Are Your Peaches Ready to Harvest? (FS-1182)

Peach maturity differs among cultivars/rootstocks, different blocks of the 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 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, DAFB can range from 100 to 130 days depending on cultivars. 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 crop load and vigor. Avoid choosing trees from the borders of any blocks, as those trees are likely encountering more traffic, wind, and light interception. Label the sampled tree and keep sampling from the same tree.

Fruit from the outer part of the tree canopy receive more sunlight and will ripen earlier than fruit located inside the canopy. Therefore, sample two or three fruits from the periphery of each marked tree. Select fruit with uniform size without visible defects. Keep sampling methods consistent and collect samples at the same time of day and measure quality within 2 hours of picking.

Color Changes

Surface color: The red skin color in most peach cultivars increases with sunlight exposure which is influenced by location in the canopy. The characteristic color 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 marketability. According to the USDA grade standard, 50% of the peaches should have no less than 1/4 of surface pink or red color (blush) to meet US. Extra No.1; while 90% of the peaches should have no less than 1/3 of surface showing blush to meet U.S. Fancy. Furthermore, the redness in peach skin is more appealing to consumers, which is a key aspect for selling in farmers markets and pick-your-own operations.

Background color: Changes in background color are widely used as one of the maturity indices for peaches. The transition from green to yellow (for yellow flesh peaches) or cream color (for white flesh peaches) is a sign of maturity for harvest if targeting for long-distance shipping or long-duration storage. In general, a fully ripened peach has dark yellow color and should be sold immediately, while orange color indicates that the fruit is overripe. Tools such as color charts (Figure 2) are useful to help estimate more defined maturity stages. However, background color measurement does not work well with solid red cultivars (i.e., ‘Sunhigh’) because they have a higher percentage of red color even before their optimal maturity and their 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.

Chlorophyll content: As fruit ripens, chlorophyll contents decrease. The DA meter is a portable device measuring the Index of Absorbance Difference (Iᴀᴅ), which is strongly related to the chlorophyll-a content in fruit skin. The Iᴀᴅ value decreases as chlorophyll degrades during ripening. The DA meter shines LED light on fruit skin and measures the amount of light reflected back (Figure 3). Peaches that are:

• immature have Iᴀᴅ > 0.9;
• ready to be harvested for the wholesale market have Iᴀᴅ between 0.4 and 0.9;
• ready to eat, sold locally through farmers markets, have Iᴀᴅ between 0 and 0.4.

IAD decreases at a much higher rate once background color goes from green to yellow/cream. Although a DA meter is a handy tool to assess maturity stage, it must be used in combination with other indices.

Fruit shape

Producers also can determine peach maturity by fruit shape and cheek fullness, 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 (Figure 4). The size of peach varies by canopy position, crop load, water and nutrient status, as well as genetics of cultivar and rootstock. Peaches that are 2.5 inches and larger in diameter appeal more to consumers and are in high demand at farmers markets.

As fruit matures, flesh firmness decreases. Firmness can be easily measured by either a hand-held penetrometer (i.e., Effegi firmness tester) or a bench stand penetrometer (i.e., Magness-Taylor pressure tester). The instruments measure the amount of force needed to penetrate the fruit flesh.

To measure, use a peeler and remove a section 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 apples). Hold the peach against a stationary, hard surface and push the plunger to the scribed line into the flesh Figure 5). It is important to insert the plunger using smooth and uniform pressure. Pay attention to the speed of penetration. It should take about two seconds to push the plunger with constant speed. Read to the nearest 0.5 lb.-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 lb.-force are recommended for wholesale and long-term storage, while peaches with a 2-4 lb.-force are considered ready to eat and are targeted for selling 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 reduced storability potential.

Firmness is not uniform in peaches, 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., Candor, Garnet Beauty) tend to soften faster at the tip, while late season cultivars (i.e., Cresthaven, Redskin) soften faster at the suture/shoulders. For the wholesale market, firmness at the tip of early season cultivars also should be measured to ensure fruit has minimum firmness (above 10 lb.-force) for transportation and storage.

Sugar is the major component of soluble solid contents (SSC) in fruit. Other components include organic acids, amino acids, phenolic compounds, and soluble pectins.

Growers can measure changes in SSC in juice using a manual or digital refractometer to estimate the sugar content (Figure 6). The device measures the light deviation when passing the juice and scales the refractive index into °Brix or SSC percent.

To measure SSC, cut two wedges from both sides of the fruit. Each wedge should be sliced longitudinally from stem end to calyx end and to the center. Use a garlic or potato-presser to squeeze juice through a cheesecloth. It is important to keep the temperature of the juice constant for each measurement. Digital and some manual refractometers have temperature compensation capability to correct for variations in temperatures.

To use a manual device (Figure 6), drop a small amount of juice on the prism. Close the lid and turn the instrument toward light. The position where the light and dark regions cross gives the reading. The digital refractometer has an internal light source and sensor which help minimize operator error. Use distilled water to rinse the prism and wipe with soft tissue paper after each measurement.

Many factors including climate, cultivar/rootstock, and orchard practices can influence SSC in peaches, making comparison between seasons difficult. Hot weather and direct sunlight increase SSC by enhanced photosynthesis, while precipitation, irrigation, and heavy crop load dilute sugar accumulation and lower SSC. Training systems and thinning practices affect fruit light exposure as well as leaf/fruit ratio, which consequentially impact SSC of fruit within the same tree. Variation in SSC content can be found even within a peach between stem and calyx ends, making it necessary to sample longitudinally across the fruit.

SSC of 10% is considered a minimum marketable quality standard. SSC in peaches continues to increase until full maturity but stays constant after harvested. Peaches with higher SSC (12% and up) are considered higher quality and preferred by the consumers. Depending on the target market, decisions regarding how late a peach can be left on a tree to accumulate SSC should be balanced with its storability.

Organic acid content decreases as peaches mature and can be measured as titratable acidity (TA). However, TA itself is usually not used as a maturity index because there are few guidelines for peach maturity based on TA. This is due to the large variation in TA among peach cultivars, which ranges from 0.2% to over 1%. Peach cultivars can be sorted into three groups based on 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). In general, white fleshed peaches are less acidic than yellow fleshed peaches.

TA measurement requires specialized laboratory instruments, such as a titrator. To measure TA, a known volume of juice is titrated with a base such as sodium hydroxide, to an end point of pH=8.2. TA is calculated based on the volume of juice, the volume of base solution used and acid milliequivalent factor of malic acid in peach.

SSC/TA is an important quality index as human’s taste is a combination of sweetness and sourness. The ratio of SSC to TA increases as peaches mature; however, there are 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 four times or more higher than standard/high acidity cultivars. Generally, a higher ratio is linked to consumers’ perception of sweetness and satisfaction. SSC/TA of 13 or higher in high-acidity cultivars and 28 or higher in low-acidity cultivars can have an over 80% customer acceptance rate.

*Background color measurement does not work well with solid red cultivars.

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