Last night, my girlfriend informed me that she made some instant (sugar free, fat free) chocolate pudding 2 nights before. When opening the fridge to finish it off, it was green! I had never heard of this before. I don’t study food chemistry, and I doubted there was a study with an explanation. But I am wrong, and my lovely girl found it for me (she’s a keeper!). So here is the scientific reason why chocolate pudding can turn green.
If you take a look at the nutrition facts panel on the pudding box, it will have the artificial dyes red, yellow, and blue in the ingredients list. Many of these dyes are “azo dyes” based on their chemical structure. Combining red, yellow, and blue obviously creates the brown coloring for the chocolate pudding. To turn green, the red color would have to be degraded.
Some microorganisms contain enzymes called azoreductases, which reduce certain azo dyes. So researchers Dykes, Timm, and von Holy (1) took bacteria from samples of dry chocolate pudding powder, unspoiled reconstituted pudding, and pudding that had turned green. They grew individual cultures (45 of them) in aerobic and anaerobic conditions and in Ponceau 4R, a red azo dye, took the 2 that were capable of clearing the dye (both only in aerobic conditions) and put them back into chocolate pudding to verify that they turned it green (this only took 12 hours at 25 degrees Celsius ~77 Fahrenheit). These 2 strains were tentatively identified (Pseudomonas paucimobilis and Klebsiella pneumoniae) and further tested on 3 more red azo dyes (Carmoisine, Allura Red, and Azo Geranine) and 1 nonazo dye (erythrosine). In culture, the strains demonstrated azoreductase activity and degraded the additional 3 azo dyes that were tested, but not the nonazo dye. So if some puddings use nonazo red dyes, they would not be susceptible to greening (will have to take a trip to the grocery store to check brand differences). Additionally, they found that the dyes were not degraded at the same rate; carmoisine was fastest, then Ponceau 4R, then Azo Geranine, and Allura Red. This means that if different red azo dyes are used in different brands, the greening rates could vary.
The authors note that no bacterial growth occurred in the dry chocolate pudding powder, so the 2 strains capable of greening the pudding may come from the milk that was used (both of the strains were reported in another study to be present in milk).
Stay tuned for a homemade experiment where I unleash my inner geek and test different brands, preparations and environments of chocolate pudding to see how often greening happens and what can be done to prevent it. I’ll be starting it next week so if you have any brand requests let me know in the comments.
Oh, and if your chocolate pudding is green, the coloring itself is safe to consume. However you’re at your own risk; these bacteria don’t seem to be pathogenic under normal circumstances, but there will be many other strains that also will be growing that weren’t studied here. It would be a signal that conditions are ripe for bacteria in general to grow, so it is probably better to be safe than sorry and toss it.
1. Dykes GA, Timm RG, & von Holy A (1994). Azoreductase activity in bacteria associated with the greening of instant chocolate puddings. Applied and environmental microbiology, 60 (8), 3027-9 PMID: 8085839