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Perhaps the title should be: Why don't all fruits containing phenol residues go brown quickly when left in the fridge?

Bananas go brown over time because of the oxidation of phenol residues.

Bananas go brown quicker in the fridge than left at room temperature. This seems to contradict the first point since the reaction should be slower at colder temperatures. Dialogue from the naked scientists briefly explains why my line of thinking might be wrong.

Emily: Well it’s a good question and the answer is that they will brown faster in the fridge. It’s mainly due to the formation of ice crystals, so if you put your banana in the fridge, the ice crystals grow, and they actually rupture the cells of the banana skin. This releases an enzyme called polyphenol oxidase and as the name suggests it acts to oxygenate phenols, which have a ring-like structure, into quinones, and these quinones can then all join together or polymerise and produce a black, brown, or red pigment called polyphenol and this is what gives it the brown colour. So, if you have your banana in the fridge, this will occur and you'll get a brown banana. But another interesting thing about bananas is that if you have them in the fruit bowl, they'll release ethene and this will make the other fruits in the bowl ripen faster because it’s a ripening hormone.

Chris: So, the question is though, if this is a chemical reaction making this brown pigment, if you slow down the reaction by lowering the temperature, then it should happen more slowly, therefore, going in the fridge should make the bananas go black slower?

Emily: I can see why you're thinking that but actually, it’s more the rupturing of the cells. So if you have a banana on your table, the cells are going to be intact, the enzymes contained, and the reaction is not happening at all whereas if you put in the fridge, the enzyme is released and the reaction can happen – although yes, it might happen slower.

At the risk of channeling people into a yes or no answer, could someone clarify for me if the following statement is true maybe with a peer-reviewed source, which I've had no luck finding:

TLDR Bananas go brown quickly in the fridge because cell lysis, caused by ice crystal formation at cold temperatures in the fridge, releases Polyphenol oxidase (PPO) into the surrounding area which freely causes more browning.

This leads me to another seeming paradox and the question I would like answered: Why doesn't this rapid browning in cool temperatures happen for other fruits and vegetables? What is special about bananas?

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    $\begingroup$ That is a good question indeed, but funding bodies are likely to be industrial, and they generally don't care much about writing papers, and often choose to go for patents instead. $\endgroup$ – AliceD Nov 20 '15 at 3:24
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    $\begingroup$ @AliceD... I am going to flag you for using the There is no money in basic research argument. I don't know that I buy Emily's argument completely. Most fridges will not go below 4°C. I have actually observed this phenomena in my freezer and seen the brown bananas (Makes really good banana bread that way), but I just don't see ice crystals forming before you get to 0°C. There has to be something else going on. Maybe the enzyme is more stable at colder temperatures? Seems counter intuitive, but maybe it is an adaptive response to frost. $\endgroup$ – AMR Nov 20 '15 at 3:38
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    $\begingroup$ Maybe it is the confined space, Ethene concentration is allowed to build up to higher concentrations in the enclosed space of the fridge. Try this experiment. Seal a banana in an airtight plastic bag at room temp in the dark, maybe with a slightly damp sponge to simulate the humidity in the fridge, leave one out, put one in the fridge, and one in the freezer and compare after 24 hours. $\endgroup$ – AMR Nov 20 '15 at 4:32
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    $\begingroup$ @AMR- I may have been unclear. What I meant was that I think chances are slim basic scientists have bothered with it. Food industries may have. $\endgroup$ – AliceD Nov 20 '15 at 5:15
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    $\begingroup$ +1. It can also be added that banana skins go completely black when frozen, even though freezing them will stop the enzymes from catalysing the reactions. The same does not happen for cut apples, despite the same explanation (polyphenol oxidases) being put forward for the browning of both fruits. $\endgroup$ – March Ho Nov 20 '15 at 9:12
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Banana fruit is very susceptible to chilling injury (CI) and this can occur at relatively high temperatures: CI may occur at 12oC and under. Typically the banana skin turns brown, but in addition there may be pitting and the fruit may not soften (Wang et al., 2013).

Oxidative Stress & ROS

One generally accepted theory it that oxidative stress, due to the accumulation of reactive oxygen species (ROS) such as H$_2$O$_2$ (hydrogen peroxide), O$_2$$^−$ (superoxide) and OH$^{\bullet}$( the hydroxyl radical), is a primary cause of CI in bananas. (Wang et al, 2003, 2016; Pongprasert et al., 2011).

ROS may give rise to lipid peroxidation, enzyme inactivation, membrane rupture and, ultimately, cell death.

One possibility is that the effect of lowering the temperature is to decrease the activity of key enzymes involved in the 'normal' plant response to ROS. In other words, it is postulated that low temperatures cause key enzymes involved in the stress response to ROS to become sluggish, allowing the build-up of reactive oxygen species that (indirectly) lead to cell death and browning (Wang et al., 2013).

However, plants also respond to cold-induced stress and a key player is the amino-acid proline (Verbruggen & Hermans, 2008; Chen et al., 2008).

Proline has a key role in maintaining plant cellular integrity in times of stress, and it has been proposed that it does this by acting as an osmolyte, by scavenging ROS, by maintaining protein structure, and by buffering pH (see Verbruggen & Hermans, 2008).

Two key biosynthetic enzymes (starting with glutamate) are pyrroline-5-carboxylate synthase and pyrroline-5-carboxylate reductase (Verbruggen & Hermans, 2008). A key degradative enzyme is proline dehydrogenase. Although an aside to this answer, it is nevertheless interesting that proline biosynthesis occurs in the cytoplasm, but proline degradation occurs in the mitochondrion (Verbruggen & Hermans, 2008)

Biochemical Aspects of Stress Response

Plants have an intricate antioxidant defense mechanism to counter the dangers of ROS.

Non-enzymatic antioxidants include ascorbic acid (Vitamin C), polyphenols, the reduced form of glutathione, and α-tocopherols (Pongprasert et al., 2011). Key enzymes of the antioxidant response include superoxide dismutase (SOD), glutathione reductase (GR), catalase, peroxidase, and ascorbate peroxidase

In addition, another important enzyme is phenylalanine ammonia-lyase (PAL). This enzyme catalyzes the conversion of phenylalanine to trans-cinnamic acid, a key intermediate in the biosynthesis of polyphenols and (many) other plant secondary metabolites (Chen et al., 2008)

As will be seen below, many of the strategies employed to counteract CI involve pretreatments designed to increase the levels of non-enzymic oxidants and to increase the expression/activity of key enzymes involved in the stress response to ROS.

The Browning Reaction

It is almost universally accepted that the browning reaction is the result of the action of (the copper-containing) polyphenol oxidase (PPO). This enzyme oxidises mono-phenols and di-phenols to ortho-quinones that non-enzymically polymerize, via a free radical mechanism, to give a dark brown or black compound(s).

In normal undamaged tissue the enzyme is latent and, furthermore, does not have physical access to its phenolic substrates due to subcellular compartmentalization (Promyou et al., 2008; Hind et al., 1995). In damaged tissues, however, the enzyme is activated and comes in contact with its substrates, perhaps through membrane rupture. The resulting brown, structurally complex polyphenol product is thought to protect plants from pathogens and herbivores (Hind et al., 1995).

Thus the brown color is the response of the plant to irreversible tissue damage as a defense mechanism against further degeneracy.

(Polyphenol oxidase is sometimes referred to as tyrosinase and catechol oxidase, and the reaction catalyzed by PPO is similar to that catalyzed by tyrosinase in the (human) formation of melanin from tyrosine).

Prevention

A number of innovative methods have been used to prevent low-temperature browning. Many attempt to induce an increased stress response to ROS prior to storage at low temperatures.

UV-C Irradiation

  • Pongprasert and co-workers have shown that irradiation with UV-C light increases the expression of phenylalanine ammonia-lyase and other key ROS stress-response enzymes, and pretreatment with UV-C seems be a very effective method in reducing CI (Pongprasert et al., 2011).

Heat Pretreatment

  • Chen and colleagues demonstrated that heat pretreatment (38oC for 3 days) induces chilling tolerance, and they attribute the effect (at least in part) to an increase in both the activity and expression of phenylalanine ammonia-lyase (PAL). (Chen et al., 2008)

Hot water

  • Promyou and associates have shown that hot water pretreatment is a practical method for preventing CI, and they attribute the effect to decreased polyphenol oxidase activity (which they call catechol oxidase) (Promyou et al., 2008).

Modified Atmosphere packaging

Nitric Oxide (NO)

  • In a detailed an innovative study, Wang and co-workers have shown that pretreatment with nitric oxide (NO), itself a free radical, greatly diminishes CI in bananas. They demonstrated that key enzymes of ROS metabolism are elevated by NO treatment. In addition, the activity of phenylalanine ammonia-lyase (PAL) is increased as is the activity of a key proline biosynthetic enzyme (pyrroline-5-carboxylate synthase), but the activity of a key enzyme of proline degradation (proline dehydrogenase), decreased in activity.

Summary

  • The browning of banana skin is due to polymer formation by the catalytic action of polyphenol oxidase on mono- and di-phenols in response to irreversible tissue damage by reactive oxygen species (ROS)
  • ROS accumulate at low temperature due to a lowering of plant defense mechanisms.
  • The brown polyphenol product is formed only when tissue damage occurs as a plant defense mechanism against further abasement by pathogen and herbivore attack.

Key References

Chen, J.Y., He, L.H., Jiang, Y.M., Wang, Y., Joyce, D.C., Ji, Z.L. and Lu, W.J. (2008). Role of phenylalanine ammonia‐lyase in heat pretreatment‐induced chilling tolerance in banana fruit. Physiologia Plantarum, 132, pp.318-328. [PMID: 18275463]

Hind, G., Marshak, D.R. and Coughlan, S.J. (1995). Spinach thylakoid polyphenol oxidase: cloning, characterization, and relation to a putative protein kinase. Biochemistry, 34, pp.8157-8164 [First page]

Nguyen, T.B.T., Ketsa, S. and van Doorn, W.G., 2004. Effect of modified atmosphere packaging on chilling-induced peel browning in banana. Postharvest Biology and Technology, 31, pp.313-317. [pdf]

Pongprasert, N., Sekozawa, Y., Sugaya, S. and Gemma, H. (2011) The role and mode of action of UV-C hormesis in reducing cellular oxidative stress and the consequential chilling injury of banana fruit peel International Food Research Journal 18, pp 741-749 [pdf]

Promyou, S., Ketsa, S. and van Doorn, W.G. (2008). Hot water treatments delay cold-induced banana peel blackening. Postharvest Biology and Technology, 48, pp.132-138. [pdf]

Verbruggen, N. & Hermans, C. (2008). Proline accumulation in plants: a review. Amino acids, 35, pp.753-759. [pdf]

Wang, Y., Luo, Z., Du, R., Liu, Y.,Ying, T. & Mao, L. (2013) Effect of Nitric Oxide on Antioxidative Response and Proline Metabolism in Banana during Cold Storage, Journal of Agricultural and Food Chemistry,61, pp 8880-8887. [PMID: 23952496]

Wang, Y., Luo, Z., Mao, L. & Ying,T. (2016) Contribution of polyamines metabolism and GABA shunt to chilling tolerance induced by nitric oxide in cold-stored banana fruit. Food Chemistry 197, pp 333-339. [PMID: 26616957]

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  • $\begingroup$ Really interesting thorough answer! Thanks. $\endgroup$ – James Sep 3 at 15:12
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What's interesting with this one is we don't really know very well the mechanism behind what's called chilling injury. It happens to a range of fruits, like bananas, peaches, avocado, or apples. The belief is that the chilling alters membrane permeability to storage vacuoles inside the plant cells. Try On Food and Cooking, pp.269, and Puig et al. (2015) for references here.

Now the membrane permeability to the storage vacuole changes (it gets leaky), and inside these vacuoles are phenolic compounds. In the cytoplasm of the plant cell, there's an active enzyme called polyphenol oxidase (PO). The reactions it catalyzes end up with a compound called melanin, a brown/black pigment:

enter image description here

Fig A. An overview of what a PO might do.

So the phenolic compounds leak out and PO goes to work catalyzing reactions that start building up the dark pigments associated with browning. Being said, I can't at this moment find a good paper that says "look, this is how the entire process happens," but I'll be attempting to read a little deeper into the matter.

Keywords: flesh browning, enzymatic browning, chilling injury, polyphenol oxidase, storage quality, storage conditions

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