Preservation of citrus fruit

Question

A decade ago [1] the possibility of using Cas9 systems to impart desirable properties to oranges seemed within reach [1]. Antifungals like Imazalil can increase shelf life of citrus but are known to disrupt hormones and are perhaps carcinogenic [2]. A more recent article suggests that Cas9 is in fact being used to overcome "biotic stresses"[3]. Problems like citrus greening, black spot, and penicillium cost growers a lot and GM seems preferable to the chemicals mentioned in [2].

Does anyone actually know how citrus is currently preserved in the U.S.? Are both fungicides or GM at work? Neither? Both?

[1] A Jia H, Wang N. Targeted Genome Editing of Sweet Orange Using Cas9/sgRNA. Prasad M, editor. PLoS ONE. 2014 [accessed 2023 Jun 6];9(4):e93806. https://dx.plos.org/10.1371/journal.pone.0093806. doi:10.1371/journal.pone.0093806

[2] Temkin et al., Hormone-disrupting fungicides found on most citrus fruit samples tested by EWG, found online in Shoppeer's Guide to Pesticides on Produce, March 2023, https://www.ewg.org/foodnews/citrus.php.

[3] Tanwir, S.E., Soares, J.M., Welker, S., Grosser, J.W., Dutt, M. (2021). Genetically Modified Citrus: Current Status, Prospects, and Future Challenges. In: Kavi Kishor, P.B., Rajam, M.V., Pullaiah, T. (eds) Genetically Modified Crops. Springer, Singapore. https://doi.org/10.1007/978-981-15-5932-7_7

Answer 1

I had a look through some web pages and came across one on shipping of oranges that indicates the storage conditions and treatments that may be applied to oranges.

This isn't necessarily about oranges in the USA, but ones from California are mentioned. It seems that there are a number of methods of maintaining shelf-life and that oranges are usually washed to remove dirt and contaminating fungal spores and then sprayed with a wax to inhibit loss of water as washing removes the natural surface waxes. To quote the article (emphasis is mine):

Quality / Duration of storage

Experience has shown that it is the care taken with preparation of the fruit for shipping which very largely determines whether individual batches withstand the rigors of transport. Such preparation for shipping is carried out in packing houses. These include:

• Post-ripening of green or unsatisfactorily colored fruit to achieve a salable peel color in ripening rooms.
• Removal of dirt, sooty mold, spraying residues and scale insects in washers.
• Finishing of oranges which do not develop the typical orange color, but instead remain pale gold, green or with green spots, in a dye bath at temperatures of 45 – 50°C. Fruit treated in this way must be marked accordingly with a stamp (color added).
• Coating with a layer of wax and treatment with preservatives and marking accordingly.
• Grading of the fruits by size (gaging), color and other external features.
• Counting, weighing and packing. Marking each package with details of number of fruit, quality class, variety and origin.
• Storage until shipment in cold stores.

Waxing to prevent loss of aroma and weight is required because the washing process removes the natural wax layer. The film of wax sprayed onto the peel only partially seals the pores so that the fruits are still able to respire.

I suspect that the wax also helps prevent fungal growth through physical inhibition of spores getting to the fruit. In addition, storage conditions play a big role here - if you don't have any other fruit around your single orange, then it might not mature further (no ethylene) and it will have adequate ventilation to prevent rotting from too much CO2:

Care of the cargo during the voyage must be aimed at controlling respiration processes (release of CO2, water vapor, ethylene and heat) in such a way that the cargo is at the desired stage of ripeness on reaching its destination. Inadequate ventilation may result in fermentation and rotting of the cargo as a result of increased CO2 levels and inadequate supply of atmospheric oxygen (see Ventilation).

Fungicides are used in parts of the world, and I would be very surprised if they weren't in use in the USA:

Fungicides are diphenyl, orthophenylphenol (OPP) and thiabendazole (TBZ). Diphenyl can be recognized from its naphthalene-like odor. The fungicides primarily prevent blue and green molds, but they do impair flavor and indication of their use is mandatory.

Answer 2

As far as processing for juice, Pectinases are used to extract more juice from the pulp. GM from Aspergillus niger or Penicillium spp.

For citrus fruit with any bitterness, naringinase is used to destroy naringin.

Technically the pectinases are: pectin methyl esterases, pectin lyases, and polygalacturonases

Info from page 141, Industrial Microbiology - An Introduction.