How do you like them Apples! The core chemistry of their internal structure

Apples are widely regarded as the most popular fruit available worldwide. They are high in fibre, antioxidants, vitamins (such as vitamin C) and minerals (potassium and calcium) and have been enjoyed by all since they were first cultivated (1). Originating from Central Asia and being first cultivated as early as 4000 years ago, apples have expanded to over 7500 different varieties worldwide (2).

While containing similar chemistry, each variety can be different in structure and mineral/elemental distribution. Additionally, within the fruit itself the chemistry can varying in both a longitudinal and lateral sense. Key regions such as the pith where seeds and carpellary bundles lie and hypanthium where much of the flesh is situated may concentrate certain elements based on position to the stem (see Fig. 1 and 2).

Figure 1. Diagram showing the longitudinal cross section of the apple from a flower evolving into a fruit. Drawing by M. Goffinet. (Taken from Lakso, A., Goffinet, M. 2013)

Figure 2. Diagram of lateral section of the apple showing gross morphology of apple fruit. ft, floral tube; s, sepal bundles; p, petal bundles; cl, outer limit of carpel or core line; dc, dorsal carpellary bundle; cb, carpellary bundles connecting dorsal with ventral carpellary bundles (Based on MacDaniels, 1940). (Taken from Herremans, E. et. Al 2015)

To identify chemical variations within apples, the M4 Tornado micro-XRF was utilised to produce detailed element maps under atmospheric conditions. To observe the internal cross-sectional chemistry, two different apple varieties (Royal Gala and Fuji) were cut both on the longitudinal (Fig. 1) and lateral (Fig. 2) planes for analysis.

Micro-XRF - The M4 TORNADO

The M4 tornado is a non-destructive, high resolution micro- X-ray fluorescence instrument that can be used to produce detailed elemental maps of a sample area, using a spot size of 25 microns. This can be done both under atmospheric conditions or under vacuum (20 mbar) within a chamber that has a sampling area of 19 x 15cm. The M4 tornado and has two detectors that enable the detection of elements from sodium (Na) to uranium (U). For more information refer to Bruker M4 Tornado . For these analyses conducted a spot size of 25µm at 100µm resolution (50kV, 400uA) was used on the cut sections of the samples.

Results

While the internal structure differed slightly between the apple varieties, both the Royal Gala and Fuji apples produced similar results. The dominant elements observed were potassium (K), calcium (Ca) and iron (Fe), these elements highlighted the key structural regions of the apples, as well as nutrient pathways leading from the core to the outer skin (Fig. 3). Other minor elements identified were zinc (Zn), copper (Cu) and titanium (Ti), these were uniformly distributed throughout both varieties of apples, including within the seeds.

From the longitudinal sections the stem, receptacle, and calyx remnants all have strong associations with heightened Fe and Ca concentrations. Meanwhile, the carpellary bundles and seed cavities were usually associated with heightened K concentrations. Potassium abundance highlights the pith/core region as well as the border of the carpels. Outwards from the border of the carpels the K and its association with Ca can be seen clearly showing some of the vascular network of the apple within the hypanthium.

The lateral sections of the apples both show K forming nutrient pathways leading from the K-rich core and carpellary bundles to the cortex/hypanthium region. Within the hypanthium region, K and Fe emphasizes the micro-structures that make up the vascular network which allows K and other vital nutrients to move through the apple. The seeds themselves are found to have a strong association with Ca and Fe, similarly with the skin of the apple itself.

Knowing the distribution of the critical elements such as Ca, Fe and K is crucial for fruit growers as knowledge about the internal chemical structure and element uptake can indicate the health of the crop, and in some cases flavour. Studies on Ca uptake by apple varieties such as Honeycrisp apples have shown lower uptake of Ca can lead to more bitter flavour (5). Additionally, knowing the elemental distribution as the apple decomposes may be key to understanding the shelf life of the fruit. Currently the apples analysed are decomposing and will be analysed again at a later date to investigate the changes that have occurred.

Figure 3. Spectral match of cup 1 lining spectrum (red) to polylactide reference spectrum (blue).

References

1) Arnarson, A. Apples 101: Nutrition Facts and Health Benefits. Retrieved on 25/07/2020 from: https://www.healthline.com/nutrition/foods/apples
2) BASF. (2020). The chemistry of apples. Retrieved on 25/07/202 from: https://www.basf.com/global/en/media/magazine/archive/issue-6/the-chemistry-of-apples.html
3) Herremans, E. et. Al. (2015). Spatial development of transport structures in apple (Malus × domestica Borkh.) fruit. Frontiers in Plant Science, Volume 6, Article 679. 
4) Lakso, A. Goffinet, M. (2013). Apple Fruit Growth. New York State Horticultural Society, New York Fruit Quarterly, Volume 21, Number 1.
5) Prengaman, K. (2018). High-res Honeycrisp: Cell structure scans offer new insight into why Honeycrisp is so prone to bitter pit. Retrieved on 25/07/2020 from: https://www.goodfruit.com/high-res-honeycrisp/

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