Sweet potatoes, with their vibrant orange flesh and naturally sweet taste, are a nutritional powerhouse. Whether baked, mashed, roasted, or fried, they’re a versatile ingredient in countless dishes. However, like many fruits and vegetables, peeled sweet potatoes are prone to discolouration. This phenomenon, often referred to as enzymatic browning, can be a source of concern for home cooks wondering if their sweet potatoes are still safe to eat.
What Causes Sweet Potatoes to Turn Brown?
The browning of sweet potatoes, just like apples or avocados, is primarily due to a natural chemical reaction called enzymatic browning. This reaction occurs when enzymes within the sweet potato are exposed to oxygen in the air.
The Role of Polyphenol Oxidase (PPO)
The main culprit behind enzymatic browning is an enzyme called polyphenol oxidase, often abbreviated as PPO. This enzyme is naturally present in sweet potatoes and other plants. When the sweet potato’s cells are damaged – as happens when you peel or cut it – PPO is released.
PPO then catalyses the oxidation of phenolic compounds, which are also naturally present in the sweet potato. This oxidation process leads to the formation of melanins, which are brown pigments. It’s the same process that causes bananas to brown when their peel is damaged.
Exposure to Oxygen: The Catalyst
While PPO and phenolic compounds are essential for browning to occur, oxygen is the crucial catalyst. Without oxygen, the reaction would proceed very slowly, if at all. This is why storing peeled sweet potatoes in water can help to prevent browning, as it limits their exposure to air.
The rate of browning also depends on factors such as temperature and pH. Higher temperatures generally speed up the enzymatic reaction, while acidic conditions can slow it down.
Is a Brown Sweet Potato Safe to Eat?
The good news is that a brown sweet potato is generally safe to eat. Enzymatic browning is primarily a cosmetic issue and does not usually affect the taste or nutritional value of the sweet potato.
However, severely browned sweet potatoes might have a slightly altered texture or flavour due to oxidation. If the sweet potato has been sitting out for an extended period, there is a risk of bacterial growth, which could make it unsafe to consume.
Therefore, it’s best to use your senses to determine whether a browned sweet potato is still good. If it smells or feels slimy, it’s best to err on the side of caution and discard it.
Tips to Prevent Sweet Potato Discolouration
Fortunately, there are several effective ways to prevent or minimise enzymatic browning in sweet potatoes. By taking a few simple steps, you can keep your sweet potatoes looking fresh and appealing.
Submerge in Water
One of the easiest and most effective methods is to submerge the peeled or cut sweet potatoes in cold water. This creates a barrier that limits their exposure to oxygen, slowing down the enzymatic browning process. Add a squeeze of lemon juice or a splash of vinegar to the water for extra protection, as the acidity helps to inhibit the PPO enzyme.
The water should be cold, as cold temperatures further slow down the enzymatic reaction. Make sure the sweet potatoes are fully submerged and store them in the refrigerator until you’re ready to use them.
Acidic Solutions
As mentioned earlier, acidic environments can inhibit the activity of PPO. Therefore, soaking sweet potatoes in a solution of water and lemon juice, vinegar, or even ascorbic acid (vitamin C) can significantly reduce browning.
For example, a solution of one tablespoon of lemon juice per cup of water can be quite effective. Soak the peeled sweet potatoes in this solution for about 10-15 minutes before storing or cooking them.
Blanching
Blanching involves briefly immersing the sweet potatoes in boiling water, followed by an immediate plunge into ice water. This process deactivates the PPO enzyme, preventing browning.
To blanch sweet potatoes, bring a pot of water to a rolling boil. Add the peeled and cut sweet potatoes and let them cook for about 2-3 minutes. Then, immediately transfer them to a bowl of ice water to stop the cooking process. Drain the sweet potatoes and store them in an airtight container in the refrigerator or freezer.
Vacuum Sealing
Vacuum sealing is another effective way to prevent enzymatic browning. By removing all the air from the packaging, you eliminate the oxygen needed for the reaction to occur.
If you have a vacuum sealer, simply place the peeled and cut sweet potatoes in a vacuum-seal bag and seal it according to the manufacturer’s instructions. Vacuum-sealed sweet potatoes can be stored in the refrigerator for several days without significant browning.
Freezing
Freezing sweet potatoes is an excellent way to preserve them for longer periods and prevent browning. However, it’s best to blanch them first to deactivate the PPO enzyme.
After blanching and cooling the sweet potatoes, spread them out in a single layer on a baking sheet and freeze them until solid. Then, transfer them to freezer bags or containers. This prevents them from clumping together and makes it easier to use only the amount you need.
Applying Oil
Coating the sweet potato surface with a thin layer of oil can create a barrier that limits oxygen exposure. This method is particularly useful when you’re planning to roast the sweet potatoes soon after peeling. However, it is less effective for longer storage.
Factors Affecting the Speed of Discolouration
Several factors can influence how quickly sweet potatoes discolour after peeling. Understanding these factors can help you to better manage and prevent browning.
Variety of Sweet Potato
Different varieties of sweet potatoes may have varying levels of PPO and phenolic compounds, which can affect their browning rate. Some varieties may be more resistant to browning than others.
Storage Conditions
Improper storage conditions can accelerate the browning process. Sweet potatoes should be stored in a cool, dark, and well-ventilated place. Avoid storing them in the refrigerator, as this can cause them to develop a hard core and alter their flavour.
pH Level
The pH level of the sweet potato itself can influence the activity of PPO. Sweet potatoes with a higher pH (less acidic) may be more prone to browning.
Temperature
As mentioned earlier, higher temperatures generally speed up enzymatic reactions. Therefore, storing peeled sweet potatoes at room temperature will cause them to brown more quickly than storing them in the refrigerator.
Using Browned Sweet Potatoes
Even if your sweet potatoes have started to brown, they can still be used in many dishes. The browning is usually just on the surface and can be easily removed or disguised.
Roasting
If you’re planning to roast the sweet potatoes, the browning will likely disappear during the cooking process. The heat will caramelise the sugars and create a rich, flavourful crust.
Mashing
For mashed sweet potatoes, you can simply cut away the browned areas before cooking. Alternatively, you can cook the sweet potatoes with the browned areas intact and then mash them. The browning will be less noticeable once the sweet potatoes are mashed.
Soups and Stews
Browned sweet potatoes can also be used in soups and stews. The browning will not significantly affect the flavour or texture of the dish.
Sweet Potato Discolouration: A Summary
Sweet potatoes do discolour after peeling due to enzymatic browning, a natural reaction between the enzyme PPO, phenolic compounds, and oxygen. While this browning is generally harmless and doesn’t affect the safety of the sweet potato, it can be prevented or minimised by using techniques such as submerging in water, using acidic solutions, blanching, vacuum sealing, or freezing. By understanding the factors that contribute to browning and using these preventative measures, you can keep your sweet potatoes looking fresh and appealing for longer. Even if your sweet potatoes do brown, they can still be safely used in a variety of dishes. Always check for signs of spoilage, like a slimy texture or unpleasant odor, before using.
| Prevention Method | Description | Effectiveness |
|---|---|---|
| Submerge in Water | Peeled sweet potatoes are placed in cold water, often with lemon juice. | High |
| Acidic Solutions | Soaking sweet potatoes in solutions with lemon juice or vinegar. | High |
| Blanching | Briefly boiling then rapidly cooling the sweet potatoes. | Very High |
| Vacuum Sealing | Removing air from packaging to limit oxygen exposure. | Very High |
| Freezing | Freezing sweet potatoes after blanching. | Very High |
Why do sweet potatoes turn brown after peeling?
Sweet potatoes, like many fruits and vegetables, undergo enzymatic browning when peeled or cut. This browning is caused by an enzyme called polyphenol oxidase (PPO), which is naturally present in the sweet potato. When the sweet potato’s cells are damaged due to peeling or cutting, PPO is exposed to oxygen in the air, triggering a chemical reaction that produces melanins. These melanins are responsible for the brown discoloration you observe on the surface of the sweet potato.
The browning process is a natural defense mechanism for the plant. However, for culinary purposes, this discoloration is often undesirable, even though it doesn’t necessarily indicate spoilage. The rate of browning depends on several factors, including the variety of sweet potato, the temperature, and the amount of oxygen available. While the color change is visually unappealing, the sweet potato is usually still safe to eat and retains its nutritional value.
Does enzymatic browning affect the nutritional value of sweet potatoes?
Enzymatic browning primarily affects the appearance of sweet potatoes, altering their color from orange to shades of brown. While there may be a slight reduction in certain nutrients, the impact is minimal and generally insignificant from a nutritional standpoint. The major vitamins and minerals, like Vitamin A, Vitamin C, and potassium, remain largely unaffected by the browning process.
The sweet potato is still perfectly nutritious and safe to consume even if it has undergone some browning. The key nutrients and overall health benefits are preserved. You can still enjoy the sweet potato without concern, though you might prefer to minimize the discoloration for aesthetic reasons when preparing meals.
How quickly do sweet potatoes brown after peeling?
The rate at which sweet potatoes brown after peeling is influenced by several factors, including the surrounding temperature, humidity levels, and the specific variety of sweet potato. Generally, browning begins relatively quickly, often within minutes of exposure to air. The discoloration will become more pronounced over time.
The higher the temperature and humidity, the faster the browning process will occur. Certain varieties of sweet potatoes may also be more prone to browning than others. To slow down the enzymatic browning, you can take steps to limit the sweet potato’s exposure to air, such as storing it in water or applying an acid like lemon juice.
What are some ways to prevent sweet potatoes from browning after peeling?
Several methods can effectively prevent or slow down the enzymatic browning of sweet potatoes after peeling. Submerging the peeled sweet potatoes in cold water is a common and simple technique. The water acts as a barrier, limiting oxygen exposure to the cut surface and thus inhibiting the PPO enzyme’s activity.
Another effective method is to coat the peeled sweet potatoes with an acidic solution, such as lemon juice, vinegar, or citric acid. The acidity inhibits the PPO enzyme’s activity and delays the browning process. Blanching the sweet potatoes in boiling water for a brief period can also deactivate the enzyme. Storing peeled sweet potatoes in airtight containers in the refrigerator will also help slow down the browning process.
Can I still cook with sweet potatoes that have already turned brown?
Yes, absolutely! Sweet potatoes that have undergone enzymatic browning are perfectly safe to cook with. The browning is a purely cosmetic issue and does not indicate spoilage or the presence of harmful substances. The flavor and texture of the sweet potato will remain largely unaffected, although there might be a very slight change in taste that is usually unnoticeable.
When cooking sweet potatoes that have browned, you can simply proceed with your recipe as usual. The browning will likely disappear during the cooking process, especially if you are baking, roasting, or boiling the sweet potatoes. If you are concerned about the appearance, you can try to trim away the most discolored areas before cooking, but this is generally not necessary.
Does cooking the sweet potato before peeling prevent browning?
Yes, cooking sweet potatoes before peeling can significantly reduce or prevent enzymatic browning. The heat from cooking deactivates the PPO enzyme responsible for the browning process. This is because enzymes are proteins, and heat can denature them, rendering them inactive.
If you bake, steam, or boil the sweet potato whole, and then peel it after it has cooled slightly, you will find that it does not brown nearly as quickly as a raw, peeled sweet potato. The cooked sweet potato cells are no longer susceptible to the enzymatic reaction with oxygen. This method is particularly useful when preparing sweet potato puree or other dishes where the appearance of the sweet potato is important.
Is there a difference in browning between different varieties of sweet potatoes?
Yes, there can be differences in the degree and speed of browning among different varieties of sweet potatoes. This variation is due to differences in the concentration of PPO enzyme and other phenolic compounds present in each variety. Varieties with higher levels of PPO will generally brown more quickly and intensely than those with lower levels.
Factors such as growing conditions, maturity at harvest, and storage methods can also influence the enzyme levels and thus the browning potential. However, regardless of the variety, the general principles of preventing or slowing enzymatic browning, such as submerging in water or applying an acid, will still be effective. So, although some varieties might brown faster than others, similar preventative measures can be taken.