19-10-2012, 04:58 PM
Design of a cryogenic grinding system for spices
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Introduction
Grinding of spices is an age-old technique like
grinding of other food materials. The main aim of spice
grinding is to obtain smaller particle size with good
product quality in terms of ¯avour and colour. In the
normal grinding process, heat is generated when energy
is used to fracture a particle into a smaller size. This
generated heat usually is detrimental to the product and
results in some loss of ¯avour and quality. The fat in
spices generally poses extra problems and is an important
consideration in grinding. During grinding, the
temperature of the product rises to a level in the range of
42±95°C (Pruthi & Misra, 1963), which varies with the
oil and moisture content of the spices, but spices lose a
signi®cant fraction of their volatile oil or ¯avouring
components due to this temperature rise. The losses of
volatile oil for dierent spices have been reported to be
in the tune of 37% for nutmeg, 14% for mace, 17% for
cinnamon and 17% for oregano (Andres, 1976). The loss
of volatile oil during grinding of caraway seed has been
reported to be 32% with an increase in grinding temperature
from ÿ17°C to 45°C (Wolf & Pahl, 1990).
Design and development
The cryogenic grinding system consists of two main
units, namely precooling unit and grinding unit.
Precooling unit
The cryogenic precooler is a cooling device made up
of a screw conveyor enclosed in a properly insulated
barrel and a system to introduce liquid nitrogen into the
barrel, thereby providing refrigeration (liquid and cold
gas) within the system. The function of the cryogenic
precooler is to remove the heat from the material before
it enters the grinder. The particle temperature must be
low enough to absorb the heat generated in the grinder
and still fracture. Cryogenic precoolers, therefore, must
have the ability to reduce the temperature of the seed
below its brittle point as well as the freezing point of its
oil, before it enters the grinder. There must be provision
to control the temperature of the precooler and the feed
rate to the grinder (Venetucci, 1980) for the obvious
purpose of controlling the grinding process.
Results and discussion
The experiments on grinding of cumin seed were conducted
at dierent temperatures from ÿ160°C to ÿ10°C.
The sieve choking, particle size distribution, speci®c
energy consumption and quality of the ®nal product were
observed using 8 number of grinderÕs rotor ribs.
Observations on choking of sieves were facilitated by
taking photographs of the sieve at grinding temperatures
of ÿ70, ÿ40 and ÿ10°C either at the completion of
grinding or when the machine was stopped due to sieve
choking. As shown in Fig. 6(a), it was observed that at
the grinding temperature of ÿ70°C, the deposition of
cumin powder was minimum. Below ÿ70°C, grinding
was smooth without any deposition and the sieve was
very clear. Accumulation of powder at ÿ40°C accounted
for about 50% area of the sieve being blocked as
shown in Fig. 6(b). At ÿ10°C (Fig. 6©) almost all the
perforations were blocked. Above ÿ70°C the grinding
experiments could not be completed successfully, because
the sieve perforations were blocked soon after
running the machine and it stopped due to overloading
with the incoming material.