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Hydrodynamic_and_acoustic_cavitation_in_fresh_milk_treatment.pdf

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DANIELE CRUDO1, VALENTINA BOSCO1, GIULIANO CAVAGLIÀ1, STEFANO MANTEGNA2, LUIGI BATTAGLIA2, GIANCARLO CRAVOTTO2*

*Corresponding author

1. E-PIC S.r.l. Torino, Via Schina 15, Torino, 10143, Italy

2. University of Torino, Dipartimento di Scienza e Tecnologia del Farmaco, Via Giuria 9, Torino, 10125, Italy

PROCESS INTENSIFICATION

Giancarlo Cravotto

Process intensification in the food industry: hydrodynamic and acoustic cavitation in fresh milk treatment

KEYWORDS: hydrodynamic cavitation, flow-ultrasound, fresh cow milk, homogenization, microorganism inactivation

AbstractCavitation phenomena, which are commonly connected to erosion effects in fluid-flow systems, can be valid non-conventional mild disinfection processes for the treatment of fresh milk, clear juices and aqueous

beverages. In this work, two flow-through reactors for hydrodynamic and ultrasonic cavitation have been tested in an attempt to achieve the simultaneous pasteurization and homogenization of fresh cow milk at low temperature and in a modified atmosphere. In this work, hydrodynamic cavitation in a loop reactor gives up to 88% microorganism inactivation when working at 6 bar pressure in a CO2 atmosphere for 30 min. Acoustic cavitation in a ultrasonic flow reactor (Sonotube®, power 370 W) gave microorganism abatement percentages of 95% in 10 min. Fast and efficient homogenization occurred in both loop reactors. An additional, and important, advantage of these techniques is the fact that they can easily be scaled-up for industrial applications.

INTRODUCTION

Fresh milk is a perishable foodstuff that requires rapid industrial treatment to make it shelf stable. The classic dairy industry processes used to produce milk at an affordable price are homogenisation and pasteurization. Standard homogenisation is used to reduce the size of fat globules and consists of forcing the liquid through a narrow gap (100–300 mm) in a homogenisation valve at an up-stream pressure of about 20–60 MPa. Thermal treatment, which

is used to reduce microbial spoilage, presents several drawbacks (protein denaturation, decrease in nutritional values etc.). These facts have prompted the development of non-thermal procedures which aim to combine both homogenisation and pasteurization steps in a single run. Hydrodynamic and acoustic cavitation can potentially address this need (1). Ultrasound and hydrodynamic cavitation technologies have increasingly been adopted

in industrial beverage and food processing. Cavitation is the mechanism by which the desired effects occur in liquid foods. Microorganism killing, enzyme activity inhibition, wine maturation, emulsi cation and crystallization all rely on the mechanism of cavitation (2).

In cavitational treatment, bubble collapse generates high- energy microenvironments and accompanying hot spots, shock waves, micro-jets and shear forces. In hydrodynamic cavitation, a liquid is forced to pass into suitable ori ces

in which the kinetic energy of the uid is ampli ed with increasing bulk pressure; when liquid pressure falls to its vapour pressure, bubbles are generated. Downstream

from the restriction, the uid decreases in velocity, recovers pressure and bubbles collapse in a con ned area. According to Shah et al. (3) and assuming linear pressure

recovery downstream from the restriction, bubbles are exposed to P∝, which represents the pressure in the liquid away from the bubbles:

where Pv is the vapour pressure of the liquid, P2 is the recovered pressure downstream from the restriction, L is the distance of pressure recovery, V is the mean velocity of the uid in the pipeline and t is time. Cavitation number, σ, is the dimensionless parameter which characterizes hydrodynamic cavitation (4):

where ρl is the density of the liquid and v0 is the mean velocity of the uid at the restriction.

Ideally, cavitation inception occurs when the cavitation number is equal to 1 and cavitation intensity increases below unity, however, in reality, cavitation inception can start

at σ >1, as dissolved gases and impurities act as points of nucleation for bubble generation (5).

To maximize the cavitation volume, a small amount of gas can also be injected into the restriction (6). Gas injection has a direct effect on nucleation and the phase of bubble collapse, which controls the intensity of the cavitation events. Gases with low solubility in the liquid medium, high polytropic constant and low thermal conductivity enhance cavitation effects (5).

In acoustic cavitation, the liquid is exposed to acoustic

55

Agro FOOD Industry Hi Tech - vol 25(1) - January/February 2014

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 Supercritical Fluid Extraction Hydrodynamic_and_acoustic_cavitation_in_fresh_milk_treatment.pdf Page 001
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