Problem 1 -
A food processing company is designing the packaging for a new product line of potato chips. The expected shelf life of the potato chips is 10 months. The bag has to hold 200 g of potato chips and 1.500 mL of nitrogen. The nitrogen gas is used as a cushion to prevent crushing the chips and to avoid lipid oxidation. The oxygen level in the bag should be less than 1% (Palk et al., 1994).
(a) The packaging generally involves three layers, each of which is made up of a few films. The outside layer is used for printing. The inside layer serves as the bather to gas, moisture, and light and provides stiffness and heat seal. The middle layer bonds the outside and inside layers together. One of the films of the inside layer is usually aluminum coated by vaporization on a polymer film. The oxygen flux, J, through
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Barrier Film Material
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Film Coating
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Permeability of Oxygen at 25oC (cc-mil)/(100 in2-day.atm)
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Water Vapor Transmission Rate 38oC, 50 to 100% RH (cc-mil)/(100 in2-day)
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|
Polyamide
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None
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2.6
|
19
|
|
Metalized
|
0.05
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0.2
|
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PET
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None
|
4.5
|
1.2
|
|
Metalized
|
0.08
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0.1
|
|
Polypropylene
|
None
|
250
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1.75
|
|
Metalized
|
3
|
0.33
|
the bather film is governed by Fick's law.
J = P(Δp/L)
where P is the permeability. p the pressure, and L the barrier film thickness. The accompanying table shows the permeability to oxygen of a neat polymer film as well as one with metal coating (Tock, 1983). Select with explanations the material you would use for the new packaging design. Note that the cost of materials is a major consideration in snack food packaging and the cost data can be found on the Internet.
(b) What should be the dimensions of the bag? Assuming a constant permeability, calculate the minimum thickness of the selected bather film to achieve a 10-month shelf life with respect to oxygen (i.e., retaining an oxygen concentration less than 1% inside the bag).
(c) Moisture content of the potato chips is another crucial parameter. Assume that the initial moisture content of potato chips is I.5wt% and should not exceed 2.5wt% at the end of shelf life. Is the bather film chosen in pan (b) sufficient for the desired product performance with respect to moisture content? The data for moisture flux per unit film thickness in the table can be assumed to be constant within the stated range of RH.
Problem 2 -
Example 5.5 shows how to determine the rotor rotation speed and its gap width to produce emulsion particles 5 pm in diameter. If it is decided that the emulsion particle diameter needs to be further reduced to 1 pm while keeping the gap width at its minimum size, what should be the rotor radius?
Problem 3 -
1. Complete Problem 1 in the text, using the following cost data. Typical dimensions of a bag holding 200 g of potato chips are 20 cm x 27 cm.
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Barrier Film Material
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Film coating
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Cost ($/kg)
|
|
Polyamide
|
None
|
3.9 - 4.4
|
|
Metallized
|
NA
|
|
PET
|
None
|
1.8 - 2.5
|
|
Metallized
|
2 - 4
|
|
Polypropylene
|
None
|
1.9 - 2.1
|
|
Metallized
|
2.4 - 2.7
|
Alibaba, March, 2014.
Also, note that the moisture content of the potato chips, given in part (c), is on a wet basis (i.e., the mass of adsorbed moisture divided by the total mass of adsorbed moisture and potato chips). Assume that the package volume remains constant, ignoring the oxygen diffusion into the bag during its lifetime. O2 in atmosphere = 21%, so the driving force for oxygen permeation is 0.21 atm.
2. Complete Problem 2 in the text. Solve for rotor speeds of 2000, 3000 and 4000 rpm (the gap should be kept at a minimum of 200 μm).
Attachment:- Example 5.5.rar