4. Storage time. Storage time, storage temperature. To ensure high quality of the
food: 1. HQL .... Course literature (SEU+Refrigeration book). • Calculator (not ...
Preservation of Food, Sustainability Examples, Course Summary Hans Jonsson
Agenda • Preservation of food • Sustainability examples • Course summary
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Agenda - Preservation of Food • • • • • • •
Basics Storage time, storage temperature Storing at low temperatures Concentration of fruit juice Freeze drying Estimation of cooling demand Freezing methods
Basics • Almost all processes in nature runs faster at higher temperatures. • This means that food is to be kept at low temperatures to allow a longer storage time. • So, the cooler the better? • Yes and no! • Since almost all food products contain water, there is a phase-change at (or around) 0 °C. Some food products are damaged by freezing! • E.g. Tomatoes, Strawberries
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Basics, cont. • Damage due to freezing is caused by the ice crystals during the freezing process. If the crystals are large, the cell membranes of the food is damaged and the texture is changed (or damaged). • It has been shown that the size of the ice crystals become smaller if the freezing process is fast. • However, some food products are quite insensitive to large ice crystals. E.g. meat, fish, etc. • The nuitrition of the food is unaffected by freezing!
Storage time, storage temperature To determine the storage time, the concept of High Quality Life (HQL) has been defined as: HQL: Time of storage before first detectable change in quality occur. The storage time depend on temperature!
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Storage time
Storage time, storage temperature To ensure high quality of the food: ⎛ τ ⎞ ⎟⎟ < 1 ∑ ⎜⎜ HQL ⎠t ⎝
Time of storage at temperature t HQL of temperature t
For the entire life span of the food, this ratio has to be summed. τ10°C τ −25°C τ8°C + + + L+ < 1 HQL10°C HQL −25°C HQL8°C where HQLt can be taken from the chart, or…
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Storage time, storage temperature The curves of the chart can be expressed by an equation, HQLt can hence be found by: HQL t = HQL ref
⎛ t ref − t ⎞ ⎜ ⎟ ⎜D ⎟ t , ref ⎠ ⋅ e⎝
Reference temperature
HQL of an arbitarily chosen reference temperature
Dt,ref is determined by choosing two points on the curve, Dt,ref is hence found with: ⎛ HQL ⎞ ⎟⎟ D t ,ref = (t ref − t ) ln⎜⎜ HQL ref ⎠ ⎝
Storage time
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Storage temperatures
Storing at low temperatures • Storing products containing water at low temperatures will result in a certain weight loss (caused by diffusion of water). The product dries. • Loss of weight means - Loss of quality - Loss of money (products sold per unit weight)
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Storing at low temperatures, cont. • This can be avoided by - Using a system with a rapid cooling which makes the surface of the products freeze. - Storing products at high humidity (reduces diffusion) - Use wrapping of products to avoid diffusion.
Use of refrigeration in food industry • Common applications of refrigeration in food industry: - Dairy products (Milk products) - Cheese - Ice cream - Breweries - Meat industry - Fishing industry - etc
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Concentration of fruit juice • Fruit juice is often concentrated in order to save weight during transportation. • This can be done by: - Concentration using a heat pump - Freeze concentration
Concentration of fruit juice Concentration using a heat pump
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Concentration of fruit juice Freeze concentration
Juice feed Ice
Heat removal
Use only about 1/7 of the energy required compared to ”Heat pump concentration” (ΔhFreeze=1/7·ΔhEvap)
Concentrated juice
Freeze drying • Very expensive • Used for special products where weight is of importance - Pharmaceutical products - ”Hiking food” • Other products: - Coffee powder - Soups - etc
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Freeze drying, cont.
Freeze drying, cont. • Product to be dried should be cut in thin slices to allow for a good water vapor transport. • Process requires a continuous evacuation of air. • Requires more energy than freeze concentration since the water is removed by sublimation
(ΔhSublimation= ΔhFreeze +ΔhEvap).
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Estimation of cooling demand Cooling demand can be estimated by: Mass of product Enthalpy change
& = m ⋅ Δh Q τ Time
Which may be rewritten as: m ⋅ c p ⋅ Δt & = Q τ How do we get the enthalpy change and the specific heat?
Estimation of cooling demand
Freezing of products involves a phase-change.
Figure 12.23a. Data also available in table 12.22.
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Estimation of cooling demand Freezing of products involves a phase-change.
Figure 12.23b
Estimation of cooling demand Products may also have ”internal heat generation” - Heat of curing (ripening)
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Freezing methods • • • •
FloFreeze GyroFreeze Freezing tunnel Liquid nitrogen freezer
FloFreeze Used for freezing small objects: Berries, peas, french fries etc
Fluidized bed Air flow
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GyroFreeze
Air flow
Air flow
Evaporator
Products to be frozen transported on conveyor belt. Used for freezing larger objects: Hamburgers, meat balls, etc
Freezing tunnel
Products to be frozen placed on carts. Sometimes products are already packed.
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Liquid nitrogen freezer
Used for freezing larger objects: Hamburgers, meat balls, etc
Agenda - Course Summary • • • •
Course objectives Course requirements Bonus points About the exam - When and where - Helping aids allowed
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Course objectives After the course the student should be able to Describe different sources of primary energy and assess their environmental impact. Describe the utilisation of energy in the present day society. Explain the physics that govern an indoor climate, and assess the changes needed to improve the indoor climate in existing buildings. Perform heating/cooling load calculations for a single family residence. Design heaters (radiators) and connecting tubing, and select a proper circulation pump.
Course objectives, cont. Design a ventilation system to provide an adequate air flow of a proper temperature and humidity. Explain the basic fundamentals of conventional refrigeration systems, including components like heat exchangers, compressors, and expansion valves. Develop and describe a computer model of a conventional cooling system. Describe different types of heating systems, and assess their applicability. Describe and analyse the function of passive systems. Briefly discuss alternative cooling processes.
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Course Requirements • One written exam, 5 credits • Seven project reports, 3 credits • Active participation at laboratory lessons, 1 credits
Project Reports • SEU-SEM1: Heating demand for a single family residence • SEU-SEM2: Design of heating systems • SEU-SEM3: Design of ventilation systems • Simulation of a refrigeration system (4 reports, SEU-CL1, SEU-CL2, SEU-CL3, SEU-CL4) You are required to submit a written report for each of the assignments. SEM1-3 are individual, CL1-4 can be carried out in groups of 2-3 students.
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Laboratory Lessons Active participation during the lab lessons: SEU-LAB1: Solar Collectors SEU-LAB2: Flow Measurements SEU-LAB3: Basic Refrigeration System SEU-LAB4: Heat Transfer from a Tube and Fin Coil For distance students, corresponding reports needs to be submitted through Bilda.
Bonus points for the exam! In order to promote continuous learning, you will get bonus points on the exam if you submit the reports before the deadline and receive a passing grade.
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Exam - When and where Tuesday December 16, 2008, 08.00-13.00 Rooms M32-36, M263, + Extra room to be announced. Brinellvägen 64. The exam starts without academic quarter!
Exam - Helping aids allowed Allowed: • Handouts available on the course homepage • Lecture notes • Summaries of Labs, Computer labs, Seminar assignments, Exercises • Course literature (SEU+Refrigeration book) • Calculator (not pre programmed) • Dictionaries, if permission given by Hans Jonsson Not allowed: • Solved problems, including instructions and solutions to: Exams, Labs, Computer labs, Seminar assignments, and Exercises. • Books, other than the above stated.
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At the exam • • • •
Fill in the exam cover properly! New question – New paper! Write neatly! Adhere to the rules! Attempts or suspicion of cheating WILL be reported! Rules can be found at: http://www.kth.se/info/kth-handboken/II/11/3bil2.html
• Bring something to eat and drink! • Read through all the questions first!
Help with questions • Wednesday December 10 between 10.30-12.30 and 13.30-14.30 • Thursday December 11 between 13.00-15.30 Session with Centra can be requested by email. Brinellvägen 68, Level 4, Office K429.
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Online Course Evaluation Available on BILDA: - Please help us improve!
Good luck on the exam!
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Agenda - Sustainability Examples • Cogeneration System in Brazil
Combined Gas Turbine Cycle and Absorption Chiller for Electricity Supply and Cooling of a Supermarket 2 10 8
4 Heat Exchanger
Absorption Chiller Thermax THW
Cooling Tower
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5 6
Micro
Turbine
3
7 1 Cooling Load
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Brazil map Natal
Rio de Janeiro
Florianópolis
Cooling Load Coverage by Abs. Chiller, yearly 70,0 60,0
[%]
50,0 40,0 30,0 20,0 10,0 0,0 Florianopolis
Rio de Janeiro
Natal
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[tonnes]
Global warming reduction in CO2 equivalents 1000,00 950,00 900,00 850,00 800,00 750,00 700,00 650,00 600,00 550,00 500,00 Florianopolis
Rio de Janeiro
Natal
Electricity [kW]
Electricity Load and Supply (December, Florianópolis)
40,00 35,00 30,00 25,00 20,00 15,00 10,00 5,00 0,00
Pload Ptot Pout
0
2
4
6
8
10 12 14 16 18 20 22 Time [Hour]
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Urhuset • House made of wood • Only a small amount of electricity used • Heated by fire wood, solar heating, and a ground heat pump
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Snow Cooling Based on Lic.Eng. Thesis by Kjell Skogsberg, Luleå Univ. Of Technology
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