2: Ohmic Heating

Question 1

What is ohmic heating?

Answer 1

• Ohmic heating is also known as resistance heating and involves direct contact of electrodes to the food.
• The heat is generated based on electrical resistance.
• Ohmic heating is ideal for liquid products.

Question 2

What are the factors that affect resistance?

Answer 2

• Temperature: Resistance decrease with increasing temp.
• Direction of electric current
• Physicochemical properties of food.

Question 3

What are the factors that affect the efficiency of ohmic heating?

Answer 3

• Electrical resistance of product (Low moisture content will be heated faster)
• Ways of product flow
• Residence time
• Distance of electrodes

Question 4

What is a product’s critical current density?

Answer 4

The maximum current density that can be carried by a product. Flash-over will likely occur if current density exceeds the critical limit.

Question 5

Why is the design of an ohmic heater product specific?

Answer 5

• Because different products will have different physicochemical properties, leading to differing heating efficiency.
• Ohmic heater design is dependent of several factors such as, product type, flow rate, heating rate, holding time, power requirement, cost and type of current (AC or DC).

Question 6

What are some advantages of ohmic heating?

Answer 6

• Rapid and even heat transfer
• High temperature can be achieved
• No hot surfaces needed
• High efficiency
• Uniform heating of liquid products with particulates
• Applicable to heating viscous liquids

Question 7

Describe the techniques to achieve optimum ohmic heating in products containing solid particulates.

Answer 7

• Pasteurize/sterilize liquid carrier and transfer the carrier to liquid containing suspended solid particulates.
• Ensure the balance between particulates and carrier liquid to ensure homogenous suspension
• Avoid turbulent flow for mixing. Turbulent flow introduces air bubbles into the liquid and reduces efficiency of heating.
• Non-homogenous liquid will heat unevenly where temperature is higher near solids.

Question 8

What are the product factors to be considered for ohmic heated products?

Answer 8

• Moisture content
• Size and shape
• Solid to liquid ratio
• Viscosity
• pH
• Thermal conductivity
• Specific heat
• Amount and type of electrolyte