Coloring and sweetening techniques are widely applied in modern agricultural production, aiming to regulate plant metabolic processes to enhance fruit coloration and sweetness rapidly. This improves fruit quality and commercial value. Below is an analysis of the mechanisms of coloring and sweetening, as well as the pros and cons of commonly used coloring products.
The Mechanism of Fruit Coloring and Sweetening
The enhancement of fruit coloration and sweetness primarily depends on the following physiological and biochemical processes:
- Pigment Synthesis
- Anthocyanin Accumulation: Anthocyanins, which are phenolic compounds, are the primary pigments responsible for fruit coloration. Their synthesis is closely associated with light, temperature, and plant hormones (e.g., ethylene).
- Chlorophyll Degradation: During fruit ripening, chlorophyll breaks down into colorless substances, creating space for the display of anthocyanins and carotenoids.
- Carotenoid Synthesis: Carotenoids play a crucial role in the coloration of certain fruits, such as citrus and tomatoes.
- Sugar Accumulation
- Enhanced Photosynthesis: Promotes photosynthesis to increase the content of soluble sugars in fruits.
- Sugar Transport: During fruit ripening, sucrose is transported to the fruit through the phloem and subsequently converted into glucose and fructose, enhancing sweetness.
- Ethylene Regulation
- Ethylene is a key hormone in promoting fruit ripening. It regulates metabolic enzymes, accelerating cell wall degradation, starch conversion, and pigment synthesis, which facilitates fruit coloring and sweetness enhancement.
Challenges in Simultaneously Achieving Coloring, Sweetening, and Weight Increase
- Coloring:
Fruit coloration involves three main steps:- Chlorophyll Degradation: Chlorophyll is gradually broken down under the action of hydrolases.
- Carotenoid Synthesis: Carotenoids are formed and increase in content.
- Anthocyanin Synthesis: Anthocyanins are produced in large quantities and continuously accumulate.
The key to fruit coloration lies in enhancing the accumulation of carotenoids and anthocyanins.
- Acid Reduction and Sweetening:
Fruit sweetness begins to accumulate during fruit formation through the photosynthetic activity of source leaves. During the ripening stage, the acidity in the fruit is converted into sugars. Tartaric acid, citric acid, and malic acid in the fruit are transformed into sugars, salts, and water, thereby increasing sweetness. - Weight Increase:
Increasing fruit weight is more challenging and requires strengthening carbon accumulation. This is mainly achieved by enhancing chlorophyll synthesis, boosting photosynthesis, and reducing dark respiration.
Achieving simultaneous effects of coloring, sweetening, and weight increase requires not only improving photosynthetic efficiency and reducing carbon consumption but also promoting the transformation of chlorophyll, tartaric acid, citric acid, and malic acid.
Types of Compounds for Coloring and Sweetening
Currently, there are three main categories of compounds used for coloring and sweetening:
1. Hormone-Based Compounds
- Abscisic Acid (ABA):
ABA halts plant growth and promotes the accumulation of assimilates in sink tissues, facilitating fruit coloration. - Ethephon:
Ethephon accelerates fruit ripening but can lead to premature aging, fruit cracking, and post-harvest decay. Negative effects on plants include dwarfing, wider and darker green leaves, increased secondary roots, and premature leaf aging. - Uniconazole:
Uniconazole strongly inhibits plant vegetative growth by blocking gibberellin synthesis, preventing excessive elongation and promoting nutrient redirection to fruits for coloration. However, it does not enhance photosynthesis, making it unsuitable for increasing yield or weight.
Key Characteristics:
Hormone-based compounds typically promote rapid ripening in a short period. However, due to insufficient carbon accumulation, fruits tend to be smaller, softer, less firm, and have significantly reduced storage life.
2. Non-Hormonal Compounds
- Diethyl Aminoethyl Hexanoate (DA-6):
DA-6 is a derivative of jasmonic acid that shortens the fruit ripening period and enhances stress response. It promotes the synthesis of ethylene and elicitors, facilitating fruit coloration and earlier maturation. - Astaxanthin:
Astaxanthin is a highly effective coloring agent. However, its poor water solubility and instability limit its performance. Moreover, it only affects pigment changes on the fruit surface and has no impact on photosynthesis or carbon accumulation, leading to coloring without sweetening.
3. Amino Acid-Based Compounds
- Phenylalanine:
Phenylalanine, an amino acid and a precursor of sweeteners, is a key raw material for the biosynthesis of anthocyanins and other flavonoids. Anthocyanins aid in fruit coloration and enhance sugar synthesis during the coloring stage, resulting in higher sugar content and lower acidity. It also stimulates proline synthesis and enzyme activity, reducing transpiration intensity and increasing respiration, which consumes plant carbon reserves for coloring and sweetening. - 5-Aminolevulinic Acid (5-ALA):
5-ALA is a naturally occurring, metabolically active substance essential for plant life. It is non-toxic, easily degradable, and leaves no residue.
5-Aminolevulinic Acid regulates various plant growth processes, such as:- Promoting chlorophyll synthesis and stabilizing photosystem II.
- Enhancing nitrogen metabolism.
- Expanding stomata, thereby improving photosynthetic efficiency, boosting carbon accumulation, and increasing soluble sugar and vitamin C content.
Additionally, 5-Aminolevulinic Acid (5-ALA) inhibits dark respiration in plants, increasing sugar accumulation, promoting fruit skin coloration, and increasing fruit weight. It significantly enhances fruit quality from both physiological and biochemical perspectives.
Although hormone products have a quick effect on color change, they often sacrifice fruit quality and are suitable for short-term needs or specific scenarios.
Non-hormone products have outstanding performance in color change, but have limited effects on sweetness and weight gain.
Amino acids (especially 5-Aminolevulinic Acid 5-ALA) can achieve a balance between color change, sweetening and weight gain by comprehensively improving the photosynthesis and carbon accumulation capacity of plants, and are the preferred direction for future applications.
When choosing color change and sweetening products, comprehensive considerations should be made based on fruit types, production goals and market demand to avoid the single pursuit of rapid maturity while ignoring the long-term value of quality improvement.
