Saturday, April 2, 2011
Friday, January 14, 2011
Ohmic Heating System
13.1 Introduction
Over the past two decades, there has been an increasing shift from batch thermal
operation towards continuous High Temperature Short Time (HTST) processing
of foods. In HTST processes, food is processed continuously through plate or
scraped surface heat exchangers at temperatures as high as 140ºC. At this
temperature, only a few seconds are needed for sterilization, during which the
products suffer only a slight quality deterioration. HTST processes rely on rapid
convection heat transfer and are thus well suited to liquid foods. They are,
however, limited in application to particulates since, for particles more than a
couple of millimeters thick, the processing time is insufficient for heat to
transfer to the center to give sterility. In ohmic heating processes, foods are
made part of an electric circuit through which alternating current flows, causing
heat to be generated within the foods due to the electrical resistance of the foods.
Therefore, in a liquid-particulate food mixture, if the electrical conductivity of
the two phases are comparable, heat could be generated at the same or
comparable rate in both phases in ohmic heating. In other conditions, heat can
also be generated faster in the particulate than in the liquid. Ohmic methods thus
offer a way of processing particulate food at the rate of HTST processes, but
without the limitation of conventional HTST on heat transfer to particulates.
13.1.1 History
The concept of ohmic heating of foods is not new. In the nineteenth century,
several processes were patented that used electrical current for heating flowable
materials. In the early twentieth century, ‘electric’ pasteurization of milk was
achieved by passing1 milk between parallel plates with a voltage difference
13
Ohmic heating
R. Ruan, X. Ye, P. Chen and C.J. Doona, University of Minnesota
and I. Taub, US Army Natick Soldier Centergenerally accepted. Consequently, the Food and Drug Administration (FDA) has no current filing of continuous ohmically processed multiphase food products.
The ohmic heating process has the promise to provide food processors with
the opportunity to produce new, high-value-added, shelf-stable products with a
quality previously unrealized with current sterilization techniques. Applications
that have been developed include aseptic processing of high-value-added readyprepared
meals for storage and distribution at ambient temperature; preheating
of food products prior to in-can sterilization; and the hygienic production of
high-value-added ready-prepared foods for storage and distribution at chilled
temperatures. Ohmic heating can also be used for heating of high-acid food
products such as tomato-based sauces prior to hot-filling, with considerable
benefits in product quality. Other potential applications include rapid heating of
liquid food products, which are difficult to heat by conventional technologies.8
Other potential future applications for ohmic heating include blanching,
evaporation, dehydration, fermentation, and extraction.
The disadvantages of ohmic heating are associated with its unique electrical
heating mechanisms. For example, the heat generation rate may be easily
affected by the electrical heterogeneity of the particle, heat channeling, complex
coupling between temperature and electrical field distributions, and particle
shape and orientation. All these make the process complex and contribute to
non-uniformity in temperature, which may be difficult to monitor and control.
13.1.4 Advantages
The advantages of ohmic heating technology claimed in the previous research
are summarized as follows.5, 9
• Heating food materials by internal heat generation without the limitation of
conventional heat transfer and some of the non-uniformity commonly
associated with microwave heating due to limited dielectric penetration.
Heating takes place volumetrically and the product does not experience a
large temperature gradient within itself as it heats.
• Higher temperature in particulates than liquid can be achieved, which is
impossible for conventional heating.
• Reducing risks of fouling on heat transfer surface and burning of the food
product, resulting in minimal mechanical damage and better nutrients and
vitamin retention.
• High energy efficiency because 90% of the electrical energy is converted into
heat.
• Optimization of capital investment and product safety as a result of high
solids loading capacity.
• Ease of process control with instant switch-on and shut-down.
• Reducing maintenance cost (no moving parts).
• Ambient-temperature storage and distribution when combined with an aseptic
filling system.
• A quiet environmentally friendly system
Over the past two decades, there has been an increasing shift from batch thermal
operation towards continuous High Temperature Short Time (HTST) processing
of foods. In HTST processes, food is processed continuously through plate or
scraped surface heat exchangers at temperatures as high as 140ºC. At this
temperature, only a few seconds are needed for sterilization, during which the
products suffer only a slight quality deterioration. HTST processes rely on rapid
convection heat transfer and are thus well suited to liquid foods. They are,
however, limited in application to particulates since, for particles more than a
couple of millimeters thick, the processing time is insufficient for heat to
transfer to the center to give sterility. In ohmic heating processes, foods are
made part of an electric circuit through which alternating current flows, causing
heat to be generated within the foods due to the electrical resistance of the foods.
Therefore, in a liquid-particulate food mixture, if the electrical conductivity of
the two phases are comparable, heat could be generated at the same or
comparable rate in both phases in ohmic heating. In other conditions, heat can
also be generated faster in the particulate than in the liquid. Ohmic methods thus
offer a way of processing particulate food at the rate of HTST processes, but
without the limitation of conventional HTST on heat transfer to particulates.
13.1.1 History
The concept of ohmic heating of foods is not new. In the nineteenth century,
several processes were patented that used electrical current for heating flowable
materials. In the early twentieth century, ‘electric’ pasteurization of milk was
achieved by passing1 milk between parallel plates with a voltage difference
13
Ohmic heating
R. Ruan, X. Ye, P. Chen and C.J. Doona, University of Minnesota
and I. Taub, US Army Natick Soldier Centergenerally accepted. Consequently, the Food and Drug Administration (FDA) has no current filing of continuous ohmically processed multiphase food products.
The ohmic heating process has the promise to provide food processors with
the opportunity to produce new, high-value-added, shelf-stable products with a
quality previously unrealized with current sterilization techniques. Applications
that have been developed include aseptic processing of high-value-added readyprepared
meals for storage and distribution at ambient temperature; preheating
of food products prior to in-can sterilization; and the hygienic production of
high-value-added ready-prepared foods for storage and distribution at chilled
temperatures. Ohmic heating can also be used for heating of high-acid food
products such as tomato-based sauces prior to hot-filling, with considerable
benefits in product quality. Other potential applications include rapid heating of
liquid food products, which are difficult to heat by conventional technologies.8
Other potential future applications for ohmic heating include blanching,
evaporation, dehydration, fermentation, and extraction.
The disadvantages of ohmic heating are associated with its unique electrical
heating mechanisms. For example, the heat generation rate may be easily
affected by the electrical heterogeneity of the particle, heat channeling, complex
coupling between temperature and electrical field distributions, and particle
shape and orientation. All these make the process complex and contribute to
non-uniformity in temperature, which may be difficult to monitor and control.
13.1.4 Advantages
The advantages of ohmic heating technology claimed in the previous research
are summarized as follows.5, 9
• Heating food materials by internal heat generation without the limitation of
conventional heat transfer and some of the non-uniformity commonly
associated with microwave heating due to limited dielectric penetration.
Heating takes place volumetrically and the product does not experience a
large temperature gradient within itself as it heats.
• Higher temperature in particulates than liquid can be achieved, which is
impossible for conventional heating.
• Reducing risks of fouling on heat transfer surface and burning of the food
product, resulting in minimal mechanical damage and better nutrients and
vitamin retention.
• High energy efficiency because 90% of the electrical energy is converted into
heat.
• Optimization of capital investment and product safety as a result of high
solids loading capacity.
• Ease of process control with instant switch-on and shut-down.
• Reducing maintenance cost (no moving parts).
• Ambient-temperature storage and distribution when combined with an aseptic
filling system.
• A quiet environmentally friendly system
Thursday, October 16, 2008
Hari pertama buat blog
salam pertemuan,
aku dilahir kan diutara malaysia, jelapang padi malaysia tempat kelahiran ku,umurku dah bleh mengundi tp belum pernah buang undi, aku dibesar dalam suasana yang penuh kasih sayang oleh mak dan abah. aku anak selapas abangku
aku dilahir kan diutara malaysia, jelapang padi malaysia tempat kelahiran ku,umurku dah bleh mengundi tp belum pernah buang undi, aku dibesar dalam suasana yang penuh kasih sayang oleh mak dan abah. aku anak selapas abangku
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