Figure 3. Using a hand-held caliper to measure the diameter of peach. Source: Yixin Cai, University of Maryland.

Figure 3. Using a hand-held caliper to measure the diameter of peach. Source: Yixin Cai, University of Maryland.

Updated: July 20, 2021
By Yixin Cai , and Macarena Farcuh

Determining Peach Fruit Maturity

The maturity stage of peach at harvest depends on its target market. Peaches for wholesale are usually harvested firm when their skin just turns yellow for better storability, while peaches sold at farmer markets or pick your own can delay harvest timing until red blush develops with softer texture and higher sweetness for better eating equality. Thus, it is key to develop peach maturity indices for an optimum fruit quality harvest depending on your target market.

Representative sample collection

Peach maturity differs among cultivars/rootstock, different blocks of orchard or even the same tree. Therefore, each cultivar and each block should be sampled separately, and representative samples should be collected in the same manner from each sampled tree. Sampling for maturity ideally takes place four weeks before the estimated time of harvest and continues until harvest. Analysis for maturity should be performed weekly and every other day when fruit is close to its anticipated maturity. Days after full bloom (DAFB) is a useful tool to estimate the harvest timing if the previous harvest dates for each block are well recorded. In general, the DAFB can range from 150-180 days depending on cultivars. Keep in mind that the actual DAFB can vary 5 to 20 days depending on the environmental conditions of the season.

The first step for sampling is to choose 10 to 20 trees per block per cultivar and rootstock. Selected trees should be representative in terms of the crop load and vigor. Avoid choosing trees from the borders of any blocks, as those trees are likely encountering more traffic, wind, and irradiation. Label the sampled tree and keep sampling from the same tree.

Fruits from outer part of tree can receive more irradiation and usually ripen earlier than fruits from the inner part. Therefore, sample 2 to 3 fruits from the periphery of each marked tree. Select fruit with uniform size without any visible defects. Keep sampling methods consistent and collect samples at the same time of day and measure quality within 2 hours of picking.

Measurement of peach fruit maturity indices

A) Color changes

Figure 1. Peach background color chart (1 to 7 scale). Image adapted from How to use a color chart. Source: UC Davis Postharvest Center.
Figure 1. Peach background color chart (1 to 7 scale). Image adapted from How to use a color chart. Source: UC Davis Postharvest Center.

Surface color: The red skin color in most peach cultivars increases with sunlight exposure which is influenced by its location in the canopy, but decreases with high temperature and excessive or insufficient nutrient availability. There is also cultivar/rootstock variation in red skin color. Therefore, the degree of red coloration is not a good indicator for maturity, although it is a key aspect for its marketability.

Background color: Changes in background color are widely used as one of the maturity indices for peaches. The break from green to yellow (for yellow flesh peaches) or cream color (for white color peaches) is a sign of maturity for harvest if targeting for long distance shipping or long duration storage. In general, a fully ripen peach has dark yellow color and should be sold immediately, while orange color indicates that the fruit is over-ripe. Tools such as color charts (Fig. 1) are useful to help estimate more defined maturity stages. However, background color measurement does not work well with solid red cultivars such as ‘Sunhigh’. They will have higher percent of red coloration even before their optimal maturity and background color change is not perceptible to the naked eye. For those cultivars, it is necessary to choose other maturity indices for accurate harvest timing. The background color cannot be used alone as a maturity indicator.

Chlorophyll content: As fruit ripens, chlorophyll contents decrease. The DA meter is a portable device measuring the Index of Absorbance Difference (IAD), which has a strong relation to the chlorophyll-a content in fruit skin. The IAD value decreases as chlorophyll degrades during ripening. The DA meter shines LED light to fruit skin and measures the amount of light reflected back. In general, an immature peach has IAD > 0.9; a peach that is ready to be harvested for the wholesale market has IAD between 0.4 and 0.9 (Fig. 2); while a ready to eat peach, sold locally through farmers markets, has IAD between 0 and 0.4. Although DA meter is a handy tool to access maturity stage, it cannot be used solely and must be used in combination with other indices.

B) Size and diameter

Peach maturity can also be determined by the fruit shape and fullness of cheeks, especially when the shoulders and suture are filled out and well developed. The diameter of a peach can be measured equatorially by a hand-held caliper at its widest part (Fig. 3). The size of peach varies by canopy position, crop load, water and nutrient status as well as genetic of cultivar and rootstock (Layne, 2010). Peaches with 2.5 inches and larger in diameter appeals more to consumers, and are highly demanded in farmers markets.

C) Flesh firmness

As fruit matures, flesh firmness decreases. Firmness can be easily measured by either a hand-held penetrometer (i.e. Effegi firmness tester). The instruments measure the amount of force needed to penetrate the fruit flesh. To measure, first use a peeler and remove a disk of skin between the size of a nickel and a quarter on both equatorial sides of the peach at a point midway between the stem end and tip. Choose a 5/16-inch (8mm) diameter plunger (the 7/16-inch one is for harder fruits such as apple). Hold the peach against a stationary, hard surface and penetrate the plunger into the flesh to the scribed line on the plunger (Fig. 4). It is important to apply a smooth and uniform penetration force. Pay attention to the speed of penetration. It should take about 2 seconds to push the plunger with constant speed. Read to the nearest 0.5 Ib-force. Measure both sides to get an average of firmness. The same person should perform the tests for each fruit for consistency.

Peaches with a flesh firmness of 10-16 Ib-force are recommended for wholesale and long-term storage, while peaches with a 2-4 Ib-force are considered ready to eat and can be sold at farmers markets and pick your own operations. In general, ready to eat peaches have a better eating quality and flavor than firmer peaches, but they are prone to damage during transportation and have a reduced storability potential.

Firmness is not uniform on peach, and the tip, suture and shoulders are considered weak points, which are prone to bruising and damage during postharvest handling. Early season cultivars (i.e. Rich May, Flavorcrest) tend to soften faster at the tip, while late season cultivars (i.e. O’ Henry, August Sun) soften faster at the suture/shoulders. For the wholesale market, firmness at the tip of early season cultivars should also be measured to ensure fruit has minimum firmness (above 10 lb-force) for transportation and storage.

    Figure 2. Using a DA meter to quantify the Index of Absorbance Difference (IAD) in peach which relates to the actual content of chlorophyll-a in the fruit skin. Source: Yixin Cai, University of Maryland.
    Figure 2. Using a DA meter to quantify the Index of Absorbance Difference (IAD) in peach which relates to the actual content of chlorophyll-a in the fruit skin. Source: Yixin Cai, University of Maryland.
    Figure 4. Using a hand-held penetrometer to measure the flesh firmness of peach. Source: Yixin Cai, University of Maryland.
    Figure 4. Using a hand-held penetrometer to measure the flesh firmness of peach. Source: Yixin Cai, University of Maryland.
    Figure 5. A hand-held manual refractometer (right) and digital refractometer (left) for measuring soluble solids contents of extracted peach juice sample. Source: Dr. Macarena Farcuh, University of Maryland.
    Figure 5. A hand-held manual refractometer (right) and digital refractometer (left) for measuring soluble solids contents of extracted peach juice sample. Source: Dr. Macarena Farcuh, University of Maryland.

    D) Soluble solid contents (SSC)

    Sugar is the major component of soluble solid contents (SSC) in fruit. Changes in SSC in juice can be measured by a manual or digital refractometer in order to estimate the sugar content (Fig. 5). The device measures the light deviation when passing the juice and scale the refractive index into °Brix or SSC percent.

    Cut two wedges from both sides of the fruit. Each wedge should be sliced longitudinal from stem end to calyx end and to the center, and use a garlic or potato-pressor to squeeze juice through a cheesecloth. It is important to keep the temperature of juice constant for each measurement. Digital and some manual refractometers have temperature compensation capability to correct. To use a manual device (Fig. 5), drop a small amount of juice to the prism. Close the lid and turn the instrument toward the light. The position where the light and dark region cross gives the reading. The digital refractometer has an internal light source and sensor, and it minimizes operator’s error. Use DI water to rinse the prism and wipe with soft tissue paper after each measurement.

    SSC in peaches continues to increase till fully mature but stays constant after harvested. Peach with higher SSC (12% up) is higher quality and receives highest consumer acceptance. Depending on the target market, decisions of how late peach can be left on tree to accumulate SSC should be made to balance with its storability.

    E) Acidity change

    Organic acids decrease as peach matures, and it can be measured as titratable acidity (TA). However, TA itself is usually not used as a maturity index because there is little guideline for peach maturity based on TA. This is due to the large variation of TA among peach cultivars, which ranges from 0.2% to over 1%. Peach cultivars can be sorted into three groups based on its acidity: low acidity (TA< 0.5 or pH>4); standard/medium acidity (TA: 0.5-0.7 or pH:3.8-4); and high acidity (TA>0.7 or pH<3.8).

    The organic acidity measurement usually requires specialized laboratory instruments, such as a titrator. TA is measured by titrating a known volume juice with a base such as sodium hydroxide to an end point of pH=8.2. TA is calculated based on the volume of juice, base used and acid milliequivalent factor of malic acid in peach.

    F) Ratio of soluble solids content over titratable acidity

    SSC/TA is an important quality index as human’s taste is a combination of sweetness and sourness. Ratio of SSC and TA increases as peach fruit develops and ripens; however, there is no suggested SSC/TA values corresponding to maturity stages due to the large variation in SSC and TA among different cultivars. It is important not to compare SSC/TA across different acidity groups as low acidity cultivars can yield a ratio 4 times or more higher than standard/high acidity cultivars. Generally, the higher ratio is linked to consumers’ sweetness perception and satisfaction.

    Summary

    Multiple maturity indices, including change in background coloration, chlorophyll contents, fruit size/diameter, firmness, sugar contents and acidity, can be applied to determine peach maturity and harvest dates. It is important to remember that none of these maturity indices alone is sufficient to indicate the maturity stage of peach. Be sure to consider using multiple indices when making harvest and marketing decisions. Moreover, the value of each maturity index is based on fruit target market. In general, peach for farmer’s market or pick your own practice is more advanced in its maturity than peach for wholesale/retail.

    References

    • Crisosto, C. H., and D. Valero. 2008. 22: Harvesting and Postharvest Handling of Peaches for the Fresh Market. The Peach: Botany, Production and Uses, CAB International, Wallingford: 575-596.
    • Fruit & Nut Research & Information Center Website, UC Davis.http://fruitsandnuts.ucdavis.edu/Weather_Services/Harvest_Prediction__About_Growing_Degree_Hours/
    • Kelley K M, Primrose R, Crassweller R, et al. Consumer peach preferences and purchasing behavior: a mixed methods study[J]. Journal of the Science of Food and Agriculture, 2016, 96(7): 2451-2461.
    • Layne, D. 2010. How to determine peach ripeness. Clemson Cooperative Extenion. https://hgic.clemson.edu/how-to-determine-peach-ripeness/
    • Marini, R. 2019. Peach Marketing: Can Peach Consumption be Increased? Penn State Extension.

    This article appears on July 15, 2021, Volume 12, Issue 4 of the Vegetable and Fruit News

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    Vegetable and Fruit News, July 2021, Vol. 12, Issue 4

    Vegetable and Fruit News is a statewide publication for the commercial vegetable and fruit industries and is published monthly during the growing season (April through October). Subscribers will receive an email with the latest edition.

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