What Is the Best Plum Harvest Maturity Index?

• Taste or sugar/acidity ratio is the plums’ most reliable harvest maturity index.
• A new plum harvest maturity indicator has been developed that combines firmness at maximum maturity with sugar content.
• No single quality parameter can be used alone as a harvest maturity index for plums.

Plum Characteristics

Plums, which are classified as climacteric fruits, have some Japanese plum cultivars that show non-climacteric and suppressed climacteric ripening patterns. These Japanese cultivars produce little ethylene and need long ripening times on and off trees. For example, “Casselman,” “Late Santa Rosa,” and “Roysum” are suppressed climacteric. Meanwhile, “Sweet Miriam” is a novel non-climacteric cultivar. All these non-climacterics and suppressed climacterics are some popular plums grown in California.

Plums have several cultivars with wide-ranging shapes, sizes, firmness, ground color changes from green to yellow, soluble solids content, acidity, and taste. Moreover, seasons, growing location, and farm conditions will influence all the quality parameters that could be used to judge harvest maturity.

Low rates of plum consumption are attributed to a lack of adequate ripening and cold storage disorders to meet consumer requirements. Plums are particularly challenging to evaluate for maturity, as all the quality parameters will not develop simultaneously. Many harvest attributes are reached before the fruit has the desired sweetness. Therefore, late harvest is recommended. It has been found that for plums, no single parameter can be used alone to indicate maturity, so two or more parameters are necessary to establish a harvest maturity index.

The harvest maturity index indicates whether a fruit is ready for harvest. Harvest maturity indices are specific values of selected quality parameters that indicate adequate maturity at harvest. The index helps in timing the harvest so that fruits meet consumer satisfaction and have an extended storage time.

According to UC Davis University, California follows a three-tier maturity system. These are:

1. Minimum maturity or US-Mature plums need artificial ethylene only for uniform ripening but not for speeding up the process.
2. Well-mature will ripen postharvest without artificial ethylene application.
3. Tree-ripe indicates maximum maturity. Plums meant for local consumption can be picked at the tree-ripe stage.

The ripening of plums progresses from the top of the tree to the bottom due to differences in light, temperature, and the canopy structure. There can be a difference of 10-14 days in ripening between uppermost and lowermost fruits. Hence, multiple harvests will be necessary. Also, plums are not susceptible to bruising as much as the other stone fruits, peaches and nectarines, and they can be harvested late.

In the case of plums, physical and chemical quality parameters are used as a reliable harvest maturity index.

Physical Harvest Maturity Indices

The physical quality parameters used as the harvest maturity index are size and shape, peel color, and firmness.

Size and shape

Stone fruits like plums are considered mature for harvest when they have attained the characteristic shape and size, with full cheeks, shoulder, and suture development. These features, along with skin color, must be used to create a reliable maturity index for plums.

Table 1 shows the length, width, and thickness of nine cultivars in Slovenia at harvest maturity.

Table 1: “Length, width, and thickness of plum fruit at physiological and harvest maturity in Slovenia,” Glišić et al. 2015. (Credits: https://www.researchgate.net/publication/319406498_ Physical_attributes_of_plum_fruit_at_physiological_and_harvest_maturity)

Peel color

External color is commonly used worldwide as a harvest maturity index for plums. As the fruit ripens, the ground color changes from green to yellow. Ground color changes are not influenced by sunlight, so they are a reliable maturity index. Overall skin color changes are used in plums and are specific to a cultivar.

In California, plum skin color is the standard parameter for fixing harvest time. A color chip guide designed for some cultivars is available to determine minimum maturity. Plums whose ground color is masked by a red or dark color before maturation also need firmness tests.

In India, the color changes from green, which is used as the harvest maturity index, to dark brown for “Kala Amritsar” and deep purple-red for “Titron.”

Firmness

Color and firmness can be used together to be plum harvest indices.

In California, a firmness test is necessary for full-color development before maturity. Firmness tests are carried out by squeezing with the hand or by a penetrometer with an 8 mm tip to predict maximum maturity, when plums can be harvested without suffering postharvest bruising. Plums with a flesh firmness of 2-3 pounds or  0.9 to 1.8 kilos are ripe, soft, and ready to eat. Firmness values in Newtons (N) and loss for 13 cultivars grown in California are given in Table 2.

In Slovenia, fruit firmness equal to 15 ± 2 N, along with full-color development, is used as a harvest maturity index.

Table 2: Plum cultivar harvest quality traits in California, Minas et al. (2015). (Credits: https://doi.org/10.3389/fpls.2015.00316)

Chemical Plum Harvest Maturity Indices

Internal biochemical quality parameters like sugar, acidity, and, to a lesser extent, phenolic contents can also be used to fix harvest time. Taste, the ratio of sweetness to acidity, is another crucial maturity index.

Soluble solids content

Soluble solids content (SSC) can be used to estimate plum maturity for harvest. Sweetness or high SSC is crucial to meet consumer acceptance. Farm growing conditions, season, and cultivars will influence SSC content, so it is not used alone as a harvest maturity index.

California’s suitable SSC content for 13 cultivars is given in Table 2.

Crisosto (2023) has developed a consumer quality index (CQI) based on SSC and minimum firmness based on critical bruising thresholds to find the correct time to harvest plums with maximum flavor and postharvest life. The scientist has developed CQI based on maximum maturity for important plum cultivars, shown in Table 3.

Table 3: “Proposed consumer quality indexes (CQIs) based on two key components: firmness (eight mm tip) and minimum SSC for different plum cultivars measured at harvest” Crisosto (2023). (Credits: https://www.mdpi.com/2311-7524/9/6/682)

Titrable acidity

Titrable acidity (TA) decreases as plums mature. The parameter is not used alone, as it can be affected by cultivars and seasons. It is, however, essential to calculate taste, a more reliable indicator of plum maturity. TA at harvest for 13 cultivars grown in California are given in Table 2.

Taste

Taste or SSC/acidity ratio is more reliable than either TA or SSC. Other quality parameters can vary considerably between cultivars at the ripe stage when the minimum sensory scores to meet consumer acceptance are met.

Chronological

In some countries, crop chronology can advise on harvest dates. In India, plums are considered to be ripe 12 weeks after the fruit set. In California, the harvest dates for 13 cultivars are given in Table 2.

Measuring Harvest Maturity Index

External color, SSC, and TA can be measured through non-destructive near-infrared (NIR) Spectroscopy. Felix Instruments Applied Food Science supplies the portable F-750 Produce Quality Meter that can be used to monitor the maturity of plums. The instrument gives accurate readings of SSC and color in real time and is easy to use without complex training in the plum orchards. It is also precise enough for further research into plum maturity indices. The tool is also suitable for use in plum ripening centers and storage, as maturity stages decide several postharvest management conditions applied to the fruits to extend their shelf life.

Learn more about how Felix Instruments Applied Food Science can support plum research, plum orchard, and postharvest management.

Sources

Bal, J.S., Bindra, A.S., Bajwa, G.S. and Minhas, P.P.S. (1990). Studies on harvesting and handling of plum. Acta Hortic. 283, 179-188. DOI: 10.17660/ActaHortic.1990.283.19

Crisosto, C. H. (1994). Stone fruit maturity indices: a descriptive. Postharvest News and Information, 5(6), 65N-68N.

Crisosto, C. H. (2023). Establishing a Consumer Quality Index for Fresh Plums (Prunus salicina Lindell). Horticulturae, 9(6), 682. https://doi.org/10.3390/horticulturae9060682

Glišić, I., Milošević, T., & Ilić, R. (2015). Physical attributes of plum fruit at physiological and harvest maturity. Retrieved from https://www.researchgate.net/publication/319406498 _Physical_attributes_of_plum_fruit_at_physiological_and_harvest_maturity

Minas, I. S., Font i Forcada, C., Dangl, G. S., Gradziel, T. M., Dandekar, A. M., & Crisosto, C. H. (2015). Discovery of non-climacteric and suppressed climacteric bud sport mutations originating from a climacteric Japanese plum cultivar (Prunus salicina Lindl.). Frontiers in Plant Science, 6, 316. https://doi.org/10.3389/fpls.2015.00316

Mitcham, E. J., Crisosto, C.H., & Kader, A.A. Recommendations for Maintaining Postharvest Quality. Retrieved from https://postharvest.ucdavis.edu/produce-facts-sheets/plum

Usenik, V., Stampar, F., & Kastelec, D. (2014). Indicators of plum maturity: When do plums become tasty? Scientia Horticulturae, 167, 127-134.