Glowing peanut butter
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The following article was originally published in the journal Antocyjan (eng. Anthocyan) (1/2017):
Surprising Properties
John Harvey Kellogg, an American physician active at the turn of the 19th and 20th centuries, is considered one of the era’s most important inventors. Together with his brother, Will Keith Kellogg, he developed methods for producing corn flakes and peanut butter [1]. Both products revolutionized American dietary habits of the time.
Peanut butter is made from the seeds of the groundnut plant Arachis hypogaea, a member of the legume family Fabaceae. Although commonly called “nuts”, these fruits are actually pods containing one to four oil-rich seeds. Interestingly, after fertilization, the flower stalk lengthens and pushes the developing pods underground, where they mature [2].
Peanut butter is especially popular in American and Dutch cuisines. It serves as a spread for sandwiches and is a key ingredient in many baked goods [3].
Not only is peanut butter flavorful, but it’s also packed with protein, vitamins, and micro- and macronutrients. As you’ll see shortly, it also exhibits a surprising optical property.
Experiment
For this demonstration, you can use any type of peanut butter — smooth or crunchy (Photo.1)
The spread has a firm consistency and is slightly moist. Under daylight, it appears light brown (Photo.2).
Instead of eating it immediately, let’s darken the room and illuminate it with ultraviolet light. A blue laser pointer works as well. Intense white light yields poorer results.
After switching off the UV lamp, you’ll observe it glowing green for several seconds (Photo.3). Allow your eyes to adjust to the dark beforehand.
This effect is quite striking, and observers are often surprised to see peanut butter glowing.
Once the demonstration is over, feel free to consume the remaining spread as intended.
Explanation
The phenomenon observed here is called phosphorescence. It involves the delayed emission of absorbed light energy. Unlike fluorescence, where emission stops almost immediately (on the order of 10–8 s) after the excitation source is removed, phosphorescence persists for seconds or longer.
In this process, an electron absorbs high-energy photons (e.g., UV light) and transitions to an excited state. This state is unstable, so after a delay, the electron returns to its ground state, releasing excess energy as photons. Due to the Stokes shift, the emitted light typically has a longer wavelength than the excitation source, because some energy is lost through thermal vibrations [4].
The delayed emission in phosphorescence arises from transitions between states of different spin multiplicity. These “forbidden” transitions have low probability, making the process slow.
Many substances exhibit phosphorescence. One example is “glow-in-the-dark” candy made by melting a sugar mixture with ethacridine lactate (rivanol) C18H21N3O4 [5].
It’s hard to pinpoint which natural compounds in peanut butter cause this glow, but they are not artificial additives. Similar effects occur in almond-based spreads. Phenolic compounds likely play a key role [6].
References:
- [1] Masło orzechowe, online: http://www.wiatrak.nl/10525/maslo-orzechowe [21.01.17] back
- [2] Strasburger E., Noll F., Schenck H., Schimper A.F.W., Botanika, Państwowe Wydawnictwo Rolnicze i Leśne, Warszawa, 1972, pp. 862 back
- [3] Kołożyn-Krajewska D., Sikora T., Poradnictwo przy sprzedaży, in: Towaroznawstwo żywności, Wyd. VII, Wydawnictwa Szkolne i Pedagogiczne Spółka Akcyjna, 2007, pp. 130 back
- [4] Gispert J.R., Coordination Chemistry, Wiley-VCH, 2008, pp. 483 back
- [5] Ples M., Świecący cukierek (eng. Glowing Candy), online: http://weirdscience.eu/%C5%9Awiec%C4%85cy%20cukierek.html [21.01.2017] back
- [6] Keartes S., Why does peanut butter glow in the dark?, online: http://nerdist.com/why-does-peanut-butter-glow-in-the-dark/ [21.01.17] back
All illustrations were created by the author.
Marek Ples