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Product and Process Technology

53 important questions on Product and Process Technology

Which types of seperation processes do exist?

- Phase separations (sedimentation/ centrifugation/ filtration/ sieving)
- Molecular separations (extraction/ evaporation/ drying)

What happens during transformation of food?

Raw materials are transformed into reaction products and by-products by (bio)chemical reactions .

Example: fermentation processes

What is the benefit of composed food products?

Increased variety in diet by providing attractive flavours, colours, aroma’s and textures.

How does irradiation work?

Irradiation is done by gamma radiation. The radiation is ionizing on molecular scale. In this way it kills bacteria cells. However it also changes the chemicals of the food product. The product tastes differently afterwards.

Why are weak acids used for the preservation of foods?

Weak acids are not charged and therefore can easily pass through the cells membrane. After that, it lowers the pH from the inside and the cell dies.

What is quick heat sterilization and what is the benefit of this preservation method?

It is a method in which a product is heated quickly to a certain temperature for a short time. This method kills all MO's but taste and vitamins remain.

What different types of cheese are there based on the way in which they are processed?

Acid cheese and Rennet cheese.

What are 3 process schemes?

- Block diagram
- Flow diagram/ chart
- P&ID (piping and intrumentation diagram)

What is the simplest way of classifying different processes and name these types of processes.

By whether the input/ output is discrete or continuous.

- Batch process (discrete-discrete)
- Semi-batch or semi-continuous (continuous-discrete) or (discrete-continuous)
- Continuous (continuous-continuous)

Which is the law that is applicable to any mass balance?

The law of conservation of energy: input = output + accumulation

What happens to the pressure in a tube if a stream of liquid is coming through?

Because of the resistance to flow in the tube, the stream slows down so loses velocity/energy and as a result the pressure drops.

What influences the resistance of a fluid?

- Fluid properties (viscosity and density)
- Length and diameter of the pipe
- Bends, valves, fittings in the pipeline
- Distance and height chance of fluid
- Power behind the stream by means of pumps!!

What happens to the resistance if power is applied to the stream by means of a pump?

A pump is a mechanical device that converts mechanical power into energy. It generates flow with enough power to overcome pressure induced by the load.

To overcome the pressure induced by the load in the pipe, overpressure is needed. To create what exactly?

- Level difference
- Flow in the pipe

Explain the difference between static and dynamic pressure.

Static pressure is the force exerted on a fluid particle from all directions. It can only be measured when the fluid is at rest of while traveling along with the fluid flow. Dynamic pressure represents the kinetic energy of the flowing fluid. Dynamic pressure is a function of the fluid velocity and its density.
Static pressure -> ΔP=ρ⋅g ⋅Δh
Dynamic pressure -> P dynamic =ρ . (v^2/2g)

What is the principal of a positive-displacement pump?

Positive displacement pumps draw fluid into a compartment at the inlet and move it to an outlet for discharge, most typically using a rotary, reciprocating, or diaphragm method to move fluid.

What is the principal of a centrifugal pump?

The most common type among centrifugal pump is the radial flow pump. These centrifugal pumps use a rotating impeller to create a vacuum in order to move fluid. The pump’s impeller rotates within the housing and reduces pressure at the inlet. This motion then drives fluid to the outside of the pump’s housing, which increases the pressure enough to send it out the discharge.

! This priciple is the same as stirring with a spoon in a cup of tea.

What is the difference between the two pumps?

The main difference between these types of pumps and centrifugal is that positive displacement pumps will move fluid at the same speed regardless of the pressure on the inlet end and centrifugal pumps will not.

Say something about the products that the pumps are used for and what force causes the liquid to be pumped out.

Centrifugal pump: Used for low viscosity liquids. The velocity of the fluid is conversed into static pressure.

Positive displacement pump: Used for high viscosity liquids. It traps an amount of liquid and than creates pressure by moving it into the discharge pipe.

What is the risk of insufficient pump capacity?

If heating is concerned: burning/ overheating or fouling
If the liquid is a suspension: sedimentation

What happens in an extrusion process?

It is a process in which the ingredients are mixed, kneaded, cooked, sheared and shaped into a food product.

How does extrusion work?

By forcing a pump able food product through a small hole (die) to form a mixture into a desired shape.

Which 6 unit operations are combined in the extrusion process?

- Mixing
- Hydrating
- Cooking
- Kneading
- Compressing
- Shaping
- Forming

What is the benefit of extrusion?

- Serves as a texturizer: makes characteristic textures
- Creates more versatility in types and shapes. This differs for the operation conditions (t/T), the die shape and the raw materials used.
- It is a continuous process so there is a big throughput.
- It has a high efficiency and reduced space on the plant floor.
- It is a low moisture process.
- There is limited and controlled heat treatment
- There are minimal waste streams

What are the different screw sections of an extruder and what happens in these sections.

Input of raw materials ->
feed section/ screw (the materials collected are fed to the extruder in every time the same portions . Screw have deep flights for easy/ rapid filling. Additives are sometimes added for preconditioning.) ->
Heating section (Decrease depth of flights for compressing and forming of the end product. This causes a higher pressure which enhances the cooking process and the structure changes.) ->
Cooling section ->
Discharge section (the product is lead to the output die and so the pressure builds up more. The water evaporates causing expansion of the product.)

What are the pro's of a single screw extruder and what the cons?

Pro's:
- Simple cooking and forming
- Relatively inexpensive
- Easier to operate

Cons:
- Doesn't work on high moisture feed (only 12-35%)
- Poor mixture ability and therefore poor heat transfer.

What are the pro's of a twin screw extruder and what the cons?

Pro's:
- Wide application in food industry
- More versatile
- Works on high moisture feed (6% to very high)

Cons:
- More expensive

Which extrusion types do exist?

- Cold extrusion
- Extrusion cooking
- Coextrusion

How do we measure temperature?

With a physical way of measuring (thermometer). The principle behind the thermometer is the law of thermodynamics which tells us that the thermometer gets the same temperature as the object measured. The temperature of the thermometer goes up until it is in equilibrium with the temperature of the measured object.

What does Zeroth law say?

Ta = Tb and Tb = Tc, then Ta = Tc.

How does a liquid thermometer work?

By capillary action.

What is the linear function of temperature when it concerns a gas?

PV/T (pressure x volume / temperature)

Which type of thermometers do exist?

- Electric resistor
- Thermocouple
- Blackbody radiation

What is the symbol for heat and what the unit?

Heat has the symbol Q. Traditionally it is measured in calories (fluids flow from hot to cold so there is a form of energy present) but it is Joules (J) in SI units.

What is the difference between heat and temperature?

Heat is energy that is transferred from one body to another as the result of a difference in temperature. Temperature is not the same as energy. It is a measure of hotness or coldness expressed in terms of any of several arbitrary scales like Celsius and Fahrenheit.

How much J in 1 calorie?

4.1868

Explain for each way of heat transfer how it works.

Radiation: Is for example electromagnetic radiation from the sun. Heat leaves a hot object without something is in between, a medium that can carry the heat. Even if there is no air that can carry the heat (as in vacuum), heat still can be transferred by radiation.

Convection: The hot medium moves. Often the matter expands like hot air. By this air flows are created.

Conduction: Only works if the heat has to be transferred over a short distance. The heat is transferred by the molecules taking over each others movements.

What is the formula for convection?

∆

What is the risk when fluids don't flow as they should?

- Risk of longer processing time
- Risk of overheating (browning, destruction of vitamins)
- Underheating (MO risk)
- Cooling to slow (MO risk)
- Fouling of equipment (equipment getting dirty)

Why should you know how fast fluids flow?

- To know what the food properties of the product you are making will be.
- To keep this into account when choosing the process design and when choosing equipment.

Why is fluid flow so important in processing?

It is an essential operation. It is part of and linking various unit operations.

Which forces will cause fluid to flow?

- Pressure
- Gravity
- Friction
- Thermal effects

What is Laminar flow?

It is a smooth flow. Laminar flow occurs when a fluid has a low flow rate and/or a high viscosity. The fluid exists out of layers that flow over each other and don't mix.
There is a high difference in residence time (along the sides of the pipe the flow goes slower).

What is Turbulent flow?

It is a rough flow above a certain flow rate. There is mixing and there is a small difference in the residence or travel time between the particles. It promotes heat transfer because there is a lower risk of overheating and it also promotes mass transfer and pressure drop. There is a higher waste of energy for this flow.

What is Reynolds number?

It is a dimensionless quantity (has no units) that shows how a fluid flows. The Re number can be calculated when velocity of a fluid is multiplied by the density of the fluid and the diameter of the pipe. That total is divided by viscosity of the fluid.

What is the average flow velocity in a tube?

It is the same as the flow rate (Qv/Atube).

What does Qv and Qm stand for? What are the SI units?

Qv = area x velocity
Unit: m3/s

Qm = density x Qv
Unit: kg/s

How do you create the optimal situation for turbulence?

Change de D in the Qv formula. Then use the velocity and put it in the formula for the Re number. Go on until turbulent flow.

For which law we use Bernoulli's principle and what does it mean?

The law of conservation of energy.
Bernoulli: potential + kinetic energy = constant

What is another meaning of potential energy?

Pressure P (measured in Pa = N/m2 = J/m3 )

What is the pressure in a fluid?

The pressure is at any depth of a container is due to the weight of the water plus the atmospheric pressure pushing on the water's surface. Pressure is equal to F (the weight of the water + the gravity) divided by the area.

Why is Venturi used for dissolving powder or mixing a liquid in a liquid flow?

The Venturi looks like an hourglas. Before the part that the diameter gets smaller the pressure is high because the same volume is pushed into a smaller tube. In the part of the tube with the small diameter the pressure is low. The pressure stays low after it widens again. Because the pressure is low in the small part it is easy to enter a fluid with a high velocity.

To which flow is Poiseuille's law applicable?

To laminar flow.

The question on the page originate from the summary of the following study material:

Food Processing

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