Article written by Madeline Wimmer, Extension Educator - Fruit Production
Iodine-starch testing for ripeness
We're reaching that time of the growing season when early University of Minnesota apple varieties like First Kiss and Zestar! are approaching their harvest window.
Many apples are showing color development, but other ripening indicators to look for include a change in flavor profile, starches converting into simple sugars, and a reduction in tartness. Because these indicators can be subjective, it’s helpful to additionally use a test that quantifies development, especially for new growers and when first growing a new variety. One method that can assist in more accurately tracking and measuring apple ripeness is to use a starch-iodine test.
What is a starch-iodine test?
As apples ripen, starch is converted into simpler sugars first within the ovaries, followed by the apple flesh until the apple reaches maturity.
Starch-iodine testing involves spraying an iodine solution on the apple flesh. The iodine solution only reacts with the starch, leaving a blue stain, and does not stain sugars like sucrose, fructose, and glucose. Apples with larger blue staining areas are generally less ripe, while more ripe apples will have less and less staining over time.
The most classic reference chart for starch testing was created by Cornell, based on the McIntosh variety (see below). More charts have come out recently for various apples, including Honeycrisp, which has a different ripening pattern than McIntosh.
Growers who anticipate storing apples for longer periods of time may want to consider harvesting apples with slightly more starch since apples continue to ripen slightly post-harvest*.
Orchards that move their apples quickly in the fall—common for Minnesota—should harvest apples when starch levels are low. This should be complemented by observing fruit flavor, aroma, and color. This includes looking at the apple background color, which is usually “Post-it note” yellow at peak ripeness, which may be too mature for long-term storage. Instead, aim to harvest apples in the mid-range of the starch chart (between stages 4 and 6).
Penetrometer and brix testing
How easily the flesh of an apple is penetrated using a penetrometer tool can be another indication of ripeness, although extremely crisp varieties may easily be penetrated regardless. Higher readings typically indicate firmer apples, which tend to be less mealy.
Brix testing, using a refractometer, is frequently practiced in wine and cider production to measure total sugars present in the fruit, which steers the final alcohol concentration after fermentation. Testing brix levels on fresh eating apples can be helpful and complementary to starch testing, but may not be the most accurate way to measure maturity, as it does not always correlate with overall ripeness.
*How much ripening occurs during storage can relate to factors like how quickly fruit is cooled, oxygen levels in storage, and the use of ethylene action inhibitors like 1-MCP.
Recommended resources for doing an iodine-starch test:
Veraison—the growth stage when grape berries begin to soften and change color—is starting in some vineyards in southern Minnesota, although some vineyards are seeing a lag in berry development.
John Thull, Vineyard Manager and Researcher at the UMN Horticultural Research Center (HRC), commented on grape development this season, noting that they have seen some vines struggle, with a handful of vines collapsing over the past few weeks—likely due to vascular injury. While the UMN HRC vineyard has seen some issues, most of their healthy vines are developing normally this year.
We’ve already discussed in this year’s newsletter the environmental stresses that occurred in the past year including heavy rains in 2024 early in the growing season, followed by drought, a warm fall, and a winter with little snow cover. John Thull also speculates that frost may have sunk deeper than usual, which could have harmed feeder roots located closer to the soil surface.
The HRC practices training new suckers to replace vines that have died back to the ground. This is one insurance measure that is only possible for non-grafted cold climate grapes, which is an advantage to growers in the Upper Midwest where seasonal conditions are variable with the chance for winters with extreme temperatures.
What is smoke taint?
Smoke taint occurs when grape berries absorb certain volatile compounds from smoke, leading to a spectrum of sensory traits in wine production, some of which are less tolerable than others.
The short summary is that Canadian wildfires are likely too far from Minnesota vineyards to be concerned. This week, I spoke with Four Daughters Winery’s wine maker and vineyard manager, Hannah Hanlon, who previously worked with California vineyards during periods where wildfires were active. Hannah acknowledged that smoke taint didn’t always show up in vineyards just because they were exposed to wildfire.
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were either taken by Madeline Wimmer or within the UMN Extension system.
This article may be shared for educational purposes with attribution to the University of Minnesota Extension. For other uses, please contact UMN Extension for permission.
- Apples
- Measuring fruit ripeness: Starch test
- Grapes
- Growth stage update: Veraison
- Bird netting and other deterrents
- Should Minnesota growers be concerned about smoke taint this year?
- Day-neutral strawberries
- Seasonal check in: Harvest is ramping up
- Management: Runner removal
- Fall-bearing raspberries
- Growth stage: Harvest is beginning
Apples
Measuring fruit ripeness: Starch test
Images: Three apples that were assessed for ripeness by the UMN Horticultural Research Center apple team, first by taste testing, followed by a starch test, which stains starches blue.
Iodine-starch testing for ripeness
We're reaching that time of the growing season when early University of Minnesota apple varieties like First Kiss and Zestar! are approaching their harvest window.
Many apples are showing color development, but other ripening indicators to look for include a change in flavor profile, starches converting into simple sugars, and a reduction in tartness. Because these indicators can be subjective, it’s helpful to additionally use a test that quantifies development, especially for new growers and when first growing a new variety. One method that can assist in more accurately tracking and measuring apple ripeness is to use a starch-iodine test.
Image: Apples are botanically considered “accessory fruits” because the edible “flesh” that we eat is derived from the hypanthium, not the ovary, which is the primary edible part for true fruits.
What is a starch-iodine test?
As apples ripen, starch is converted into simpler sugars first within the ovaries, followed by the apple flesh until the apple reaches maturity.
Starch-iodine testing involves spraying an iodine solution on the apple flesh. The iodine solution only reacts with the starch, leaving a blue stain, and does not stain sugars like sucrose, fructose, and glucose. Apples with larger blue staining areas are generally less ripe, while more ripe apples will have less and less staining over time.
The most classic reference chart for starch testing was created by Cornell, based on the McIntosh variety (see below). More charts have come out recently for various apples, including Honeycrisp, which has a different ripening pattern than McIntosh.
Image: The Cornell starch-iodine index chart shows a gradient of starch staining percentages within the apple core and flesh as the McIntosh apples ripen.
Growers who anticipate storing apples for longer periods of time may want to consider harvesting apples with slightly more starch since apples continue to ripen slightly post-harvest*.
Orchards that move their apples quickly in the fall—common for Minnesota—should harvest apples when starch levels are low. This should be complemented by observing fruit flavor, aroma, and color. This includes looking at the apple background color, which is usually “Post-it note” yellow at peak ripeness, which may be too mature for long-term storage. Instead, aim to harvest apples in the mid-range of the starch chart (between stages 4 and 6).
Penetrometer and brix testing
Images: A penetrometer is used by first removing the apple skin and applying the penetrometer to the apple flesh (left). Refractometers can be used to measure sugar level in cider apple production, primarily (right).
How easily the flesh of an apple is penetrated using a penetrometer tool can be another indication of ripeness, although extremely crisp varieties may easily be penetrated regardless. Higher readings typically indicate firmer apples, which tend to be less mealy.
Brix testing, using a refractometer, is frequently practiced in wine and cider production to measure total sugars present in the fruit, which steers the final alcohol concentration after fermentation. Testing brix levels on fresh eating apples can be helpful and complementary to starch testing, but may not be the most accurate way to measure maturity, as it does not always correlate with overall ripeness.
*How much ripening occurs during storage can relate to factors like how quickly fruit is cooled, oxygen levels in storage, and the use of ethylene action inhibitors like 1-MCP.
Recommended resources for doing an iodine-starch test:
- Check Apple Ripeness with the Starch Iodine Test (UMN Fruit and Veg News article)
- Honeycrisp specific index chart, and other apple cultivars (Michigan State University Extension)
- Making an iodine solution (Virginia Tech)
Grapes
Growth stage update: Veraison
Images: Frontenac Gris (left) and St. Croix (right) are entering veraison at Little Hill Berry Farm (Zone 4b) and Four Daughters Vineyard (Zone 5a).
Veraison—the growth stage when grape berries begin to soften and change color—is starting in some vineyards in southern Minnesota, although some vineyards are seeing a lag in berry development.
John Thull, Vineyard Manager and Researcher at the UMN Horticultural Research Center (HRC), commented on grape development this season, noting that they have seen some vines struggle, with a handful of vines collapsing over the past few weeks—likely due to vascular injury. While the UMN HRC vineyard has seen some issues, most of their healthy vines are developing normally this year.
We’ve already discussed in this year’s newsletter the environmental stresses that occurred in the past year including heavy rains in 2024 early in the growing season, followed by drought, a warm fall, and a winter with little snow cover. John Thull also speculates that frost may have sunk deeper than usual, which could have harmed feeder roots located closer to the soil surface.
The HRC practices training new suckers to replace vines that have died back to the ground. This is one insurance measure that is only possible for non-grafted cold climate grapes, which is an advantage to growers in the Upper Midwest where seasonal conditions are variable with the chance for winters with extreme temperatures.
Should Minnesota growers be concerned about smoke taint this year?
Depending on where you’re located in Minnesota, clear skies have been covered by wildfire smoke drift recently. This might lead some growers to wonder if smoke taint could be an issue here like it has been for California vineyards located near active wildfires in previous years.What is smoke taint?
Smoke taint occurs when grape berries absorb certain volatile compounds from smoke, leading to a spectrum of sensory traits in wine production, some of which are less tolerable than others.
The short summary is that Canadian wildfires are likely too far from Minnesota vineyards to be concerned. This week, I spoke with Four Daughters Winery’s wine maker and vineyard manager, Hannah Hanlon, who previously worked with California vineyards during periods where wildfires were active. Hannah acknowledged that smoke taint didn’t always show up in vineyards just because they were exposed to wildfire.
Based on the literature there are a number of factors that influence the likelihood of smoke taint, which also includes the growth stage when grapes are exposed (post-veraison being a higher risk period), as well as how long they are exposed to smoke, the smoke concentration and composition, among other factors.
To learn more, check out the following resources:
Bird management is a key part of grape production in the Upper Midwest, and there are numerous solutions to either deterring or excluding birds from vineyards. Of all the bird management approaches that exist, bird netting is the most secure and effective method.
When deciding which type of netting to use and how to apply it, there are a number of factors to consider that will help make the process simpler to install and remove (e.g., label nets by length or assigned row when storing, and don’t wear a shirt with buttons!)
This week in UMN Fruit and Veg News, we wrote a separate article to share the details:
Day-neutral strawberries are different from June-bearing strawberries as they produce fruit throughout the growing season and tend to peak in production in August and September in Minnesota. Two farms that largely produce day-neutral strawberries, Little Hill Berry Farm and Twin Cities Berry Company (TCBC), are seeing production ramping up this week.
Both farms grow their berries in smaller-sized high tunnels, also known as caterpillar tunnels, which help mitigate environmental risks like excessive rainfall and hail. Andy Petran, founder of TCBC, focuses on high density production and grows strawberries in soilless medium using fertigation (i.e., table-top production). Andy mentioned the season is going well, as they were able to plant their fruits eight days earlier this year, and one of their goals for 2026 production is to dissipate some of the high tunnel mid-season heat by adding exhaust fans to the tunnel.
Strawberries are an herbaceous perennial. They die back to the ground as they go dormant in the fall and emerge again in the spring, never becoming woody (similar to rhubarb). Strawberries are sometimes referred to as “vines” because of their production of runners, or stolons, which produce new daughter plants.
In June-bearing strawberry production, runners are allowed to root within the planting aisle to establish what is referred to as matted row production. In contrast, day-neutral strawberries are planted as individual plants within a row and runners are removed regularly to encourage the plants to put their energy into fruiting. This can be time consuming, but an important step for production.
To learn more about day-neutral strawberry production, check out our UMN web pages:
Fall-bearing raspberries produce fruit on primocanes that emerge at the beginning of the growing season. This differs from summer-bearing varieties that only produce fruit on one-year-old canes, known as floricanes. Harvest season is beginning in southern Minnesota locations and will continue into fall, especially when grown within a high tunnel environment.
The fall-bearing harvest season is longer than for summer-bearing raspberries, but they are generally easier to manage, as they are pruned to the ground each year. This also simplifies disease management for conditions like spur blight, which can spread from floricanes to primocanes.
Because of the long harvest window, it’s important to monitor for spotted wing drosophila (SWD), especially during years with high humidity into July and August.
To learn more, check out the following resources:
- Smoke taint (Australian Wine Research Institute)
- Wildfire impacts on CA grapes and wine (University of California Davis)
Bird netting and other deterrents
Images: 1a) Rigid, plastic mesh netting with 3/4in mesh demonstrating how netting can be lifted and temporarily clipped above clusters to make harvesting easier; b) close up showing a yellow garden spider for size comparison; c) netting clips can fasten any type of net and can be clipped below grapevines. 2a) An alternative over-the-row knit mesh netting is more flexible, can be applied mechanically (2b), and can additionally be clipped underneath vines to keep it in place.
Bird management is a key part of grape production in the Upper Midwest, and there are numerous solutions to either deterring or excluding birds from vineyards. Of all the bird management approaches that exist, bird netting is the most secure and effective method.
When deciding which type of netting to use and how to apply it, there are a number of factors to consider that will help make the process simpler to install and remove (e.g., label nets by length or assigned row when storing, and don’t wear a shirt with buttons!)
This week in UMN Fruit and Veg News, we wrote a separate article to share the details:
Day-neutral strawberries
Harvest season ramping up in southern Minnesota
Images: A container of day-neutral strawberries (left photo); and a day-neutral strawberry plant flowering and setting fruit in a caterpillar tunnel (right photo), which is an example of a modified growing environment that can help produce quality fruits and lengthen the production season.
Day-neutral strawberries are different from June-bearing strawberries as they produce fruit throughout the growing season and tend to peak in production in August and September in Minnesota. Two farms that largely produce day-neutral strawberries, Little Hill Berry Farm and Twin Cities Berry Company (TCBC), are seeing production ramping up this week.
Both farms grow their berries in smaller-sized high tunnels, also known as caterpillar tunnels, which help mitigate environmental risks like excessive rainfall and hail. Andy Petran, founder of TCBC, focuses on high density production and grows strawberries in soilless medium using fertigation (i.e., table-top production). Andy mentioned the season is going well, as they were able to plant their fruits eight days earlier this year, and one of their goals for 2026 production is to dissipate some of the high tunnel mid-season heat by adding exhaust fans to the tunnel.
Management: Runner removal
Image: Strawberry stolons (runners) are cut and removed to encourage day-neutral strawberry plants to put their energy into fruit production. Photo taken by UMN graduate student Christina Perez.
Strawberries are an herbaceous perennial. They die back to the ground as they go dormant in the fall and emerge again in the spring, never becoming woody (similar to rhubarb). Strawberries are sometimes referred to as “vines” because of their production of runners, or stolons, which produce new daughter plants.
In June-bearing strawberry production, runners are allowed to root within the planting aisle to establish what is referred to as matted row production. In contrast, day-neutral strawberries are planted as individual plants within a row and runners are removed regularly to encourage the plants to put their energy into fruiting. This can be time consuming, but an important step for production.
To learn more about day-neutral strawberry production, check out our UMN web pages:
- Day-neutral strawberries (UMN webpage)
- Day-neutral strawberry planting: Key points (UMN Fruit and Veg News article)
Fall-bearing raspberries
Growth stage: Harvest is beginning
Image: Fall-bearing raspberries are at the start of their harvest season at Little Hill Berry Farm, located outside of Northfield, MN (Zone 4b).
Fall-bearing raspberries produce fruit on primocanes that emerge at the beginning of the growing season. This differs from summer-bearing varieties that only produce fruit on one-year-old canes, known as floricanes. Harvest season is beginning in southern Minnesota locations and will continue into fall, especially when grown within a high tunnel environment.
The fall-bearing harvest season is longer than for summer-bearing raspberries, but they are generally easier to manage, as they are pruned to the ground each year. This also simplifies disease management for conditions like spur blight, which can spread from floricanes to primocanes.
Because of the long harvest window, it’s important to monitor for spotted wing drosophila (SWD), especially during years with high humidity into July and August.
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were either taken by Madeline Wimmer or within the UMN Extension system.
This article may be shared for educational purposes with attribution to the University of Minnesota Extension. For other uses, please contact UMN Extension for permission.
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