Analyzing orange: peel and pulp separately or as whole?

Up to this point, I’ve focused on the optimization of QuEChERS salts and dSPE cleanup with fairly homogenous matrices. So, what about oranges? Should I peel them and analyze only the pulp? After all, that is the edible part. The peel is used as well – pressed into essential oils or scraped into zest. Besides, there are uses where the peel and the pulp are used together in some products; such as orange juice, where squeezed oranges lose the majority of the flavor due to preservation and additional flavor (made from the peel) needs to be added back into the juice. So, there is a need for analysis for individual parts of orange as well as for orange as a whole.

Orange pulp

Let’s start with the orange pulp. It is a matrix that has high water and sugar content and low pH (3.3 – 4.2). Because pH plays a role in pesticide stability, the buffered QuEChERS salts (AOAC or EN) are preferable. When we look at the data collected after spiking orange pulp with the QuEChERS Performance mix (#31152) and extracting with all three types of extraction salts (Fig 1), there are higher responses for several pesticides, especially when buffered salts are used. Overall, the EN salts (#25849) were chosen because most of the pesticides exhibited higher recoveries.
Figure 1: Comparison of total pesticides' recoveries in orange pulp using different QuEChERS salts

 

When it comes to dSPE, the high sugar content suggests that we should use high PSA (primary secondary amine) content. But is there a benefit in additional sorbents? We tested five dSPE tubes: #26124 (150 mg MgSO₄, 50 mg PSA), #26125 (150 mg MgSO₄, 50 mg PSA, 50 mg C18-EC), #26217 (150 mg MgSO₄, 25 mg PSA, 2.5 mg GCB), #26218 (150 mg MgSO₄, 25 mg PSA, 7.5 mg GCB), and #26243 (150 mg MgSO₄, 50 mg PSA, 50 mg C18-EC, 7.5 mg GCB). Varying levels of PSA, C18-EC (end-capped) and GCB (graphitized carbon black) provided a good range to evaluate whether the PSA alone (#26124) was enough for this analysis. The comparison shown in Figure 2 shows that there is either no difference in responses or the dSPE with PSA alone has an edge.

Figure 2: Comparison of total pesticides' recoveries in orange pulp using different dSPE cleanups

But is this combination (EN salts and PSA only dSPE) good for peel, too?

Orange Peel

While orange peel feels fairly dry, it actually contains approximately 75% water.¹ However, these measurements are done gravimetrically and the presence of essential oils could potentially inflate these results. Nevertheless, the commodity needs hydration for extraction. I’ve chosen to add an additional 40% of water, i.e. 4 mL for every 10 g of orange peel. The acid content in the peel is lower, therefore, the extraction salts don’t need to be buffered. When the three types of QuEChERS salts were compared (Figure 3), the unbuffered (#25847) and EN salts (#25849) performed better than the AOAC salts (#25851) for several pesticides. The EN salts were chosen again since they resulted in the best recoveries for our chosen pesticides.

Figure 3: Comparison of total pesticides' recoveries in orange peel using different QuEChERS salts

The selection of dSPE is quite different than for the orange pulp. Thanks to the presence of essential oils and absence of high sugar content, the dSPE selected for pulp (#26124, 150 mg MgSO₄, 50 mg PSA) was compared to dSPE containing some level of end-capped C18 (#26125 – 50 mg C18-EC, #26216 – 25 mg C18-EC, #26243 – 50 mg C18-EC and 7.5 mg GCB). Figure 4 shows the comparison of pesticides’ responses. While most of the pesticides have similar peak averages, #26216 and #26125 have a higher response for several of them.

Figure 4: Comparison of total pesticides' recoveries in orange peel using different dSPE cleanups

So, to answer the question, the same combination of QuEChERS salts and dSPE clean up can be used, however, dSPE with C18-EC added performs better for several pesticides.

Now the next question is: what would be best for the whole orange?

Whole orange

So far, we established two methods: one for pulp and one for the peel. The ratio of pulp to peel is approximately 40:60 w/w¹ and the water content is >80% w/w.² In theory, oranges shouldn’t need hydration before QuEChERS because the water content is high enough. However, due to the peel, some level of hydration can be beneficial, especially if samples are stored in the freezer between homogenization and extraction.

Figure 5: Comparison of total pesticides' recoveries in a whole orange using two different dSPE cleanups

Figure 5 shows a comparison of the clean-up methods chosen for pulp and peel, #26124 (50 mg PSA) and #26216 (25 mg PSA and 25 mg C18-EC) respectively. As you can see, there is very little difference between the responses. However, if we examine the orange specific pesticides (i.e. 2-phenyl phenol, carbaryl, chlorpyrifos, imazalil, and thiabendazole)³, the dSPE with added C18-EC (#26216) has higher responses for several pesticides (Figure 6). This confirms that the peel has an effect on the overall pesticide recovery.

Comparison of specific pesticides' recoveries in a whole orange using two different dSPE cleanups

1) https://pdfs.semanticscholar.org/16a9/2ed2fc116e57e35c1aa66651524d4b5128c8.pdf

2) https://fdc.nal.usda.gov/fdc-app.html#/food-details/327501/nutrients

3) http://www.whatsonmyfood.org/food.jsp?food=OG