruit ripeness. With knowledge of their natural development process and the right tools and method to track berry ripening, a vineyard can be confident in its ability to harvest a quality crop. In this article, we’ll talk about fruit ripening, what it means to collect a representative sample of berries, and how to measure fruit sugar levels, pH, and titratable acidity.
Grape berries generally go through three developmental stages for ripening:
One grape berry, or even one grape cluster cannot represent a vineyard block or a single cultivar due to variation in canopy conditions and natural variation in ripening. Collecting a representative sample involves understanding: 1) grapevine training systems and management practices impact fruit development and 2) collecting 1-2 berries per cluster and rotating where a berry is sampled will lead to the variation needed to collect a representative sample. While there exist other methods that work for getting a representative sample of berries, we will focus on the suggested method below for the length of this article.
1) Consistent canopy management practices are important because grapes that are grown without any canopy management will be shaded and generally have higher acid levels and lower sugar levels than ones that are fully exposed to sunlight. If only exposed clusters were sampled, for example, the tester could be misled into thinking the crop is more ripe than it actually is and end up harvesting prematurely.
2) The following guidelines for sample sizes and from where on a cluster the berries are sampled are suggestions—not strict rules—but having a rough idea of how to collect a representative sample is a good practice to be familiar with. To collect a representative sample, simply rotate between picking berries on the front and back of the cluster, as well as from its four regions: left shoulder, right shoulder, center, and cluster tail (see below).
Additional considerations for berry sampling
Steps for sampling and preparing samples for analysis
After the juice has been collected, there are typically three different juice chemistry traits that can be analyzed for ripeness: sugar levels (i.e., soluble solids concentration), pH, and titratable acidity.
Sugars/soluble solids concentration (SSC):
Grape juice sugar levels are estimated by measuring the soluble solids concentration (SSC; measured in degrees of Brix) using a refractometer as the tool for measurement. The amount of sugar that eventually becomes alcohol in the final wine product translates to about one degree Brix becoming about 0.55% alcohol. Which means that a 22° Brix wine grape juice batch could have an estimated final alcohol content of 12.10%, depending on the yeast efficiency and other conditions present during fermentation. Thus, the optimal ranges for juice Brix levels can vary between wine grape cultivars and the desired wine style and method used. Having an idea about what type of wine will be made with any single or blend of cultivars can set the goal for fruit/juice Brix levels at harvest.
Using a hydrometer to measure specific gravity is an additional method for estimating sugar in a liquid, but a larger sample is usually required making it more practical to use a hand-held refractometer for measuring sugar levels. Measuring the specific gravity can be most helpful after harvest, before fermentation begins.
Additional reading:
How to use a refractometer (University of California Davis)
Estimating grape maturity by total soluble solids (Iowa State University Extension)
*Where does the term Brix originate? The Brix unit (i.e., degrees Brix) was named after the last name of the person who developed methods to measure sugar content in juice. One degree brix represents one gram of sucrose in one hundred grams of a solution. Refractometers measure brix by measuring the amount of light that bends when passing through a liquid.
While measuring soluble solids, pH, and titratable acidity tend to be the most common ways to quantify fruit ripeness, qualitative indicators are still important. Remember to taste what is out there and visually evaluate how your crop is doing as a whole. Even the taste of the grape seeds can indicate how close harvest is, as riper grapes develop a nutty, or toast-like flavor. With a mixture of quantitative and qualitative approaches, year after year, it becomes easier to know when your crop is at its optimum ripeness for harvesting.
With all of this in mind, it is important to note that sometimes optimal fruit ripeness is not the final call for when it is time to harvest. Extreme weather conditions, like hail, which threaten the crop, disease progressions, or even labor availability can determine an earlier harvest than anticipated.
Finally, for those who are new to measuring fruit ripeness and for whom this information feels too technical or too complex, the best thing to do is take it slow and take it one step at a time. Adopt practices as you can as they are relevant to your situation and goals.
Grapes are a fruit crop with distinct, measurable ways to determine fruit ripeness. With knowledge of their natural development process and the right tools and method to track berry ripening, a vineyard can be confident in its ability to harvest a quality crop. In this article, we’ll talk about fruit ripening, what it means to collect a representative sample of berries, and how to measure fruit sugar levels, pH, and titratable acidity.
Grape berries generally go through three developmental stages for ripening:
- Stage I: Cells within the berry begin to divide and multiply, which leads to an increase in berry size.
- Stage II: A lot of energy is focused on embryo development within the grape seeds during this phase. Stage II ends when berries begin to change color at veraison.
- Stage III: At veraison, stage III is characterized in the fruit by a loss of chlorophyll, increase in sugar levels, reduction in acid levels, and berry softening. Additionally, the seeds continue to mature and eventually have a “nutty” taste when sampled.
Sampling
Collecting a representative sampleOne grape berry, or even one grape cluster cannot represent a vineyard block or a single cultivar due to variation in canopy conditions and natural variation in ripening. Collecting a representative sample involves understanding: 1) grapevine training systems and management practices impact fruit development and 2) collecting 1-2 berries per cluster and rotating where a berry is sampled will lead to the variation needed to collect a representative sample. While there exist other methods that work for getting a representative sample of berries, we will focus on the suggested method below for the length of this article.
1) Consistent canopy management practices are important because grapes that are grown without any canopy management will be shaded and generally have higher acid levels and lower sugar levels than ones that are fully exposed to sunlight. If only exposed clusters were sampled, for example, the tester could be misled into thinking the crop is more ripe than it actually is and end up harvesting prematurely.
2) The following guidelines for sample sizes and from where on a cluster the berries are sampled are suggestions—not strict rules—but having a rough idea of how to collect a representative sample is a good practice to be familiar with. To collect a representative sample, simply rotate between picking berries on the front and back of the cluster, as well as from its four regions: left shoulder, right shoulder, center, and cluster tail (see below).
Image: Sample randomly from the front and backside of a grape cluster as well as these four regions: center, left shoulder, right shoulder, and tail, to get a representative grape sample across a vineyard region. This should not be overthought, but something to keep in mind.
Additional considerations for berry sampling
- Outlier berries: Avoid selecting berries that are not representative of the majority of grapes within a vineyard. These berries could be diseased, or damaged from insect pests, birds, hail, etc.
- Environmental factors: Beyond canopy density and the cluster position, it may be desirable to separate samples within a single grape cultivar if vineyard blocks are located on different topographies, soils, training systems, etc.
- Rain: Avoid sampling during or immediately after a rain event, as rainwater can dilute the sugar concentration and cause berry swelling.
Tools and supplies for fruit sampling
- Plastic sandwich bags labeled with name of grape cultivar, or region being sampled. Large cups or containers may also be used.
- Handheld refractometer for testing sugar levels in the field
- Water bottle or spray bottle and a towel or paper towels to rinse and dry the refractometer as needed if measuring sugar levels in the vineyard.
- Juice collection containers can be labeled and used to bring juice samples to a location where samples will be processed and analyzed.
- Additionally, a cooler with ice for samples that will be refrigerated before processing is helpful when transporting samples a long distance.
- Notebook and pencil to record results. Notes can be kept each growing season to compare to last year’s trends- this is where the magic starts to happen!
Steps for sampling and preparing samples for analysis
- Grape berries can be sampled weekly starting around 3 weeks before the anticipated harvest.
- Choose the number of berries needed to represent the area being sampled (usually between 15-30 berries).
- Collect a representative sample of berries for the variety or region being sampled .
- Place berries into the plastic zip lock bag.
- Either refrigerate berries if processing samples at a later time, or crush berries within the zip lock bag to extract the juice.
Video: Vineyard/fruit sampling for quality and maturity, Part I: In the field. In this video, University of Minnesota Enologist Drew Horton discusses how to monitor grapes in the field as harvest approaches. In this video, Drew mentions the flying blind technique, which encourages samplers to pay less attention to which clusters are being selected for berry sampling.
Analyzing juice for sugar levels, pH, and titratable acidity
After the juice has been collected, there are typically three different juice chemistry traits that can be analyzed for ripeness: sugar levels (i.e., soluble solids concentration), pH, and titratable acidity.
Sugars/soluble solids concentration (SSC):
Grape juice sugar levels are estimated by measuring the soluble solids concentration (SSC; measured in degrees of Brix) using a refractometer as the tool for measurement. The amount of sugar that eventually becomes alcohol in the final wine product translates to about one degree Brix becoming about 0.55% alcohol. Which means that a 22° Brix wine grape juice batch could have an estimated final alcohol content of 12.10%, depending on the yeast efficiency and other conditions present during fermentation. Thus, the optimal ranges for juice Brix levels can vary between wine grape cultivars and the desired wine style and method used. Having an idea about what type of wine will be made with any single or blend of cultivars can set the goal for fruit/juice Brix levels at harvest.
Using a hydrometer to measure specific gravity is an additional method for estimating sugar in a liquid, but a larger sample is usually required making it more practical to use a hand-held refractometer for measuring sugar levels. Measuring the specific gravity can be most helpful after harvest, before fermentation begins.
Additional reading:
How to use a refractometer (University of California Davis)
Estimating grape maturity by total soluble solids (Iowa State University Extension)
*Where does the term Brix originate? The Brix unit (i.e., degrees Brix) was named after the last name of the person who developed methods to measure sugar content in juice. One degree brix represents one gram of sucrose in one hundred grams of a solution. Refractometers measure brix by measuring the amount of light that bends when passing through a liquid.
pH
Having an optimum juice pH is important to the final wine stability and prevents unwanted microbial growth, which can lead to wine faults. The pH technically measures how acidic or basic a solution is based on the concentration of hydrogen ions and values range from 0-14 with 0 values being the most acidic and 14 being the most basic. Measuring juice pH is helpful for the reasons listed above, but knowing the titratable acidity is still important. Not all acids are the same strength, and titratable acidity indicates the total amount of acids present in the juice, regardless of their strength. Juice pH of cold climate wine grapes typically range from 3.0 to 3.5, depending on varieties, growing conditions and intended wine styles. Measuring juice pH can be done using a pH meter; today, a wide range of pH meters exist on the market for this purpose, and the key to maintaining an accurate meter depends on how well it is calibrated and cared for over time.
Note: Can pH strips be used to measure juice pH? pH strips are usually inexpensive, easy to use, convenient for in-field measurements, and can indicate juice pH levels. Most brands can measure juice pH values between 2.7 and 4.3.Their down fall is that the strips have a more limited ability to accurately measure juice pH, and the reading itself can be challenging to determine for darker pigmented wine grapes.
Additional reading:
pH Analysis (University of California Davis)
Estimating grape maturity with potential power of hydrogen (pH) (Iowa State University Extension)
Having an optimum juice pH is important to the final wine stability and prevents unwanted microbial growth, which can lead to wine faults. The pH technically measures how acidic or basic a solution is based on the concentration of hydrogen ions and values range from 0-14 with 0 values being the most acidic and 14 being the most basic. Measuring juice pH is helpful for the reasons listed above, but knowing the titratable acidity is still important. Not all acids are the same strength, and titratable acidity indicates the total amount of acids present in the juice, regardless of their strength. Juice pH of cold climate wine grapes typically range from 3.0 to 3.5, depending on varieties, growing conditions and intended wine styles. Measuring juice pH can be done using a pH meter; today, a wide range of pH meters exist on the market for this purpose, and the key to maintaining an accurate meter depends on how well it is calibrated and cared for over time.
Note: Can pH strips be used to measure juice pH? pH strips are usually inexpensive, easy to use, convenient for in-field measurements, and can indicate juice pH levels. Most brands can measure juice pH values between 2.7 and 4.3.Their down fall is that the strips have a more limited ability to accurately measure juice pH, and the reading itself can be challenging to determine for darker pigmented wine grapes.
Additional reading:
pH Analysis (University of California Davis)
Estimating grape maturity with potential power of hydrogen (pH) (Iowa State University Extension)
Titratable acidity (TA):
The final element to measuring grape ripeness involves going beyond knowing how acidic or basic the juice is by measuring acid levels using a process known as titration**. Titratable acidity (TA) is not the same as total acidity, although these two traits are sometimes used synonymously in winemaking literature. Total acidity represents the total amount of fixed, or stable acids (e.g., tartaric, malic, citric) and volatile, or aromatic acids (e.g., acetic, butyric, sulfurous) in a solution, whereas TA reflects the amount of acid that can be neutralized by an alkaline solution—it is literally measuring how many protons react with the alkaline solution***. While total acidity can be measured using methods like spectrometry or chromatography, it’s not practical for grape growers and winemakers. Thus TA is an industry standard. There are a wide range of different titration methods, some of which are less mechanized than others. Check out the document below under, “Additional reading” to learn more about the process of measuring TA.
Note on cold climate grapes: Titratable acidity of cold climate grape juice typically ranges from 8 to 15 g/L depending on varieties, growing conditions, and intended wine styles.
Additional reading:
Measuring grape maturity by titratable acidity (Iowa State University Extension)
**What is titration? Titration is an analytical practice that determines the concentration of a dissolved substance within a liquid solution. The titrant is slowly added to the solution until it reaches the point of equivalence, which could be indicated by a change in color, or other other property. In the case of titrating for acids in a solution, the titrant is usually something that is basic, or alkaline like sodium hydroxide (NaOH) and usually uses phenolphthalein or a fixed end point pH (8.2) as an indicator.
*** Titratable acidity only captures 70-80% of what is anticipated to be the actual concentrations of the acid being measured.
Additional ripeness indicators
The final element to measuring grape ripeness involves going beyond knowing how acidic or basic the juice is by measuring acid levels using a process known as titration**. Titratable acidity (TA) is not the same as total acidity, although these two traits are sometimes used synonymously in winemaking literature. Total acidity represents the total amount of fixed, or stable acids (e.g., tartaric, malic, citric) and volatile, or aromatic acids (e.g., acetic, butyric, sulfurous) in a solution, whereas TA reflects the amount of acid that can be neutralized by an alkaline solution—it is literally measuring how many protons react with the alkaline solution***. While total acidity can be measured using methods like spectrometry or chromatography, it’s not practical for grape growers and winemakers. Thus TA is an industry standard. There are a wide range of different titration methods, some of which are less mechanized than others. Check out the document below under, “Additional reading” to learn more about the process of measuring TA.
Note on cold climate grapes: Titratable acidity of cold climate grape juice typically ranges from 8 to 15 g/L depending on varieties, growing conditions, and intended wine styles.
Additional reading:
Measuring grape maturity by titratable acidity (Iowa State University Extension)
**What is titration? Titration is an analytical practice that determines the concentration of a dissolved substance within a liquid solution. The titrant is slowly added to the solution until it reaches the point of equivalence, which could be indicated by a change in color, or other other property. In the case of titrating for acids in a solution, the titrant is usually something that is basic, or alkaline like sodium hydroxide (NaOH) and usually uses phenolphthalein or a fixed end point pH (8.2) as an indicator.
*** Titratable acidity only captures 70-80% of what is anticipated to be the actual concentrations of the acid being measured.
Video: Vineyard/fruit sampling for quality and maturity, Part II: In the lab. In this video, University of Minnesota Enologist Drew Horton discusses how to process and analyze grape samples to measure sugar levels/soluble solids, pH, and titratable acidity to determine fruit ripeness for harvest.
Additional ripeness indicators
Image: a close-up photo of showing the variation in color of ripe Frontenac Gris clusters at harvest.
While measuring soluble solids, pH, and titratable acidity tend to be the most common ways to quantify fruit ripeness, qualitative indicators are still important. Remember to taste what is out there and visually evaluate how your crop is doing as a whole. Even the taste of the grape seeds can indicate how close harvest is, as riper grapes develop a nutty, or toast-like flavor. With a mixture of quantitative and qualitative approaches, year after year, it becomes easier to know when your crop is at its optimum ripeness for harvesting.
With all of this in mind, it is important to note that sometimes optimal fruit ripeness is not the final call for when it is time to harvest. Extreme weather conditions, like hail, which threaten the crop, disease progressions, or even labor availability can determine an earlier harvest than anticipated.
Finally, for those who are new to measuring fruit ripeness and for whom this information feels too technical or too complex, the best thing to do is take it slow and take it one step at a time. Adopt practices as you can as they are relevant to your situation and goals.
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