Cooling spice is a kind of spice with cooling effect, including natural cooling spice and synthetic cooling spice two categories. Natural cooling spices are taken from nature and maintain the original plant aroma characteristics of the spice, usually crushed, distilled, extracted and other physical or chemical methods of processing and production; synthetic cooling spices is the use of chemical raw materials, prepared by chemical synthesis method of the monomer spices.

Menthol

Menthol is the most common natural cooling spice in nature, with cool sense of outbreak of fast, fresh aroma and other characteristics, coupled with green and natural products in recent years is more and more popular, so the demand for menthol in the end product has a gradual upward trend. However, menthol also has its own application defects, such as a bitter taste when the dosage is large, strong volatility, and a short duration of coolness.

In response to these shortcomings, many application experts have made certain improvements to menthol. For example, the German company Simules invented the “spice preparations containing menthol” in the use of sugar alcohol raw materials and sweeteners to mask the bitterness of menthol, including erythritol, xylitol, stevia glycosides and aspartame, etc.. In addition, they also added essential oils in the product, spice ingredients to reduce the burning sensation produced by menthol in the mouth.

Liquid flavoring agents are also used in the “cooling compositions” invented by the Swiss company Chihuahuaten to improve the bitterness of menthol. The China Tobacco Corporation has also added sweeteners to menthol cigarettes to mask the bitterness of menthol. Similar bitter masking methods are also widely used in refreshing beverage products. Currently, the commonly used bitter masking agents for menthol are sucralose, neotame and aspartame.

In addition, Anhui Aidi Spice Co., Ltd. has documented the preparation process of a low-bitterness menthol product in its patent “A low-bitterness levo-menthol, preparation method and application”. The product has the needle-like crystal shape of menthol, and compared with ordinary menthol, the bitter taste in the mouth is reduced by 50%~70%.

Because menthol can produce a good cooling effect on the skin, it is also widely used in products in the fields of medicine, daily chemicals and so on. For example, the “Aloe Vera Cooling Cream Gel” produced by Suzhou Shengbai Health Co., Ltd. contains menthol, which can provide cooling and pain relief; the eye drops also contain a small amount of menthol, which can provide antiseptic, anti-inflammatory and cooling effects [2]. Currently, the results of the coolness test show that the use of menthol in topical creams and ointments can obtain a faster coolness outbreak effect, and the coolness effect of menthol on the skin is significantly longer than that in the mouth.

The WS range

Since 1970, Wilkinson Sword has conducted extensive research into synthetic coolants, during which time they have developed some 1,200 cool-sensing compounds, known as the WS range of products. These include the later commercially successful WS-23, WS-3 and WS-5 products.

I. WS-23 Products

WS-23 was developed and synthesized by Wilkinson Sword in the early 1980s. The company used diisopropylpropionitrile (DIPPN) as raw material, first hydrolyzed to generate diisopropylpropionic acid, then added sulfoxide chloride to generate acyl chloride, and finally reacted with methylamine to produce the target product [3].

In 2002, International Flavors and Fragrances (IFF) introduced a new process for the preparation of WS-23 in its patent, which also used diisopropylpropionitrile as raw material, added excess glycerol triacetate, and finally added dimethyl sulfate, and distilled to produce the target product, with a yield of up to 70%. In addition, Millennium has also proposed a new synthesis method, which utilizes diisopropylpropionitrile with methanol, dimethyl carbonate and other compounds, and reacts under an acidic catalyst to produce the product, with yields as high as 90%.

The most significant advantages of WS-23 over menthol are the long duration of the cooling sensation and the absence of bitterness in the mouth. However, WS-23 lacks the explosive coolness of menthol and its special minty flavor, but even so, WS-23 is still widely used in confectionery, beverage and other products. In some refreshing candy products that require a high degree of coolness, the amount of WS-23 added can be as high as 0.3% to 0.5%, while menthol will produce bitterness after the amount of menthol added is greater than 0.08%. In addition, WS-23 can destroy the crystalline crystal form after crushing, forming finer particles similar to powder, making it easier to add them to the pressed candy products, and most of the products such as pressed candy and ring candy currently sold in the market use WS-23 as a cooling agent.

In addition, WS-23 has good solubility in hot water, whereas most other commercially available WS series products are insoluble in water. WS-23 can be directly dissolved in water at room temperature after dissolving with propylene glycol, so WS-23 is also one of the commonly used cooling agents for mouthwash and beverages.

Second, WS-3 products

The traditional synthesis process of coolant WS-3 is to use p-fumaric acid chloride and ethylamine to make it by direct reaction, while Millennium has proposed a new synthesis route, which is made by a one-step reaction between p-fumaric acid chloride and triethyl phosphate catalyzed by polyphosphoric acid. Among them, the lower the production cost of p-fuckyl-3-carbonitrile, the more advantageous the product price is [4].

WS-3 is a long-lasting coolant, and the coolness intensity of WS-3 tested by Millennium Company is 1.5 times of that of menthol, and the duration of coolness is 20~30 min, and the coolness effect site is mainly in the throat, and there is also coolness in the mouth, so WS-3 is often used in lubricating sugar products. However, it should be noted that WS-3 has the same bitter taste as menthol, so it is necessary to optimize the dosage in specific use, and a small amount of sweetener can be added to cover the bitter taste when the dosage is large. In addition, WS-3 is difficult to dissolve in water, so if it is used in beverages, mouthwash and other products, it is necessary to dissolve WS-3 in propylene glycol, and then add Tween or Span solubilizer before use.

WS-5 and WS-12 products

WS-5 is also called N(- ethoxycarbonylmethyl)-p-alkane-3-carboxamide, its appearance is a white powder solid, the cooling sensation is mainly concentrated in the mouth and throat, the cooling intensity is about 3~4 times that of menthol, which is the variety with the highest cooling intensity in the WS series.WS-5 is mainly used in hard candies, lubricating candies, toothpastes, etc., and WS-5 is contained in black toothpastes available in the market at present. In addition, when WS-5 is used in combination with menthol oil or menthol, the cooling effect will be significantly enhanced.

WS-12 is also known as N(- 4-methoxyphenyl)-p-menthyl-3-carboxamide. Millennium’s test results show that WS-12 does not have the bitterness, burning and pungency of menthol, but produces a refreshing and prolonged cooling sensation that is impactful, refreshing, and cools the entire mucous membranes of the oral cavity and throat.

In addition, WS-12 has the characteristic of high temperature resistance, sustained high temperature of 200℃ will not affect the cooling strength of the product. However, during the application process, there is basically no cooling sensation when WS-12 is tasted directly, and there is a patent report that a cooling sensation will be produced in the mouth after WS-12 is ground into a powder with a very small particle size using a ball mill.

Menthol Esters and Carbonyl Reduction Series Products

I. Menthol Glutarate

Glutaric acid monomenthyl ester is a cooling agent produced by direct esterification of natural menthol with the corresponding dicarboxylic acid or dicarboxylic anhydride, and it has been reported in the literature that the reaction between menthol and glutaric anhydride can be used to obtain a high yield. The coolness of mono-menthyl glutarate is softer and lasts longer, and it is recognized as a safe coolant by the Food Flavor and Extract Manufacturers Association of America (FEMA) (FEMA 4006).In 2002, IFF had a patent describing that mono-menthyl glutarate mixed with di-menthyl glutarate in the ratio of 70:30 can give a better coolness [5].

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Menthyl Lactate

Menthyl lactate is mainly synthesized by esterification of fermented lactic acid and natural menthol as the main raw materials, and the conversion rate can reach 75.5% [6]. Menthyl lactate is mainly used in personal care products such as body wash, wet wipes and talcum powder. Compared with menthol, menthyl lactate is milder in terms of cooling effect and sensation, and the cooling sensation lasts longer.

Menthol lactate is also widely used in tobacco flavors. Due to its sweet and cooling aroma, menthyl lactate can be added to tobacco flavors to increase the sweetness of tobacco and significantly improve the taste of cigarettes. In particular, the raw materials of menthyl lactate are natural ingredients, volatile at high temperatures, safe and harmless to human beings, at the same time will make the flavored products more stable.

Third, menthol ketone glycerol acetonide

Menthol ketone glycerol acetonide is the use of acid catalysts will be the dehydration of menthol ketone and glycerol condensation to form the coolant. Compared with menthyl lactate, menthyl ketone glycerol ketone cool feeling is more gentle, and non-irritating to the skin, so it is often used in shampoo, body wash and shaving products and other personal care products. In addition, menthone glycerol ketone can also be used in the formulation of daily chemicals, tobacco and cooling flavors as a cooling stabilizer for menthol flavors [7].

Comparison of coolness of various cool flavors

Millennium has given data on the coolness intensity of various cool flavors by measuring their thresholds [8]. The cool strength of menthol [(-)-Menthol] was defined as 100, WS-23 as 75, both WS-3 and WS-12 as 150, and WS-5 as 400.In the menthol ester and carbon-condensed series of products, the cool strength of menthyl ketone glycerol condensed ketone (Frescolat MGA) is about 40, the cool strength of menthyl lactate ester (Frescolat ML ) has a coolness intensity of 43, see Figure 1.

In addition, several isomers of menthol had low coolness intensities, for example, D-neomenthol [(-)-Neomenthol], D-isomenthol [(-)-Isomenthol], L-neoisomenthol [(+)-Neoisomenthol] had a coolness intensity of 3, and L-isomenthol [(+)-Neoisomenthol ] also had a coolness intensity of only 11, and D-neoisomenthol [(-)-Neoisomenthol] had a coolness intensity of about 13.

The development trend of cold flavor spices

For the development trend of cold flavor spices, I believe that it includes the research and development of monomer products and the application of compound products in two aspects. First of all, at present, more than a thousand kinds of monomer synthetic cool flavor spices have been developed, of which only a few dozens have been successfully commercialized, and only 20~30 kinds are commonly used in food, cosmetic, and flavor [7], therefore, in the future, new food-grade and cosmetic-grade monomer cool spices will continue to be commercialized and be widely used. In addition, the application research of cooling fragrances will also become a new development direction, it is now known that some of the cooling fragrances will be used in combination to achieve synergistic effect, while water-soluble cooling fragrances, anti low eutectic cooling fragrances and so on will also become the direction of the research and development of application engineers.