Food Process Design I at Universität Hohenheim | Flashcards & Summaries

Lernmaterialien für Food Process Design I an der Universität Hohenheim

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TESTE DEIN WISSEN

Which type of heat exchanger do you use for a system containing large particles?

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TESTE DEIN WISSEN

CSTR (continuous stirred tank reactor)

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TESTE DEIN WISSEN

How does the heat transfer coefficient in a tube change over the length and why?

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TESTE DEIN WISSEN

not sure

When the fluid enters the tube, the flow profile is a "block" (no difference between wall and centre velocity). Therefore the transfer coefficient is higher (?) at that point (beginning of tube). Later on, the flow profile changes to a parabolic (curve like) shape with different velocities at the wall and the centre. This then leads to a different coefficient than from the beginning.

Does that sound right?

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TESTE DEIN WISSEN

How does the sorption isotherm shift for a higher temperature and why?

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TESTE DEIN WISSEN

The shift is to a higher aw value with the same product moisture (with higher temperature). This is because warmer air can hold more water than cold air, due to a higher binding energy.

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TESTE DEIN WISSEN

Explain the process steps and stages during freeze drying. Does the vacuum pump need to be running during the entire process?

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TESTE DEIN WISSEN

1) Freezing --> freezing rate influences crystal size

2) Drawing vacuum

3) Apply heat for drying by heating plates (--> sublimation ice)

4) Discharge


Vacuum pump doesn't need to be running during the entire process, only when pressure is getting higher again (but just to be sure: let it run all the time)

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TESTE DEIN WISSEN

Which water activity is desired to have a microbiological stable product?

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TESTE DEIN WISSEN

0.63 in general

(0.7 molds, 0.75 yeasts, 0.8 bacteria)

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TESTE DEIN WISSEN

What is the desired water activity of most food powders and why?

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TESTE DEIN WISSEN

For most food powders a aw value of 0.3 is desired, because here all the reactions are at the lowest point

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TESTE DEIN WISSEN

What do you need to solve a partial different equation of first, second and third order?

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TESTE DEIN WISSEN

separation of the variables; first order system; initial and boundary conditions

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TESTE DEIN WISSEN

Why are there deviations from the ideal behaviour of diffusion in food?

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TESTE DEIN WISSEN

complex structure (muliphasees, gels, ...)

- intermolecular interactions

- variation of the chemical composition

- parallel chemical reactions


-> matrices can change during storage and processing


=> not ideal condition => instative processes ?

Lösung ausblenden
TESTE DEIN WISSEN

What kind of mass transfer mechanisms are there?

Name dryers with that heat transfer technology.

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TESTE DEIN WISSEN

Convection:

-tray/ cabinet dryer

-fluid bed dryer

-spray dryer

-tunnle dryer

Contact/conduction:

-drum dryer

-freeze dryer

-mixer dryer

Radiation:

-freeze dryer

Lösung ausblenden
TESTE DEIN WISSEN

What type of heat exchanger do you use for products with high solid contents?

Lösung anzeigen
TESTE DEIN WISSEN

CSTR (continuous stirred tank reactor)

Lösung ausblenden
TESTE DEIN WISSEN

What is the ''Stefan-flux''?

Under which diffusion conditions does it occur and how does it affect the diffusion?

Lösung anzeigen
TESTE DEIN WISSEN

Stefan-flux is a compensating volume flow. The difference in the volume flux from one side to the other is leading to a pressure difference which leads to a flow which is interfering with the diffusion process.

It can occur through:



Single sided diffusion:


One sided exchange of one component through a semi-permeable wall or away from a source (e.g. dry ice sublimating, water evaporating)

Non equimolar diffusion:


Exchange of any non equal amount (volume) of substances in inverse directions of two components (e.g. coke made out of coal)

Lösung ausblenden
TESTE DEIN WISSEN

How can the resulting particle size be influenced in spray drying?

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TESTE DEIN WISSEN

- mass flux of evaporating water as low as possible (preconcentration) --> increase in viscosity --> decreasing pumpability and increasing droplet size

- Agglomeration (fines separated and returned, leading to collision with drying droplets --> increase in particle size)

- feed entering the nozzle --> higher viscosity --> larger particle

Lösung ausblenden
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Q:

Which type of heat exchanger do you use for a system containing large particles?

A:

CSTR (continuous stirred tank reactor)

Q:

How does the heat transfer coefficient in a tube change over the length and why?

A:

not sure

When the fluid enters the tube, the flow profile is a "block" (no difference between wall and centre velocity). Therefore the transfer coefficient is higher (?) at that point (beginning of tube). Later on, the flow profile changes to a parabolic (curve like) shape with different velocities at the wall and the centre. This then leads to a different coefficient than from the beginning.

Does that sound right?

Q:

How does the sorption isotherm shift for a higher temperature and why?

A:

The shift is to a higher aw value with the same product moisture (with higher temperature). This is because warmer air can hold more water than cold air, due to a higher binding energy.

Q:

Explain the process steps and stages during freeze drying. Does the vacuum pump need to be running during the entire process?

A:

1) Freezing --> freezing rate influences crystal size

2) Drawing vacuum

3) Apply heat for drying by heating plates (--> sublimation ice)

4) Discharge


Vacuum pump doesn't need to be running during the entire process, only when pressure is getting higher again (but just to be sure: let it run all the time)

Q:

Which water activity is desired to have a microbiological stable product?

A:

0.63 in general

(0.7 molds, 0.75 yeasts, 0.8 bacteria)

Mehr Karteikarten anzeigen
Q:

What is the desired water activity of most food powders and why?

A:

For most food powders a aw value of 0.3 is desired, because here all the reactions are at the lowest point

Q:

What do you need to solve a partial different equation of first, second and third order?

A:

separation of the variables; first order system; initial and boundary conditions

Q:

Why are there deviations from the ideal behaviour of diffusion in food?

A:

complex structure (muliphasees, gels, ...)

- intermolecular interactions

- variation of the chemical composition

- parallel chemical reactions


-> matrices can change during storage and processing


=> not ideal condition => instative processes ?

Q:

What kind of mass transfer mechanisms are there?

Name dryers with that heat transfer technology.

A:

Convection:

-tray/ cabinet dryer

-fluid bed dryer

-spray dryer

-tunnle dryer

Contact/conduction:

-drum dryer

-freeze dryer

-mixer dryer

Radiation:

-freeze dryer

Q:

What type of heat exchanger do you use for products with high solid contents?

A:

CSTR (continuous stirred tank reactor)

Q:

What is the ''Stefan-flux''?

Under which diffusion conditions does it occur and how does it affect the diffusion?

A:

Stefan-flux is a compensating volume flow. The difference in the volume flux from one side to the other is leading to a pressure difference which leads to a flow which is interfering with the diffusion process.

It can occur through:



Single sided diffusion:


One sided exchange of one component through a semi-permeable wall or away from a source (e.g. dry ice sublimating, water evaporating)

Non equimolar diffusion:


Exchange of any non equal amount (volume) of substances in inverse directions of two components (e.g. coke made out of coal)

Q:

How can the resulting particle size be influenced in spray drying?

A:

- mass flux of evaporating water as low as possible (preconcentration) --> increase in viscosity --> decreasing pumpability and increasing droplet size

- Agglomeration (fines separated and returned, leading to collision with drying droplets --> increase in particle size)

- feed entering the nozzle --> higher viscosity --> larger particle

Food Process Design I

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