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Steps to Ripen California PPN for Receivers & Distributors

About Stone Fruit Maturity and Ripening

California peaches, plums and nectarines are harvested when they have reached maturity. Mature fruit will ripen successfully off the tree, but is still firm enough to pack and ship. Judging maturity is complex and can include evaluating color, size, firmness, sugar content and acidity. Once fruit is harvested, ripening begins.

As California peaches, plums and nectarines ripen, several changes take place. Green color, acidity and firmness decrease as yellow color, flavor and aromas increase. As acidity decreases, the ratio of sugar to acid increases and the fruit tastes sweeter. The fruit’s red color and soluble solids (sugars) remain the same as the fruit ripens. For this reason, measuring fruit’s sugar content will not determine its ripeness.

Measuring firmness is the best way to evaluate peach, plum and nectarine ripeness. Firmness is measured in pounds of pressure. Peaches and nectarines are typically picked at a range of 14 to 16 pounds pressure, while plums are picked in the 10 to 12 pound pressure range. Stone fruit ships well down to the “transfer point” range – for peaches and nectarines, a 6- to 8-pound pressure range, and plums to a 4-to 5-pound pressure range.

When consumers shop for peaches, plums and nectarines, most are looking for fruit in the transfer point range – fruit that’s firm or firm with a little give, with a fragrant aroma, which they can enjoy then or quickly ripen to their personal preference. Using a ripening protocol to provide fruit at this level can increase consumer acceptance and increase purchase frequency.

A Ripening Protocol for Receivers and Distributors

The procedures are designed to enable warehouse quality control managers or store produce personnel to ripen stone fruits efficiently and profitably. The protocol in this section was developed by Dr. Carlos Crisosto, pomologist from UC/Davis at the Kearney Agricultural Center of the University of California.

Following a ripening protocol will reduce shrink from internal breakdown caused by chilling injury and provide fruit the way most consumers want to find it in the store – firm or firm with a little give.

Steps to Ripen California Peaches, Plums and Nectarines

1. Prepare ideal fruit ripening conditions

Multiple temperature zones between 51° and 77°F (10.6°C to 25°C) can be used in stone fruit ripening. Having more than one zone will allow greater ripening flexibility. However, a single temperature zone can also be used to ripen stone fruits.

The rate of softening is fairly slow at 59°F (15°C), faster at 68°F (20°C) and faster still at 77°F (25°C). Even though ripening is faster at 77°F (25°C), the fruit continues to produce its own heat and may get too warm for safe ripening. At temperatures higher than 77°F (25°C), ripening becomes irregular; fruit may become mushy, and “off” flavors may develop. To ripen fruit quickly, aim for 68°F (20°C). For slower ripening, use a lower temperature, but not below 51°F (10.6°C).

Adequate air circulation is necessary to ensure uniform fruit temperature and regulate the amount of carbon dioxide present. Pallets should be placed at least 18 inches from walls and 6 inches from each other on all sides to provide good air circulation.

Because boxes on the inside of a pallet take longer to come up to room temperature than boxes on the outside, fans can be used to provide air circulation to warm the inside of pallets more quickly. A forced-air system, like that in a banana room, provides the fastest warming and most uniform temperature. Switch from forced air back to regular air circulation as soon as the inside of the pallet has reached the desired temperature.

If fruit is ripening in a closed room, it is important to open the room or vent it periodically for an air change so that the carbon dioxide produced by the fruits does not accumulate. Too much carbon dioxide may produce “off” flavors in the fruit and stop the ripening process.

2. Check initial fruit firmness

Checking initial flesh firmness on arrival will indicate where stone fruit is in the ripening process. Firmness is measured using a penetrometer with an 8 mm (5/16”) tip. Either a hand-held or drill press-mounted instrument can be used. The drill press-mounted instrument is more costly, but more accurate and consistent.

How to measure fruit firmness:

• A. Upon arrival, select 10 fruits and warm them to room temperature (68°-70°F). (Cold fruit will give an artificially high reading.)
• B. Remove a nickel-sized slice of skin on each cheek of the fruit with a knife or potato peeler.
• C. Hold fruit against a hard, stationary surface such as a tabletop or, in a drill press-mounted system, the base.
• D. Place the penetrometer tip against the exposed fruit flesh, and with slow, constant pressure, push the penetrometer tip into the fruit to the depth of the inscribed line on the end of the tip.
• E. Record the reading to the nearest half-pound and reset the instrument. Repeat the test on each sample prepared.
• F. Total the pressure readings and divide by the number of measurements taken to find the average firmness for the whole lot.

3. Determine the rate of softening at the holding temperature

To check the holding temperature, take the pulp temperature of the fruit using a probe thermometer. Push the probe into the fruit and wait for the reading to stabilize.

The rate of fruit softening, measured in pounds of firmness lost per day, varies among peach, nectarine and plum varieties. Determine the rate of softening at the holding temperature as shown in Tables 1,2 and 3.

4. Calculate days to the transfer point

The transfer point is the degree of firmness at which the fruit is ready to move to stores or restaurants.

In general, peaches and nectarines will arrive at the warehouse at about 10 to 14 pounds pressure, while plums will arrive at about 8 to 12 pounds.

For peaches and nectarines, the transfer point should be in the 6- to 8-pound pressure range, and for plums, in the 4-to 5-pound pressure range.

The transfer point formula:

To calculate the number of days to the transfer point, first figure the change needed in pounds of pressure. Subtract the transfer point firmness from the initial firmness to determine the desired firmness change.

Then calculate the number of days needed to achieve that change by dividing the firmness change by the rate of softening per day at the holding temperature

For example: A load of O’Henry peaches arrive with an average firmness of 12 pounds. This company’s transfer point for peaches is 8 pounds pressure. The ripening area maintains fruit at a pulp temperature of 68°F (20°C). At that temperature, O’Henry softens at the rate of about 2 pounds per day. This lot of O’Henry would be ready for transfer to the stores in two days after bringing the fruit up to 68°F (20°C).

5. Keep ripening records

To track the ripening process, keep a log sheet. Record fruit’s arrival date, variety, lot number, quantity, initial temperature and firmness, and note the time expected to the transfer point. As the fruit ripens, measure pulp temperature and pressure at regular intervals until the fruit is ready to be shipped out. These log sheets will support the development of a knowledge base on specific varieties and ripening conditions.

6. Coordinate with other stakeholders, stores and accounts

Before implementing the ripening protocol, all parties involved – the warehouse logistics team, category managers, buyers, quality control personnel and store produce managers – should understand what is necessary to make the new system work as well as the sales benefits of ripe fruit.

As the ripening process is completed and fruit is shipped to its final destination, work with receivers and other fruit handlers to capture feedback, including fruit’s arrival conditions, any problems encountered, market life and consumer reaction, in order to optimize the ripening process. This information can be combined with that gathered in log sheets in order to improve the program for the future.