Raspberry Ketone: What Is It?

Raspberry ketone [1-2] is the primary aromatic component of raspberry fruit, characterized by a distinctive sweet fruity aroma and flavor. Raspberry ketone is an internationally recognized safe synthetic fragrance, featuring an elegant fruity aroma with excellent fragrance and taste. In China, GB 2760-1996 specifies raspberry ketone as an approved food flavor (for baked goods, candies, beverages, etc., with a usage range of 40–320 mg/kg), serving to enhance aroma and sweetness.

Raspberry ketone can be used to formulate fragrances for raspberry, grape, pineapple, peach, plum, strawberry, red currant, jasmine, gardenia, and tuberose, among others. It can also be used as a modifier or fixative in large quantities in cosmetic fragrances, food fragrances, personal care fragrances, and tobacco fragrances. Raspberry ketone has certain whitening and anti-inflammatory effects and is widely used in cosmetics formulation. As a fine chemical intermediate, raspberry ketone can be used in the synthesis of drugs, dyes, and pesticides. In agriculture, raspberry ketone is also an insect attractant. Raspberry ketone is widely used both domestically and internationally, has a high price, and possesses significant economic value.

1 Physical and chemical properties

Raspberry ketone, also known as Rubus ketone, has the chemical names: p-hydroxybenzyl ketone, 4-(4-hydroxyphenyl)-2-butanone, 4-(p-hydroxyphenyl)-2-butanone, 4-hydroxybenzyl propanone, etc. Its trade names include Frambinone, Oxyphenylone, and Oxanone. CAS number: 5471-51-2, molecular formula: C10H12O2, molecular weight: 164.22. It appears as a lustrous, colorless or white crystalline or granular solid, with a melting point of 82–83 °C (varying in different literature) and a boiling point of 161 °C (0.67 kPa). It is insoluble in water and petroleum ether but soluble in ethanol, ether, and volatile oils. It exhibits the chemical properties of ketones and phenolic compounds.

Raspberry ketone forms stable complexes with Ni(NO₃)₂ at pH 6.260 to 6.865, with a coordination ratio of 1:1 and a coordination constant of 8.520. Spectrophotometric analysis is conducted at λ_(max) = 228 nm [3].

Its standard curve follows the Lambert-Beer law in the range of (8.152 × 10⁻⁶) to (7.337 × 10⁻⁵) mol/L, with a correlation coefficient of r = 0.9666, recovery rates of 96.40% to 102.80%, with an RSD of 0.190%.

Raspberry ketone infrared spectrum (ν/cm⁻¹): 3300 (benzene ring -OH), 3030, 2940 (benzene ring C-H), 1700 (C=O), 1610, 1590, 1520, 1450 ( benzene ring C=C), 850, 800 (benzene ring para substitution); Nuclear Magnetic Resonance Spectroscopy (CDCl₃), δ: 2.0 (s, -CH₃), 2.6 (t, -CH₂), 2.8 (t, -CH₂), 6.9 (s, ph-OH), 7.2, 7.7 (pH-H).

2 Current Status

Natural raspberry ketone is found in the juice of raspberries (Rubus frutescens) and blackberries, with a mass ratio of approximately (0.1 × 10^(−6)) to (0.2 × 10^(−6)) in raspberry juice. It was first discovered in 1918 and not confirmed as the primary aromatic compound in raspberries until 1957. Due to its extremely low content and difficulty in isolation, it remains impossible to produce “natural” raspberry ketone in large quantities from natural sources for commercial purposes.

Raspberry ketone was first synthesized by Firmenich Company and added to raspberry flavorings, enabling the company's raspberry flavorings to dominate the global market. The late 1960s marked the most active period for research into raspberry ketone synthesis methods. The acetate ester of raspberry ketone, 4-p-acetoxyphenyl-2-butanone, is an insect attractant also known as fly pheromone, possessing a melon-like fragrance. It serves as a pheromone for Asian melon flies and exhibits attractant properties toward them. Fly attractant hydrolyzes to produce raspberry ketone. At the time, mature pineapples in Hawaii and Guam were threatened by melon flies, prompting the U.S. Department of Agriculture to show great interest in fly attractant, which was produced using raspberry ketone as a raw material.

Some fine chemical companies have dedicated themselves to the research and development of melon ketone, refining it for use as a fragrance. Initially, melon ketone production was concentrated in Europe, with companies such as Firmenich, Birmingham Chemicals, PFW, Givaudan, and Original De Long being the primary suppliers of raspberry ketone. Subsequently, due to the infestation of melon flies on melons, pineapples, and other fruit crops in Asia, countries such as Japan and China invested significant resources into researching the production of raspberry ketone and fly attractant. Currently, Japan's Takasago International Corporation and some domestic fragrance factories remain the primary producers.

Countries worldwide place great emphasis on the synthetic research of raspberry ketone and its analogues. China's research and development efforts have primarily focused on the synthesis of raspberry ketone, with significant gaps remaining in the synthesis and application of its analogues.

3 Biochemical Synthesis Research

Bioconversion methods [4-6] for synthesizing raspberry ketone offer advantages such as broad raw material sources, high reaction specificity, and mild reaction conditions, making them an important research and development area in green biochemical synthesis. Industrial application research on the biosynthetic synthesis of raspberry ketone began in the late 20th century or early 21st century, primarily focusing on three aspects: genetic engineering for the selection and breeding of superior strains; optimization of culture medium composition and regulation of fermentation process conditions to enhance production efficiency; and selection and optimization of post-production purification processes. Combining the advantages of these methods to establish a process route that is easy to operate, requires minimal investment, is environmentally friendly, and produces high-purity products is a topic that requires further research.

The biological synthesis methods for raspberry ketone include oxidation dehydrogenation, hydrogenation reduction, and precursor synthesis, among others.

3.1 Oxidation Dehydrogenation Method

DUMONT Benoit et al. [7] disclosed a process for preparing raspberry ketone by oxidizing and dehydrogenating rhodiol using alcohol-dehydrogenating microorganisms. Specifically, rhodiol was produced using β-glucosidase under the action of xylose, and then converted into raspberry ketone.

FALCONNIER Brigitte [8] invented a biological conversion method for producing raspberry ketone. Yeast strains with α, β-glucosidase activity and secondary alcohol dehydrogenase activity were used to convert alcohol into ketone. After conversion, raspberry ketone was separated.

KOSJEK Birgit et al. [9] developed a “green” oxidation method using the conversion of 4-(p-hydroxyphenyl)-2-butanol (duodenol) to 4-(p-hydroxyphenyl)-2-butanone (raspberry ketone) as a model reaction. Different freeze-dried cells of the Rhodococcus genus were screened for oxidative reactions. Rhodococcus IFO3730 and R. Ruber DSM 44541 were able to use acetone as a hydrogen acceptor during hydrogen transfer. This oxidative reaction could be carried out at substrate concentrations as high as 500 g/L.

3.2 Hydrogenation-reduction method

FUGANTI Claudio et al. [10] studied the use of different microorganisms to produce raspberry ketone from 4-hydroxybenzyl acetone.

BEEKWILDER Martinus Julius et al. [11-12] based their invention on the remarkable discovery that chalcone synthase possesses benzyl acetone synthase (BAS) activity. Host cells produce chalcone synthase (CHS) and 4-coumaric acid coenzyme A ligase (4CL), with one or both being heterologous cells. Methods for conferring benzene-2-carboxaldehyde synthesis activity to CHS proteins include exposing CHS microbial cells to an environment, preferably Escherichia coli.

Benzocyclopropane is reduced to raspberry ketone by benzocyclopropane reductase (BAR) in bacterial cells. Host cells provide raspberry ketone precursors, primarily benzocyclopropane or coumaric acid. The raspberry CHS, tobacco 4CL, and raspberry BAR gene sequences and peptides have been described as vectors containing CHS and 4CL sequences, primarily combined with the phenylalanine ammonia-lyase (PAL) gene or the cinnamic acid-4-hydroxylase (C4H) gene. Transgenic Escherichia coli demonstrated that raspberry CHS cDNA and tobacco 4CL cDNA can produce raspberry ketone from coumaric acid. Other CHS cDNA, such as those from grape, Arabidopsis, snapdragon, alfalfa, corn, and coriander, can replace raspberry CHS cDNA.

BEEKWILDER Jules et al. [13] focused on the identification, application, and relevance of genes to raspberry ketone synthesis. Candidate genes were isolated from raspberries and other plants, introduced into bacterial and yeast expression systems, and the expression conditions were characterized. Raspberry ketone yields reached up to 5 mg/L. The results lay a solid foundation for the production of a potential renewable natural fragrance compound.

A solid foundation for the production of a potential renewable natural fragrance compound.

3.3 Synthesis of precursors

ZORN H et al. [14] produced 4-(4-hydroxyphenyl)-2-butanone from isolated cells of the basidiomycete Nidula niveo-tomentosa, supplemented with 13C-labeled phenylalanine and 13C-labeled glucose. A new method for stable isotope analysis of metabolic products was employed, coupled with gas chromatography-emission detection and gas chromatography-mass spectrometry for detecting labeled transformation products. Partial extension of the benzoate side chains followed a poly-β-ketone scheme. Acetyl coenzyme A carboxylase inhibitors alter the spectra of benzyl compounds.

FISCHER-ZORN Manuela et al. [15] reviewed the biosynthesis of raspberry ketone and related compounds via Nidula niveo-tomentosa. The D and ¹³C isotope labeling of precursors and metabolites was identified using GC-MS and radiometric detection. Using phenylacetic acid as a precursor, Nidula niveo-tomentosa demonstrated the ability to extend side chains, leading to the conclusion that butyl side chains are formed through reactions with propionyl-CoA. The biosynthetic pathways of phenylmalonic acid derivatives in Nidula niveo-tomentosa and Rubus plants were compared.

FERON G et al. [16] used acetone as the donor and p-hydroxybenzaldehyde as the acceptor to study the enzymatic catalysis of the hydroxyaldehyde condensation reaction for the preparation of p-hydroxybenzyl acetone (p-hydroxybenzyl acetone). Bacterial functional test results showed that the bioconversion range was 15–160 mg/L after 21 h. 2-Deoxyribose-5-phosphate aldolase (DERA) has demonstrated the ability to produce p-hydroxybenzyl acetone.

4 Conclusion

In today's trend toward simplicity and naturalness, people are increasingly seeking green products. Although the current production of furfural via biological conversion is limited, the demand for natural furfural is growing rapidly. For the fragrance and food industries, utilizing biotransformation to produce precursors through simple processes and synthesize important compounds holds significant importance. A thorough understanding of the synthesis of raspberry ketone in vivo, involving genes and enzymes, is essential. Obtaining this information will enable the design of more effective microbial fermentation processes for raspberry ketone production. The development and production of natural raspberry ketone hold immense potential for future growth.

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