Assessing variability in Navel orange fruit maturity

On-tree flavour differences: assessing variability in Navel orange fruit maturity - a snapshot

Roberto Marques, Steven Falivene, Andrew Creek, and Lorraine Spohr, DPI

October 2015

Determining the right time to harvest is important to ensure fruit have desirable flavour attributes. Industry currently undertakes some fruit maturity tests to determine the timing of harvest to achieve a product to match consumer preferences. Maturity is a key factor determining flavour in ripe citrus fruit, with the increase of soluble sugars (Brix) and reduced organic acids being the most important developmental traits. In general, the external appearance of fruit is a major factor in encouraging consumers to make a purchase. However, internal quality (e.g. flavour) is a key driver towards stimulating repeat sales and increasing consumption.

The variability in Navel orange maturity within a tree and between trees is not well understood. NSW DPI recently undertook a preliminary study to look at this variability. We collected Navel fruit at about 2–3 weeks before expected harvest from two commercial orchards, one at Leeton ('Washington' trees on Trifoliata) and another at Ellerslie ('Barnfield' trees on Citrange). At each orchard, we picked 32 fruit from each of five uniform adjacent trees for a total of 160 fruit in each location. We took fruit from different sections of the canopy and individually labelled them according to their position on the canopy:

• Depth: outside (up to 30 cm) or inside (an arms' length or about 50 cm) of the canopy
• Height: top (1.4–2.0 m) or bottom (0.5–1.2 m) section of the canopy
• Quadrant: north-east (NE), north-west (NW), south-east (SE), or south-west (SW).

We individually assessed the fruit for quality at the NSW DPI postharvest laboratory at Ourimbah, including percentage of yellow skin, Brix, and titratable acidity (TA), so the BrimA/Australian Citrus Standard (ACS) could be determined.

Key results

• Fruit from the outside and top part of the canopy had up to 24% (Leeton) and 28% (Ellerslie) higher ACS than fruit from the inside and bottom part (as shown in Figure 1). Those differences were mainly due to a higher Brix (9–13%) combined with lower TA (16–22%) in the outside/top fruit (Table 1).
• The variability within the tree in fruit ACS varied according to location: it was mostly due to canopy height at Leeton, but mostly due to canopy depth at Ellerslie.
  Outside/top fruit also had 19–34% more yellow on the skin, suggesting faster maturation of those fruit.
• The impact of canopy quadrant in fruit ACS was considerably smaller than canopy height or depth at either location. Fruit from the west side of the tree (i.e. NW and SW) generally had up to 5% higher ACS and 8% more yellow on the skin than fruit from the east side (i.e. NE and SE) at either location. There was little difference in ACS between fruit from the north and south sides of the canopy.
• Fruit from Ellerslie exhibited significant differences between trees (i.e. up to 26% variation in fruit ACS and 27% for yellow on the skin). In contrast, there was little variation (less than 4% in fruit ACS) from the Leeton site.
• The variations were generally larger in fruit from Ellerslie than from Leeton. However, the patterns of variation in ACS, Brix, TA, and skin yellow affected by canopy depth, height and side, were quite consistent for both locations.
• The percentage of fruit with the minimum ACS (e.g. 90 for Navels) was 99% for Leeton and 86% for Ellerslie.

The above results are an initial snapshot highlighting the complexity of maturity variation in Navel oranges. Additional work is required with more representative samples to better understand variability at different stages of fruit maturation, across various locations and blocks within locations, for other varieties and across seasons. Understanding variability in maturation will help to develop improved sampling techniques for each area to better reflect a block's suitability for harvest, thus contributing to delivering better tasting fruit to market to meet consumer preferences. It also opens up the opportunity for looking at the economics and practical aspects of select picking options for markets that pay a premium for higher brix fruit.

Table 1: Quality characteristics of fruit harvested from different sections of the canopy. Values are the average of 80 (for canopy depth, height, and side) or 32 (for tree) fruit from 5 adjacent trees at each location (total of 160 fruit per location).