Microwave assisted extraction of food colorants: principle, mechanism, extraction technique and applications

Introduction

Importance of Natural Food Colorants Food color is a key factor influencing consumer choices and preferences. Colorants, which can be dyes, pigments, or chemicals, are used to impart color to food, medications, cosmetics, or the body, either alone or in combination. They also enhance food safety, quality, and sensory attributes. There are two main types: synthetic colorants (e.g., tartrazine, sunset yellow) and natural colorants from plants, such as chlorophylls, carotenoids, betalains, and flavonoids. Natural pigments can be either lipidsoluble (e.g., carotenoids) or water-soluble (e.g., anthocyanins, betalains). Plant-derived pigments are increasingly favored over synthetic ones due to their therapeutic properties and lower risk of toxicity. Unlike synthetic colors, which can cause health issues like asthma, eczema, headaches, and hyperactivity, natural pigments offer a safer alternative with fewer allergens and enhanced nutritional benefits. Synthetic pigments, some of which are deemed harmful by authorities like the European Food Safety Authority, carry risks of adverse effects and require warning labels. The growing consumer preference for healthy eating makes research into natural food colorants essential, despite challenges such as instability and poor absorption associated with natural pigments. Extraction Methods for Natural pigments Microwave-assisted extraction (MAE) is a popular non-conventional method for solid-liquid extraction due to its ease, controlled heating, and rapid extraction times (15-20 minutes). It requires less solvent and offers higher yields, precision, and accuracy, especially for trace compounds like pesticide residues and heavy metals. MAE combined with enzyme-assisted aqueous two-phase extraction (MEAATPE) has shown a 71.14% higher extraction yield compared to other methods like Soxhlet extraction. For extracting lutein from marigold flowers, optimal conditions include 28% ethanol, 20% ammonium sulfate, 0.45 U/g enzyme concentration, 45 °C, 150 minutes anthocyanidins include cyanidin, malvidin, and delphinidin. Their conjugated bonds are essential for the development of red, blue, and purple colors in plants.

Heterocyclic derivatives of nitrogen

Betalains are nitrogen-based plant pigments derived from tyrosine, comprising two main types: yellow-orange betaxanthins and reddish-violet betacyanins. Betanin, a common betacyanin, is found in beets, while violasexanthin and indicaxanthin are present in cactus pears and yellow roots. Betaxanthins and betacyanins produce colors ranging from orange to red, with absorption maxima at 480 nm and 536 nm, respectively. Betacyanins can undergo acylglycosylation and hydroxyl glycosylation, whereas betaxanthins do not show glycosylation.

Major Food Pigments in flower Plant waste/Parts

Chrysanthemum morifolium Ramat flowers contain bioactive compounds like flavonoids, carotenoids, and caffeoylquinic acids, which exhibit antioxidant, antiinflammatory, antibacterial, and antifungal properties. The plant, native to China, features a variety of pigments across its colored flowers, including flavones, carotenoids, and anthocyanins. Clitoria ternatea, known as the blue pea, has high levels of anthocyanins and is used for its vibrant blue color in food and packaging. Marigold petals are rich in lutein and polyphenols, with lutein esters making up 70-79% of the total. The cockspur coral flower contains anthocyanins and various phenolic compounds with potential health benefits. Roselle (Hibiscus sabdariffa L.) is a tropical plant known for its carotenoids and anthocyanins, used extensively in food products for its color and antioxidants. In MAE, the solvent transfers heat to the solid matrix, leading to the evaporation of moisture, elevated vapor pressure, and ruptured cell walls that release pigments. This method offers a significant boost in extraction rates and efficiency compared to traditional techniques. Additionally, MAE can be adapted to solventfree systems like in-situ microwave-generated hydro distillation, although using water may introduce challenges like microbial growth and hydrolysis of plant metabolites.

Effect of Solid to Liquid Ratio

Natural deep eutectic solvents (NADESs) were tested for extracting curcuminoids, with a 1:1 citric acid to glucose ratio and 15% water at 50 °C, 0.1/10 g/mL solid/liquid ratio, and 30 minutes yielding the highest results compared to conventional solvents. For annatto seed pigments, microwave-assisted extraction was compared to hot aqueous extraction. Optimal conditions for bixin and norbixin extraction were found to be a 1:1 seed-to-water ratio, 30 minutes at 60 °C with microwave assistance, producing 0.625% bixin and 3.008% norbixin. Increasing solvent-tomaterial ratios beyond optimal levels can lead to uneven distribution and reduced efficiency.

Microwave Power& Temperature

Microwave power and temperature in microwave-assisted extraction (MAE) are closely linked, as the extraction matrix heats up when absorbing microwave energy. This increases the solvent's ability to solubilize solute compounds, enhancing solvent penetration and matrix wetting, which leads to higher pigment yields. However, while increasing microwave power generally boosts extraction yield, it can reach a point where further increases yield diminishing returns or even reduced efficiency due to thermal degradation of compounds. For instance, in extracting anthocyanins from Cockspur Coral flowers, the optimal conditions were a 1:15 feedstock-to-solvent ratio, 600 W power, and 3 minutes of extraction, yielding the highest anthocyanin content. Meanwhile, the highest total phenolic content (132.73 g/L) was achieved with 12 minutes of extraction at the same power level.

Characteristics of the Plant Matrix

The type of plant material used significantly affects extraction efficiency for compounds like nicotine, phenols, and volatile oils. Factors such as plant species, age, and storage conditions influence pigment concentration and accessibility. Understanding plant cellular structure is crucial for effective extraction, as secondary metabolites accumulate in specific tissues or organelles. For instance, traditional phenolic extraction from onions is less effective compared to solvent-free microwave hydro diffusion and gravity (MHG) techniques, which enhance extraction by disrupting cell walls and vacuoles. Key plant characteristics influencing extraction include: Types of plant cell Plant cells, tissue, and areas of the plant matrix containing bioactive components such as oil glands, bark, etc. are commonly found to have thick walls. Whether a plant is secretory or not, microwave technique can easily destroy it because it releases the bioactive contents of the cell when it is exposed to microwave flux [80] .

Plant cell components

Plant leaves and shoots are where plant components like phenols are stored. In order to extract phenol from the plant matrix, the cellular components such as guard cells, epidermal cells, and central vacuoles are separated [81] . Covalent attachment of bioactive components Complex attachment of polyphenol-like bioactive components to plant cell walls is observed through glycosidic linkage [82] . Amount of water The amount of water or moisture retained inside the matrix can significantly improve microwave absorption and promote heating by changing the polarity of the solvent. The physical structure of the matrix may alter as a result of moisture.

Particle Size and Shape

The plant material's particle size should be minuscule, between 100 μm and 2 mm, in order to maximize the sample's contact surface with the solvent [83] . The plant material will then be more surface area for the solvent to penetrate, break the cell, and enhance mass transfer if it is smaller in size [84] . This will result in an extraction that is more effective [85] . assert that the plant material's form is important. After suspended in the solvent, its irregular shape becomes round and smooth. Because the secondary metabolites must diffuse through the stagnant layer the used solvent creates around the plant material, transport may be slowed down.

Effect of Stirring

Agitation is a positive addition to the MAE, improving it even further. This is because there is a direct impact on mass transfer. The extraction efficiency can be increased by continuously agitating the concentrated active compounds, which lowers the mass transfer barrier. By encouraging the release and dissolution of the target compounds, stirring aids in accelerating the extraction rate [86] . Because it involves continuously agitating or stirring the solvent-sample mixture while using a microwave, this step in the extraction process is very important. Agitation can be achieved in several ways, such as using an ultrasonic bath, a mechanical stirrer, or a magnetic stirrer, depending on the size of the extraction.The used solvent then forms a stagnant layer around the plant material, which could slow down transport because the secondary metabolites need to diffuse through it.

Limitations of Micro-wave

Fruits

The ideal parameters for watermelon were 500 W, 0.5 mm, and 120 s, and 400 W, 0.5 mm, and 120 s for muskmelon shells.

Yield of carotenoids was found to be 590. 85 2022) [57] Chlorella biomass Microalgae hexane: acetone (1:1), & hexane: ethanol (7:3). For acetone optimum microwave power and time, 130 W and 1.69 min, for hexane extraction time was 1 min and microwave power of 100 W.

Acetone yielded 0.061 ± 0.003 mg/g of total carotenoids, hexane yielded 0.00173 ± 0.005 mg/g, and methanol yielded 0.0371 ± 0.004 mg/g. Sarma, M.K., Saha, K., Choudhury, R., Pabbi, S., Madamwar, D. and Subudhi, S. (2022) [22] (2017) [31] Amaranthus tricolour leaves

Leaf

For betacyanin conditions were 450W microwave power, 90 °C temperature, and 15 minutes & for betaxanthin: microwave power of 200 W, a temperature of 60 °C, and an extraction duration of 15 minutes.

The yield obtained for betacyanin was (71.95 mg/g of dw) and for betaxanthin was (42.30 mg/g dw).

Sharma, A., Mazumdar, B., and Keshav, A. (2021) [33] Red dragon fruit Fruit

Optimal conditions had solvent to material ratio of 1:30 (w/v), a temperature range of 60°C, a pH value of 2.25.

total yield of betacyanin was 0.4120 mg/100 g.

Saman, W., Mulyadi, A., and Wijana, S. (2023) [37] Recent [17] . There is a growing trend of anthocyanin-based colorants in some mixed fruit juices and yoghurt drinks. A few food manufacturers did use artificial coloring in their goods. Excessive consumption of these artificial colors may have negative effects.

Carotenoids

Carotenoids, known for their health benefits and antioxidant properties, are commonly used in food for coloring and nutritional enhancement. They contribute vibrant colors to fruits and vegetables-like β-carotene in yellow foods, lycopene in red fruits, and zeaxanthin in orange fruits. Zeaxanthin, in particular, is valued for its potent pigmentation abilities, often added to foods to achieve true color. Carotenoids are fat-soluble pigments found in fish, algae, fruits, and vegetables, and are also utilized in various food products as colorants and active ingredients.

Chlorophyll

Chlorophyll, a green pigment found in plants like nettles and peppermint, varies from dark to olive green. It is highly sensitive to pH, oxygen, temperature, and light, often turning brown in acidic conditions. Chlorophyll is soluble in oil and ethanol but not in water. There are two main types: chlorophyll a, which is green-blue, and chlorophyll b, which is yellow-green. It is used as a natural colorant in various food products such as beverages, jams, candies, and pickles.

Betalains

Betalains are being explored for their use as natural colorants, antioxidants, and antimicrobials. Studies have shown that red beet extract can be used in jelly and ice sherbets, providing a synthetic-like red color and enhancing product acceptability. Betalains also improve color stability in ice cream for up to 180 days at 20 °C. However, betalains from Rivina humilis used in banana juice and fruit spreads showed only 40% stability after six months at 5 °C. Additionally, betaxanthins from yellow pitaya have been used to color drinks and jelly candies.

Research gaps and scope in future

As consumers become more health-conscious, there's growing demand for natural food colorants due to their health benefits compared to synthetic alternatives.

Microwave-assisted extraction (MAE) is a promising, ecofriendly method for extracting natural pigments, offering higher efficiency, lower solvent use, and shorter extraction times than traditional methods. While MAE is widely used in food, pharmaceutical, and cosmetic industries, challenges remain, including pigment stability, industrial scalability, and the need for improved purification methods. Future research should focus on addressing these issues to enhance the quality and applicability of MAE-extracted pigments.

Conclusion

Extraction using microwave has been proven as a sustainable and non-conventional extraction method for extraction of natural pigments from various sources such as plants, microalgae sources, animals and many other sources. Due to the minimal solvent consumption, quick extraction times, and high extraction efficiency, this method is ideal for environmental detection, food inspection, and agricultural sample analysis. The mechanism or principle of MAE method has been designed in such a way that it is able to penetrate deep into tissues along with selective and heating which can be controlled. Despite of its numerous benefits and applications provided the various variables such as extraction time, solvent ratio, extraction time, microwave power need to be further optimised and investigated further to improve extraction efficiency at a large scale rather than only limiting to laboratory research and extraction analysis.

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