1. General Overview
- Most fruits and vegetables contain organic acids in excess of the requirement of the TCA cycle and other metabolic pathways.
- The excess organic acids are stored in the vacuole.
- Some fruits may contain up to 5 g per 100 g fresh weight.
Examples of high organic acid fruits:
- Lemon
- Lime
- Passion fruit
- Black currant
2. Major Organic Acids in Plants
Dominant acids:
- Citric acid – Citrus fruits
- Malic acid – Banana
Other important acids:
- Tartaric acid – Grapes
- Oxalic acid – Spinach
- Isocitric acid – Blackberries
3. Importance of Organic Acids
- Play critical roles in plant metabolism.
- Contribute significantly to fruit taste.
- The sugar–acid balance determines desirable flavor quality.
4. Fruits & Vegetables Rich in Major Acids
Citric Acid-rich:
- Berries
- Citrus
- Guava
- Pear
- Pineapple
- Tomato
- Beetroot
- Leafy vegetables
- Legumes
- Potato
- Plum
Malic Acid-rich:
- Apple
- Banana
- Cherry
- Melon
- Onion
- Broccoli
- Carrot
- Celery
- Lettuce
5. Examples of Acids in Daily Life
- Citrus fruits – Ascorbic acid and citric acid
- Vinegar – Acetic acid
- Toilet cleaners – Hydrochloric acid
- Carbonated drinks – Carbonic and phosphoric acids
6. Folic Acid
- Involved in RNA synthesis.
- Deficiency causes anemia and fetal spina bifida.
- Food fortification is a major prevention strategy.
- Green vegetables are rich sources.
- Intensity of green color often correlates with folic acid content.
- Proper handling and storage are important to prevent loss.
7. Fatty Acids of Concern
Erucic Acid
- Found in rapeseed and mustards (Brassica species).
- Associated with cardiac injury in experimental animals.
- Chemical structure: 22:1 ω9 (cis-13-docosenoic acid).
- High content in mustard oil and certain rapeseed oils.
Oleic Acid
- Monounsaturated omega-9 fatty acid.
- Common in olive oil and many plant oils.
- Chemical notation: 18:1 cis-9.
8. Industrial Production of Organic Acids
Acetic Acid Fermentation
- Produced by oxidation of ethanol by Acetobacter species.
- Vinegar contains 4–8% acetic acid.
Lactic Acid Fermentation
- Produced from carbohydrates by Lactobacilli.
- Substrates: corn starch, molasses, whey.
Gluconic Acid Fermentation
- Produced from glucose by Aspergillus niger or Penicillium species.
- Used in pharmaceuticals.
Citric Acid Fermentation
- Produced by Aspergillus niger.
- Derived from molasses, sucrose, starch hydrolysates.
- Widely used in food and pharmaceutical industries.
Itaconic Acid Fermentation
- Produced by Aspergillus terreus.
- Used in detergent resins.
Gibberellic Acid Fermentation
- Produced by Gibberella fujikuroi.
- Important plant growth regulator.
9. Kavalactones
- Produced in roots of kava (Piper methysticum).
- Allelopathic effects on lettuce, radish, barnyard grass.
- Act as pre-emergent herbicides.
- Used traditionally as calming beverage.
10. Other Important Compounds
- β-carotene – Orange pigment in carrots and many plants.