Photooxidation In Food

Photooxidation in foods is a type of oxidative reaction triggered by light exposure — particularly from ultraviolet (UV) and visible light. This process leads to the deterioration of food quality by breaking down fats, pigments, vitamins, and proteins. 

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🌿 How does photooxidation work?

1. Light absorption:

• When foods are exposed to light, photosensitizers (like chlorophyll, riboflavin, or myoglobin) absorb light energy (Min & Boff, 2002).
• These excited triplet molecules transfer energy to atmospheric triplet oxygen, producing singlet oxygen — a highly reactive form of oxygen.

2. Oxidation reactions:

• Singlet oxygen reacts with the double bonds of unsaturated fats in the food, starting a chain reaction of lipid oxidation.
• It can also break down pigments (like carotenoids), vitamins (like vitamin C and E), and even proteins, leading to off-flavours, discolouration, and nutrient loss.
• Singlet oxygen oxidation is 1450 times faster than oxidation by triplet oxygen (Min & Boff, 2002).

3. Radical formation:

• The process often creates free radicals (like peroxyl radicals) that continue damaging the food, accelerating spoilage.

Photooxidation Through Type 1 and Type II Reaction Pathways

Photooxidation happens through two main reaction pathways — Type I and Type II — each involving different mechanisms for generating reactive oxygen species (ROS). 

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🌿 Type I Photooxidation: The Radical Pathway

How it works:

1. Light absorption:
   A photosensitizer such as riboflavin, chlorophyll, or myoglobin in foods absorbs light energy and enters an excited and reactive state.

2. Electron transfer:
   The excited photosensitizer reacts directly with surrounding molecules like unsaturated fats or amino acids, transferring an electron or hydrogen atom.

3. Radical formation:
   This creates free radicals — reactive species like:

   • Superoxide anion (O₂⁻•)
   • Hydroxyl radical (•OH)
   • Lipid radicals (R•)

4. Propagation:
   These radicals attack lipids, proteins, and pigments — starting a chain reaction of oxidation, leading to the breakdown of food components.

End products:

• Lipid peroxides → break down into aldehydes and ketones → cause rancid, cardboard-like off-flavours.
• Protein radicals → produce sulphur compounds → lead to metallic or sulphurous odours.

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🌞 Type II Photooxidation: The Singlet Oxygen Pathway

How it works:

1. Light absorption:
   Again, a photosensitizer absorbs light and enters an excited state.

2. Energy transfer:
   Instead of directly reacting with food molecules, the excited photosensitizer transfers energy to molecular oxygen (O₂).

3. Singlet oxygen formation:
   This excites oxygen into a highly reactive form called singlet oxygen (¹O₂) — an energized form of oxygen that’s much more reactive than its ground state.

4. Oxidation:
   Singlet oxygen reacts with:

   • Unsaturated fatty acids → forming hydroperoxides that degrade into volatile compounds.
   • Pigments (like carotenoids) → causing colour fading.
   • Proteins (like methionine) → creating sulphur off-flavours.

End products:

• Lipid hydroperoxides → cause rancid odours and flavour deterioration.
• Pigment breakdown → leads to colour loss in foods like cheese or meat.

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🔥 Key differences:

Aspect	Type I Reaction	Type II Reaction
Key trigger	Electron or hydrogen transfer	Energy transfer to oxygen
Reactive species	Free radicals (O₂⁻•, •OH, R•)	Singlet oxygen (¹O₂)
Targets	Lipids, proteins, pigments	Unsaturated fats, pigments, proteins
Main products	Lipid peroxides, protein radicals	Lipid hydroperoxides, aldehydes
Flavour impact	Rancid, metallic, or sulfur notes	Cardboard, stale, or bitter tastes
Reaction speed	Slower but propagates in a chain reaction	Fast and direct oxidation
Common photosensitizers	Riboflavin, chlorophyll, myoglobin	Riboflavin, chlorophyll

 

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🍫 Which foods are most affected?

1. Dairy products (milk, cheese, butter):

   • Riboflavin (vitamin B2) absorbs light, triggering oxidation of milk fats.
   • Results: Rancid, cardboard-like flavours and yellowing of butter.

2. Meats (especially cured meats and fish):

   • Myoglobin absorbs light, oxidizing fats and proteins.
   • Results: Colour fading (from bright red to brown) and off-odours like a metallic or fishy smell.

3. Oils and fats (vegetable oils, salad dressings):

   • Unsaturated fats oxidize quickly in light.
   • Results: Rancidity and a bitter, stale taste.

4. Snacks and baked goods:

   • Fats and flavour compounds (like vanillin) break down under light.
   • Results: Stale aroma and loss of flavour intensity.

5. Beverages (juices, wines, and beers):

   • Pigments (like anthocyanins in wine) degrade, causing colour fading.
   • Vitamin C also oxidizes, reducing nutritional value.

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🧪 Effects of photooxidation on flavour:

• Rancid, cardboard notes (from oxidized fats)
• Metallic or fishy tastes (from degraded proteins or heme (haem) pigments)
• Bitter or stale off-flavours (from breakdown of natural flavour compounds like vanillin or carotenoids)

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🔒 How to prevent photooxidation:

1. Packaging:

   • Opaque or UV-blocking materials (like tinted glass, foil-lined bags, or dark plastic) help reduce light exposure.
   • Vacuum sealing limits oxygen, slowing the oxidative process.

2. Antioxidants:

   • Natural: Vitamin E (tocopherols), rosemary extract, and ascorbic acid scavenge free radicals. Many of these antioxidants also quench singlet oxygen (Lee & Min, 1992; Hirayama et al., 1994).
   • Synthetic: TBHQ, BHA, and BHT stabilize fats by stopping oxidation chains.

3. Storage conditions:

   • Keep foods in dark, cool environments — minimizing light and heat exposure reduces reaction rates.

Photooxidation Of Cheese

🌞 How does photooxidation happen in cheese?

Cheese contains several components that are vulnerable to light-induced oxidation, including:

1. Lipids (fats):

   • Cheese, especially high-fat types like cheddar or brie, contains unsaturated fatty acids.
   • When exposed to light, photosensitizers like riboflavin (vitamin B2) absorb light energy, creating singlet oxygen.
   • This singlet oxygen reacts with unsaturated fats, breaking them down into hydroperoxides — which then degrade into aldehydes and ketones — causing off-flavours.

2. Proteins (casein and whey):

   • Light can degrade amino acids like tryptophan and methionine.
   • This leads to the production of sulphur compounds — contributing to pungent, off-odours often described as “cabbage-like” or “burnt hair.”

3. Pigments and vitamins:

   • Beta-carotene (in cheeses made from cow’s milk) can also oxidize, causing a loss of the creamy yellow colour — turning it pale or grey.
   • Vitamin C and E levels can degrade, reducing their natural antioxidant protection.

4. In food systems such as cheese:

   • Riboflavin acts as a photosensitizer, triggering Type II reactions that oxidize lipids and cause off-flavours.
   • Fatty acids might also undergo Type I reactions when free radicals form — causing a ripple effect of flavour and colour degradation.

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🍽️ What are the flavour and aroma changes?

Light exposure creates a range of unpleasant flavours in cheese due to photooxidation:

• Rancid or cardboard-like notes — from oxidized fats.
• Metallic or fishy flavours — due to protein degradation.
• Sulphurous, cabbage-like odours — from breakdown of methionine and tryptophan.
• Bitter taste — caused by the formation of certain free fatty acids and protein breakdown products.

These off-flavours can develop within hours of light exposure — especially in transparent packaging.

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🏡 Which cheeses are most at risk?

1. High-fat cheeses:

   • Cheddar, gouda, and brie have a higher lipid content, making them prone to fat oxidation.

2. Soft and semi-soft cheeses:

   • Ricotta, cream cheese, and mozzarella have more water activity — allowing free radicals to move more easily and accelerate oxidation.

3. Surface-ripened cheeses:

   • Cheeses with rinds (like brie or camembert) may experience pigment degradation — fading their typical hue.

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🛡️ How to prevent photooxidation in cheese:

1. Packaging:

   • Opaque or UV-blocking packaging (such as foil wraps or dark plastic) protects against light penetration.
   • Vacuum-sealed bags reduce oxygen exposure, slowing oxidation.

2. Natural antioxidants:

   • Adding tocopherols (vitamin E), ascorbic acid (vitamin C), or rosemary extract can stabilize fats and slow oxidative reactions.
   • These are sometimes used in processed cheeses.

3. Storage:

   • Keeping cheese in a dark, cool place (like a fridge or cellar) reduces light and heat exposure, which otherwise speed up oxidation.

References

Hirayama, O., Nakamura, K., Hamada, S., & Kobayasi, Y. (1994). Singlet oxygen quenching ability of naturally occurring carotenoids. Lipids, 29, pp. 149-150.

Lee, J. Y., & Choe, E. O. (2003). Lipid oxidation and tocopherol contents change in full-fat soy flour during storage. Food Science and Biotechnology, 12(5), pp. 504-507.

Min, D.B., Boff, J.M. (2002). Lipid oxidation of edible oil. In: CC Akoh, DB Min, editors. Food lipids. New York : Marcel Dekker. p 335–63