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Introduction
Proper maturity at harvest is essential to marketing good quality stone fruit. Immature fruits are more subject to shrivelling, internal breakdown, mechanical damage and inferior quality when ripe. Over-mature fruits are likely to become soft and mealy and attain insipid flavour soon after harvest. So, what is the right level of maturity? This largely depends on the variety, the intended market domestic ‘ready to eat’ or export and the buyer specifications. Growers must pick when the fruit has the maturity characteristics as set by the purchaser. Failure to do so means reduced income.
A number of tests have been developed to assist growers in making an informed decision for correct harvest times.
Shape and Size
Stone fruit size and shape are affected by: variety, seasonal conditions, crop loads and orchard variability. Constant measuring of fruit is vital as to determine when the fruit size has reached a marketable size. Size can be measured using a Cranston Fruit Sizing Loop (Plate 1).
Plate 1. Cranston Fruit Sizing Loop
When fruit shoulders and sutures are well developed and filled out, stone fruit are considered mature. To be dependable this maturity measure needs to be coupled with other tests such as skin colour.
Colour
As fruit matures and ripens, colour changes from green to red or yellow (Plate 2b and 3). Red colour in nectarines/peaches depends on exposure to light, therefore the position of the individual fruit and training of trees will greatly effect the development of the red pigment in the skin. Excessive quantities of Nitrogen can also have a deleterious effect on skin colour as well as internal/keeping quality. Change in background colour of the fruit is not affected by sunlight and thus is a more dependable measure of maturity. However, this test is less affective on some of the newer varieties that have been bred to appear well coloured. The red pigmentation obscures the background colour change.
Although, probably the most common method of assessing fruit readiness to harvest, skin colour is not considered the most accurate indicator of maturity. However, experienced growers often use background colour as a means of assessing harvest maturity of peaches and nectarines.
Plate 2. (a) Shape and size: Note the difference in shape of immature (left) and mature fruit (right) the mature fruit is full around the stem end and the more rounded appearance in general. (b) Background skin colour: Immature fruit is greener (left) and the mature fruit is more yellow (right).
Plate 3. Apricots picked the same day showing the variance in size and colour, immature (left) and mature (right).
Flesh Firmness
Flesh firmness can be a very good indicator of maturity. This also relates closely to the eating experience for the consumer. Peaches harvested with pressure readings too high will generally offer a dry mealy texture when consumed.
Stone fruit firmness is affected by variety, crop loads, nutrition, pruning and seasonal conditions.
Flesh firmness is measured using a penetrometer with an 11-mm diameter plunger (Plate 4). The resistance required too drive a plunger though the flesh is measured in kilograms.
For nectarines, appropriate export pressures would be between 4-5kg while domestic pressures would be between 5-6kg. However, it is best to refer to the purchasers to determine the desired pressures, if they don’t know, ask them to find out.
Using a penetrometer:
It is important to take readings from the fruit within an hour that it is collected. Fruit will be firmer in the morning than the afternoon, so it is propitious to collect fruit at the same time every day, preferably morning.
About 10 days before the expected harvest date begin to take samples of fruit from the orchard. For stone fruit it will be necessary to take samples every couple of days as pressures will change rapidly at this stage. Take 15-20 samples of fruit from trees across the block, sample from both sides of trees, inside and out side the trees, and from top and bottom. Take the samples back to the packing shed for testing.
With a sharp knife slice an area of skin from the cheek of the fruit about the size of a 10-cent piece and no more than 1-2mm deep.
Holding the fruit firmly on a bench gradually plunge the penetrometer fitted with an 11mm tip into the fruit until the line is reached on the tip. Sharp movements while plunging will not give accurate readings. Read the gauge and record, press release button and continue taking readings on opposite cheeks of each fruit to find an average.
Do not discard the fruit, they can be used for testing the sugar content with a refractometer.
Plate 4. Penetrometer Kits commonly contain, gauge, knife/peeler, 8 mm tip and 11 mm tip.
Soluble Solids Content
Soluble solids content (SSC), also referred to as Total Soluble Solids (TSS), is a measure of the sugar, organic acid and other soluble components in the juice of the fruit. SSC is measured using a refractometer to determine the percentage of pure sucrose, the major constituent in fruit juice is expressed as °Brix. Variety and cultural practices affect SSC. SSC is important because it tells you when the fruit is sweet enough to eat. It can be used with other indices, such as firmness.
Using a refractometer
This is best done in the shed where the temperature is most stable. Most refractometers are adjusted to give correct readings at around 20º C, this can be adjusted during the calibration process, refer to users manual.
While taking the penetrometer readings a couple of drops of juice can be collected on the ring at the top of the tip.
Put these drops onto the glass window located at the front of the refractometer under a fold back flap.
Close the flap on the juice and ensure that the prism has a full thin layer of juice, hold it up to the light and look through the eyepiece as you would a telescope.
Plate 5. Refractometer use’s light to determine SSC
Plate 6. Typical view, looking through the refractometer, reading approx 16° Brix.
The window on the refractometer must be cleaned with a new tissue between each reading to maintain accuracy.
Record readings so that an average can be calculated.
Aim for 12-13º Brix for fresh market early nectarines. Sugar levels don’t increase significantly once the fruit is removed from the tree. 16º Brix as in the above view, would not be an achievable level if harvested to early.
As for flesh firmness check with your buyer, for there preferred ranges.
Titratable acidity
As Stone fruits mature the level of acidity drops, this, as with most of our indices is affected by all the common variances mentioned so far. Measuring this on farm, in most cases would not be possible due to the complicated nature of the test. Although the ratio of SSC:acid content has been found to be more closely related to the quality of the fruit than just acid or SSC content alone.
Examples of maturity test results
Table 1: Showing results of tests
| Fruit | Size mm | Firm East | Firm West | Brixº East | Brixº West |
| Nectarine 1 | 56 | 8.75 | 7.5 | 12 | 12 |
| Nectarine 2 | 51 | 11.5 | 10.5 | 11 | 13 |
| Nectarine 3 | 56 | 7.5 | 4.5 | 14 | 15 |
| Nectarine 4 | 56.5 | 7.5 | 8.5 | 14.5 | 14 |
| Apricot 1 | 47.5 | 9.5 | 7 | 11 | 13 |
| Apricot 2 | 0 | 0 | 1 | 15 | 19 |
East and west refers to different sides of the fruit. Firmness readings taken with an 11mm tip.
Note: Nectarine 2 is significantly less mature and in order to reach a larger size apricot 2 has become over mature.
Conclusion
There are many tools and associated parameters that can be used to guarantee a minimum maturity for your fruit, to ensure a pleasant eating experience for your consumers.
Experience
Will be your most useful tool and this will come from keeping good records on how your trees behave in your orchard.
Minimum maturity
Use the parameters presented in this Agriculture note as a guide. It is most important that you discuss with your market, what they require as a minimum maturity because many supermarkets have already set minimum maturity guides.
Further reading
- “Stonefruit maturity indices: a descriptive review” Carlos H Crisosto (Post harvest news and information 1994, Vol.5, No.6, pp64n-68n)
- “Factors in fresh market stone fruit quality” Carlos H Crisosto, F. Gordon Mitchell, and Scott Johnson. (Post harvest news and information 1995, Vol.6, No.2, pp17n-21n)
- “Non-destructive determination of internal quality in peaches and nectarines” D.C. Slaughter (American society of engineers 1995, Vol 38(2), pp617-623)
- “Post harvest physiology of peaches and nectarines” R.E. Lill, E.M. Donoghue, and G.A. King. (Horticultural reviews 1989, Vol.11, pp-413-452)
- “Methods for determining maturity and quality” University of California (Peaches, plums and nectarines, growing and handling for a fresh market, Pub 3331, 1989, pp195-196
The previous version of this note was published in December 2003.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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