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ABSTRACT

Spices are valuable not only in terms of their flavor, taste and preservation but also for their medicinal properties which are to be preserved during the processing. The traditional grinding system of spices causes a loss of volatile oil compounds, natural flavor and aroma. The natural components of the spices can be saved by adopting cryogenic technology that involves the use of cryogenic liquid (liquid nitrogen) which maintains low temperature during grinding. The cryogenic grinding system prevents loss of flavor, aroma profile, volatile oil and other medicinally important compounds results in the production of superior quality ground powder.

Spices are known for their pungent flavour, taste and the medicinal properties. They are used as an essential ingredient for imparting taste and flavour in food preparations. India is a major producer and exporter of a large variety of spices like black pepper, clove, cinnamon, cumin, coriander, turmeric, garlic, etc. (Barnwal et al., 2014). The medicinal importance of spices is well described in Ayurveda and other scriptures of Indian civilization. Besides, these are used as a preservative in food preservation because of their anti-microbial and anti-fungal properties. Nowadays, people prefer Ayurvedic medicines because of their no side effects over allopathic medicines. Consequently, the demand of pharma-grade (high quality) spice powder is increasing day by day. The quality of spices is evaluated from its intrinsic (chemical quality like retention of volatile oil, alkaloids, oleoresins etc.) and extrinsic (physical quality like color, shape, appearance, texture, etc.) characteristics. Generally, the quality of spices is depended on their essential oil, which is present inside the cells or tissues. The availability of constituents such as oil inside cells, fragrance and flavoring components are increased when it is converted into powder form. The powder can be easily converted into tablets form and mixing with other powders or liquids, conveniently. Grinding is an important unit operation in which the size of the material is reduced and the surface area of particles increased to get powder. Mostly, hammer mill is used for grinding of spices. In this traditional method, only 1% of the applied energy is used for size reduction operation whereas remaining energy is dissipated in raising the temperature of the ground material. Moreover, during grinding, the temperature inside the grinder increases up-to 42-93°C, which adversely affected the quality of spice powder in terms of loss of volatile oil, color and flavoring constituents (Singh and Goswami, 1997).

 

The spices also contain fatty oils that are melt due to heat and oil oozes out during grinding due to which the ground material becomes sticky that causes chocking of sieves and thus, hinders operation of the grinder. Thermal damage is reported as one of the main limitations of the conventional grinding process (Meghwal and Goswami, 2010); hence, it was suggested to perform the grinding

 

under controlled temperature conditions. It was also suggested that a better product can be obtained by reducing the temperature of two rubbing surfaces (Malkin and Guo, 2007). The temperature rise of the product can be minimized to some extent by circulating cold air or water around the grinder (Murthy et al., 1996; Singh and Goswami, 1999). But this technique is not sufficient enough to significantly reduce the temperature rise of the product to a safe level thus, damages the quality characteristics. 

The control of temperature during grinding is necessary to preserve the quality of spice powders. A novel technology named cryogenic grinding has been developed to control the high temperature during grinding in which cryogens (liquid nitrogen) are used to ground the spices below their brittle temperature. The grinding of spices at low temperature makes superior quality product, while maintaining appearance, medicinal and aromatic properties of spice with less power consumption. This technology has been scientifically proved to be a suitable method for adding value to ground spices in terms of improved flavor and its medicinal properties. The superiority of quality of cryogenically ground spice over conventionally ground is shown in figure 1 (it is in pdf version)

 

 Cryogenics

Cryogenics is defined as a branch of engineering specializing in technical operations at very low temperature, generally below -50°C. Cryogenic liquids are those which boil at cryogenic temperature and atmospheric pressure. The word “cryogenics” originated from the Greek words “Kryos” means frost and “Genic” means to produce. Cryogens such as liquid nitrogen, oxygen, hydrogen, helium etc. lower the generated heat during the grinding process. They are gases at ambient temperature and pressure and these gases must be cooled below ambient temperature to liquefy them. Liquid forms of hydrogen, helium, nitrogen, oxygen, inert gases, air, methane, carbon dioxide, etc. are common cryogens. Liquid nitrogen (LN₂) is most commonly used for grinding purpose. It is an inert, odourless, colourless, non-flammable, non-corrosive liquid and extremely cold (boiling point -195.8°C). In cryogenic grinding, LN₂ cools the raw material and grinder to operating temperature and removes the heat generated during grinding. Cryogens are stored in a vessel called dewar flask that provides good insulation.

Cryogenic grinding

Cryogenic grinding means grinding at cryogenic temperature, it is also known as cryo milling or freezer grinding. To obtain high-quality spice powder, cryogenic grinding systems cool the spices before feeding to the grinder and maintain the cryogenic temperature in the grinding zone.

The cryogenic grinding system has two main components:

ü   Pre-cooling unit

ü   Grinding unit (pin mill and a hammer mill)

 

The pre-cooling unit consists of a screw conveyor assembly, an air compressor, liquid nitrogen (LN₂) Dewar, a power transmission arrangement and control panel. The screw conveyor enclosed in a properly insulated barrel and liquid nitrogen is introduced into the barrel through distributer, thereby providing refrigeration (liquid and cold gas) within the system. The temperature should be low enough to absorb the heat generated in the grinder. The pre-cooling unit, therefore, must have the ability to reduce the temperature of the material below its brittle point as well as the freezing point of its oil, before it enters the grinder. There must be a system to control the temperature of the pre-cooler as well as feed rate to the grinder for controlling the grinding process.

The grinding unit consists of a rotor that contains some ribs surrounded by a sieve. The size of the final powdered product depends upon the aperture size of the sieve. The grinding of spices is done by compression, impact, cutting or combination of these techniques. The brittle material can be easily reduced by compression and impact, while tough material needs combined action of impact, cutting and shearing.

A cryogenic grinding system for spices is available at ICAR-CIPHET, Ludhiana to get superior quality spice powder. In this system, the whole spice is fed in the cryogenic pre-cooler inlet and the LN₂ is allowed to travel through the pre-cooler that cools the spice to desired operating temperature. The cryogenically cooled spice from pre-cooler outlet enters into grinder inlet where the spices are ground and high-quality spice powder is collected at the outlet of grinder in the collection system.

Consumption of liquid nitrogen and the operating costs are important considerations and matters of concern for a cryogenic pre-cooling system. The liquid nitrogen losses can be minimized to a great extent by proper consideration of the design and insulation of the pre-cooler. The pre-cooling unit is designed in a manner to prevent the spice from being heated up during the grinding process and should pre-cool the spice before the actual starting of the grinding operation. Thus, the pre-cooling unit should be designed to match with a commercially available grinder (a pin mill and a hammer mill) that could withstand low-temperature operations.

 

 

 

Consumption of liquid nitrogen and the operating costs are important considerations and matters of concern for a cryogenic pre-cooling system. The liquid nitrogen losses can be minimized to a great extent by proper consideration of the design and insulation of the pre-cooler. The pre-cooling unit is designed in a manner to prevent the spice from being heated up during the grinding process and should pre-cool the spice before the actual starting of the grinding operation. Thus, the pre-cooling unit should be designed to match with a commercially available grinder (a pin mill and a hammer mill) that could withstand low-temperature operations.

 

Effect of cryogenic grinding on properties of spices

Several studies have proved that the cryogenically ground spices powder is superior in terms of quality attributes as compared to absently ground powder.

In cryogenic grinding of black pepper, Murthy et al. (1996) observed an increase in retention of volatile oil from 1.35 to 1.91% when the grinding temperature was decreased from -50 to -120°C. Similarly, during cryogenic grinding of cloves, the retention of volatile oil increased from 13.16 to 13.31% when the temperature was declined from -50 to -110°C (Goswami,2000). Barnwal et al. (2014) compared the grinding characteristics of fenugreek powder under cryogenic and ambient grinding conditions and reported that the cryogenically ground fenugreek powder has fine particle size, better colour and low energy requirement.  Liu et al. (2013) stored black, white and green pepper powder individually at 4°C temperature for 6 months and found that cryogenic grinding was better preserving the main potent aroma constituents than hammer milling, but the concentrations of the main aroma constituents were dramatically reduced during storage.

The lower grinding temperature provided the best quality of powder concerning the mineral composition, volatile oil retention, colour, particle size, size distribution and a specific surface area along with good water activity and flowability at a lower energy requirement. Moreover, the fineness and homogeneity of the cryogenically ground powder particle were increased due to a reduction in the grinding temperature and the powder obtained at -120°C was observed to be more homogeneously distributed  (Ghodki and Goswami,2016). The use of cryogenic technology for grinding of spices has been proved to be an appropriate technique with negligible loss of volatile content and improved colour and grinding operation (Saxena et al., 2013).

 Advantages

The cost of cryo-ground spice is high as compared to conventionally ground spice due to higher investment in setting up the plant but a significant number of advantages are provided by the cryogenic grinding which is listed below (Meghwal and Goswami, 2010; Singh and Goswami, 1997; Purthi, 1980; Singh and Goswami, 2000; Barnwal et al., 2016):

ü   Superior quality product in terms of volatile oil, color, particle size, fineness modulus

ü   Reduction in the oxidation of spice oil and increased in stability

ü   More uniform dispersal of flavour throughout the product

ü   Composition of essential and total oil of powder spice is similar to that of intact spice

ü   Ability to grind soft or elastic material

ü   Increased productivity

ü   Better appearance of spice powder

ü   Retention of medicinal properties of the spice

ü   Finer, uniform and consistent powder production

ü   Low Power Consumption

ü   No clogging of sieve

ü   Freedom from contamination

ü   Insignificant formation of dust leads to no pollution

ü   Minimum thermal fatigue and risk of fire

 Conclusion

Cryogenic grinding is a useful technology for adding value to the spices. The deterioration of quality attributes in the conventional grinding method is overcome by grinding the spices in cryogenic conditions. The physicochemical compositions along with medicinal properties of the spices are preserved to a great extent. The application of cryogenic grinding is a suitable technique for grinding of spices with negligible loss of volatile compounds with improved colour and grinding operation.

 References

Baranwal, P., Kumar, P., Singh, K.K., Mohite, A.S. (2016). Selected engineering properties of cryogenic and ambient ground black pepper. Journal of Food Processing and Preservation, ISSN 1745-4549.

Barnwal, P., Mohite, A., Singh, K.K., Kumar, P., Zachariah, T.J., Saxena. S.N. (2014). Effect of cryogenic and ambient grinding on grinding characteristics of cinnamon and turmeric. International Journal of Seed Spices, 42(2): 26-31.

Barnwal, P., Singh, K.K., Mohite, A., Sharma, A., Saxena, S.N. (2014). Influence of cryogenic and ambient grinding on grinding characteristics of fenugreek powder: A comprehensive study. Journal of Food Processing and Preservation, 39: 1243-1250.

Ghodki, B.M. and Goswami, T.K. (2016). Effect of grinding temperatures on particle and physicochemical characteristics of black pepper powder. Powder Technology, 299: 168-177.

Goswami, T. (2000). Cryogrinding of cloves. Journal of Food Processing Preservation, 24:57-71.

Liu, H., Zeng, F., Wang, Q., Ou, S., Tan, L. and Gu, F. (2013). The effect of cryogenic grinding and hammer milling on the flavour quality of ground pepper (Piper nigrumL.). Food Chemistry, 141: 3402-3408.

Malkin, S., Guo, C. (2007). Thermal analysis of grinding. Annuals of the CIRP 56(2), Connecticut, USA.     

Meghwal, M., Goswami, T.K. (2010). Cryogenic grinding of spices is a novel approach whereas ambient grinding needs improvement. Continental Journal of Food Science and Technology, 4: 24-37.              

Murthy, C.T., Krishnamurthy, N., Ramesh, T., Srinivasa Rao, P.N. (1996). Effect of grinding methods on the retention of black pepper volatiles. Journal of Food Science and Technology, 33(4): 299-301.

Pruthi, J.S. (1980). Spices and Condiments: Chemistry, Microbiology, Technology. Advances in Food Research Supplement, 4. Academic Press, Inc., New York.

Saxena, R., Rathore, S.S., Barnwal, P., Soni, A., Sharma, L., Saxena, S.N. (2013). Effect of cryogenic grinding on recovery of diosgenin content in fenugreek (Trigonellafornum- graecum L.) genotypes. International Journal of Seed Spices, 3(1): 26-30.

Singh, K.K., Goswami, T.K. (1997). Studies on cryogenic grinding of spices. IIT Kharagpur (India).

Singh, K.K., Goswami, T.K. (1999). Design of cryogenic system for spices. Journal of Food Engineering, 39: 359-368.

Singh, K.K., Goswani, T.K. (2000). Thermal properties of cumin seed. Journal of Food Engineering, 45: 181-187.

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Is cryo-grinding better than conventional grinding.pdf