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Science in kitchen: backing and conservation of food
Prof.ssa Ernesta De Masi

Preface
The kitchen is certainly for a student an interesting place to be explored. We used in the laboratory, for the most activities, the kitchen tools, allowing the scientific exploration of daily phenomena: a such approach to the study of science turns out motivating and exciting.

The teachers team, working on the project, devised a lot of experiments that, coming from the ideas of common sense of the pupils, bring them to the construction of meaningful scientific knowledge as the temperature concept, the energy transfer, the efficiency of thermal machines, the chemical transformations of the foods.

The project involves many subjects:
· Physics: measurement of the efficiency of the moka (coffee machine), the operating curve of a pressure cooker, measurement of the wavelength in the microwave oven, making the ice cream using liquid nitrogen
· Biochemistry: composition of the foods and they transformations
· History: researches on typical foods and traditional process in the conservation and the baking.

The proposed experiments, qualitative and quantitative, carried out in laboratory but also in classroom or at home, privilege the amusing aspect and the method of learning by discovery with a strong involvement of the perceptive and emotional aspects.

Tema affrontato
Il tema affrontato è quello della conservazione e cottura dei cibi. Il lavoro ha coinvolto le seguenti discipline: fisica, scienze, chimica, italiano, storia, disegno e inglese (alcuni materiali sono stati prodotti in questa lingua).

Contenuti innovativi
Il progetto è da ritenersi innovativo per i seguenti motivi:
-L’approccio agli argomenti trattati è sempre di tipo problematico: si parte dall’osservazione di fenomeni che si riscontrano nella vita di tutti i giorni e dalle difficoltà di apprendimento degli alunni. Le lezioni sono di tipo dialogato: si parte da domande stimolo o da esperimenti di tipo esplorativo il cui scopo è quello di motivare ed incuriosire gli alunni ed, in qualche caso, di mettere in crisi le misconoscenze dell’allievo.
-La metodologia prevalentemente applicata è quella costruttivista.
-Gli argomenti sono stati trattati da un punto di vista globale, superando la classica suddivisione per discipline.
-E’ stato privilegiato un apprendimento collaborativo e, quando possibile, è stato favorito l’aspetto ludico e di scoperta.
-Si è fatto largo uso delle nuove tecnologie: le lezioni sono state preparate in forma di ipertesto, gli esperimenti proposti prevedono largamente l’uso di strumentazione on-line, gli alunni sono stati invitati ad usare INTERNET per le loro ricerche, a comunicare tra loro con l’uso di posta elettronica e a consegnare i loro lavori sia in formato cartaceo che elettronico.
-Gli alunni stessi hanno prodotto manufatti costruiti con materiale di facile reperibilità per eseguire gli esperimenti proposti.
-L’esperienza, anche se articolata, e per certi aspetti complessa, è facilmente esportabile in altri contesti sia per la cura con cui sono stati prodotti i materiali didattici, sia per la semplicità delle attività sperimentali proposte, che in alcuni casi si possono svolgere anche a casa.

Modalità di verifica per la validazione della proposta e dei suoi obiettivi
Alla fine di ciascun gruppo di lezioni (unità didattica), agli alunni sono stati proposti dei test di verifica, i cui risultati sono stati analizzati statisticamente. Il docente, per le attività più significative, ha compilato una scheda diario di bordo e schede di osservazione degli alunni.

Organizzazione del lavoro
Il lavoro è stato portato avanti congiuntamente da più insegnanti: alcune lezioni sono avvenute in compresenza. Si è inserita anche l’insegnante di italiano e storia che ha condotto con gli alunni ricerche storiche sull’uso del ghiaccio, in particolare nella nostra città e raccolto espressioni e modi di dire sul freddo nella lingua italiana. La professoressa di fisica ha coordinato il lavoro.
Sono state effettuate numerose riunioni di coordinamento. Gli alunni stessi sono stati impegnati nella sistemazione multimediale dei materiali.

Cooking system.
From microwave oven to interferometer measurement
The magnetron is the microwaves generator and trough a wave guide they are sent to the baking chamber. The sidewalls of the baking chamber reflect the microwaves so to have an uniform distribution on the food. A rotary dish is used to improve this uniformity. The result from the interaction between the food molecules and the microwaves is the heating of the food and the following increase of temperature.

Baking chamber
1. Wave guide
2. Magnetron
3. Rotare dish

Wave length measurement Cooking cheese without rotary dish.

… over the tradition
Induction cooking: no flame
Heating is from eddy current generated in the bottom plate of the pot by magnetic induction.

Induction cooker parts
1 Bottom plate
2 Pyroceram
3 Elettromagnetic field
4 Converter
5 Coil for induction
6 Power supply
7 Control switch

As the heating resistances, the coils are assembled under the cooker plate.

To prepare a scientific coffee
How the moka, coffee machine, is made

Lower water tank with relief valve
The funnel has to be filled by coffee powde
Upper tank and in the centre is a pipe opened on the top
On the top of lower tank there is a filter sustained by a rubber gasket

How it works
When we put the coffee machine on the hot plate, the heat supplied from a power source warms the tank and the water.
The steam coming from the heating of the water increases the pressure in the lower tank.

This pressure push on the water going up trough the little pipe under the filter where is the powder coffee. Finally it arrives in the upper tank, as the famous drink.

The safety valve, installed on the lower tank, is to limit the maximum pressure at a value less than the cracking pressure. For instance, it can happen in the case of high pressing of the coffee powder.

The experiment
We calculated efficiency of the moka that we can consider a thermal machine because there is a conversion from thermal energy to mechanical energy.
We define the efficiency in this application as the ratio between the useful energy and the supplied energy to the machine to produce the drink:
η = Eu / Es
The useful energy is of two terms:
1. the potential energy variation when water goes up from the lower tank to the upper tank
2. the thermal energy because (we drink the hot coffee).

The supplied energy Es is of four terms:
1. Heat supplied to the water inside the lower tank to raise the temperature to the working value.
2. Work to the friction when the water cross trough the coffee powder.
3. The heat to produce the vapour raising the pressure that rises he water
4. The heat to warm the moka
To do the measurement, we installed in the lower tank of the moka, the probe temperature and the pressure transducer.
We weighed the water inside the lower tank, the quantity of the coffee produced and its temperature. We measure besides the distance B1B2 between the centroid of lower tank and the top of the pipe in theupper tank.

Conservation with the heat
The enzymes and the micro organisms are much sensitive to the heat, therefore one of the most effective methods in order to extend the duration of the food is the high temperatures treatment. The enzymes are deactivated in a few minutes in a range of temperature between the 50–80 °C, because they are constituted from proteins denaturating to these temperatures.

The micro organisms are particularly sensitive to the warm wet conditions, that facilitates the coagulation of proteins. Most resistant are the spores, like the Clostridium botulinum (the botulino) resisting to warm wet conditions up to 20 minutes at 120 °C, while the most of the vegetative shapes die at 60 –70 °C. The chemical composition, but over all the low PH, influences very much the heat resistance of the micro organisms.

The acid food, with pH less than 4.5, prevents the development of the organisms spores (like the anthrax and the botulino), and needs therefore of treatments much lighter, because the other sources of contamination are less resistant, as you can see on the following table where it is written the Thermal Dead Time: that is the time necessary to kill a micro organism at a certain temperature.

MICROORGANISMS	MINUTES	DEGEES CELSIUS
Salmonella typhosa	4,3	60
Staphilococcus aureua	18,8	60
Escherichia coli	20-30	57
Streptococcus thermophilus	15	70-75
Lactobacillus bulgaricus	30	71

SPORE DI:
Bacillus anthracis	1,7	100
Bacillus subtilis	15-20	100
Clostridium botulinum	100-330	100
Clostridium calidotolerans	520	100

Conservation with the cold
The low temperatures is important to conserve longer the food because slows down the chemical and enzymatic reactions. The cold slows down, until arresting, the enzymatic activity but not deactives the enzymes acquiring their property when the temperature increases. Because of the slowing down of the enzymatic activity, also the development of the microbes is slowed down: at -18 °C it can be considered null.
The most of the microbes is not killed from the cold. The food conserved with the cold, therefore, must have two important characteristics:
- they must be very cleaned from the hygienic point of view;
- they mustn’t have change of temperature during the conservation

Ice cream by liquid nitrogen
The boiling point of nitrogen, at 760 mmHg, is -196°C and for that it is possible to make ice cream by liquid nitrogen.
Pouring it on mixed ingredients of ice cream, these become solid very fast because they exchange an high quantity of heat for high temperature difference.

The pressure in the kitchen
Pressure cooker - How it is made
The pot is cylindrical vessel closed by an hermetic cover.
There are two valves: the working valve is to have constant pressure ejecting the steam in excess, the safety valve is opened to avoid the burst if the working valve doesn’t work for failure.
How it works
The boiling point of the water, on the sea level, then at 760 mmHg, is 100°C.
During the boiling process, the water temperature is constant. The heat exchanged is only to have the water vaporization but it doesn’t increase the temperature.
If there is hermetic seal, the pressure inside the pot can increase as the temperature in order to the saturation curve.
In the pot, the water begins to boil when the working valve is opened from the vapour pressure of the water being inside.
In this condition the pressure cannot further increase and for that not even the temperature.
The experiment
The working valve is a gravity valve, then we can calculate the pressure weighing the little piston and measuring its section, the temperature is measured by a thermometer. The result agrees to the saturation curve

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