4.1. ANÁLISIS DESCRIPTIVO
4.1.3. Valoración posturográfica
Only differences to equipment and methods reported in Chapter 6 are stated here.
8.4.1 Sample Preparation
The carrots available locally were 2 to 4 cm. in diameter with a ratio of length to diameter of 4 - 5 (i.e. about 1 3 to 20 cm. in length). To represent the infinite cylindrical shape, each sample was prepared as followed:
(1) The carrot selected was as close to the infinite cylindrical shape as was practically possible. Since it was not perfectly homogeneous or cylindrical, it was peeled and then rounded (using a razor) until the size and shape were uniform. The diameter and length were measured using vernier calipers. The weight and volume (using a water displacement technique) were recorded.
(3) Two copper-constanan thermocouples (28-30SWG) were inserted from each end to the desired depth (about 4 - 8 cm. from the end) in the central axis of the carrot as
indicated in Figure 8. 1 . Another thermocouple was inserted along the radius to the central axis at the mid point of the carrot. The diameter of each carrot was measured at the same level as each of these three thermocouples using vernier calipers. Since the carrots were not peIfectly uniform, even after peeling, the mean of two diameters measured at right angles to each other was calculated for each measurement point. (4) Polystyrene end caps were applied to both ends of the carrot to minimise end effects.
8.4.2 Weight Loss, Water Content, and Surface Water Activity Measurement
The water content of a thin slice of peeled carrot was measured at the end of each cooling experiment by oven drying. The weight of the whole carrot was measured both immediately before chilling (in the incubator, because it was still necessary to maintain uniform initial temperature), and after cooling in the air tunnel, by weighing the total assembly of thermocouple wire, polystyrene end caps etc. However, when a sample was weighed in the incubator, an accurate reading of the true weight was difficult to obtain because of inteIference of the incubator air circulation with the weight measurement system. Further, because only small weight changes were occurring, and due to the awkward shape of the sample, the uncertainty in percentage weight loss data recorded in this manner was significant. Another difficulty was that because weight loss does not stop when steady state is achieved a single percentage weight loss value is much less valuable than a continuing weight change versus time history. Therefore although data are reported in Chapter 9, their usefulness was limited.
Before chilling (in the incubator) and at
f:h
e conclusion of the cooling trial, thin slices of peeled carrot were taken andaw
measured using a Water Activity System model CX-2. The CX-2 uses the cooled mirror (dewpoint) technique for measuring water activity. Because this is a primary measurement of relative humidity based on dew point, no calibration needs to be peIformed. The temperature of the sample should be within 2-3 degrees of the CX-2 temperature so cold samples were rewarmed before measurements were taken. Mter adjusting for temperature, the error in the CX-2 reading should be within ±O.3% ofaw•
8.4.3 Temperature and Relative Humidity Measurement
Temperature and air velocity measurement were described in Section 6.4. The methods used
for relative humidity measurement were different from those in Chapter 6.
A Squirrel Series 1 206 data logger was used. The humidity probes were Capacitive Humidity
Probes in which the sensor is a small plastic capacitor inside a ptfe membrane filter, with a protective guard. Circuits inside the probe handle provide a voltage output proportional to relative humidity. After calibration against saturated salt solution measurement accuracy was
±2 % below 80 % relative humidity and ±3 % above 80 %.
8.4.4 Cooling Trials
( 1) The carrot sample was wrapped with a plastic film which had a very low permeability
to water vapour.
(2) This sample was placed in a thermostatically controlled incubator for 8 - 10 hours to
equilibrate to the required initial temperature.
(3) The refrigeration system and fans in the air tunnel were started and operation stabilised at the desired conditions.
(4) The relative humidity was stabilised at the desired value.
(5) The air velocity in the air tunnel was measured after the con�tions in the air tunnel
are stable.
(6) Sample was weighed.
(7) The test sample was transferred from the incubator in an insulated container. (8) Thermocouples from the product were connected to the data logger.
(9) The plastic film was removed immediately before placement of the product in the air
tunnel.
(10) Sample rotation by a sample oscillator (to minimise position variation of heat transfer conditions) was commenced.
(1 1) Chilling was continued for 3.5 hours to achieve apparent equilibration.
(1 2) The air velocity in the air tunnel was measured prior to sample removal.
(1 3) Samples was reweighed.
(14) Although there were three temperature readings, temperature-time data obtained from the thermocouple which gave the slowest rate of temperature change were used, irrespective of whether this thermocouple was at the position where the largest diameter measurement was made. This selection was made on the basis that the slowest cooling thermocouple was most central in the carrot, and positioning error was more significant than diameter measurement uncertainty.
8.4.5 Heat Transfer Coefficient Measurement
Heat transfer coefficients were determined by cooling every sample in a separate trial which preceded the evaporative cooling trial. All steps described in Section 8.4.4 were applied other than sample weighing and removal of the plastic film.
8.4.6 Experimental Design
Ideally, the conditions used should cover wide ranges likely to occur in practice. However due to the small sample size two difficulties were encountered. Firstly, due to the low thermal mass it was difficult to maintain the sample at a uniform initial temperature if this was well above ambient. A top limit of about lOoC above ambient (30°C) was therefore imposed. Secondly, the total cooling time was short « 3.5 hours) and for very rapid cooling conditions (high air velocity) the time for sample set up became an unacceptably large part of the total experimental time. Therefore the velocity was restricted a maximum of about 3
ms·l. The ranges sought were set at:
air velocity (m S·I) =
relative humidity air temperature (OC)
initial temperature (OC)
= = = 0.5 - 3.0 0.75 - 0.95 0 - 10 20 - 30
Using a normal factorial design, for four variables the number of runs was 16 to which two additional runs at the centre point were added (Table 8. 1). Thus, in total 1 8 runs were planned. The time required to move from one set of experimental conditions to another was considerable, and when runs for heat transfer coefficient measurement were included 36 trials
were required. Further, each pair of trials (with and without a plastic film present) required preparation of a new sample. Therefore to avoid excessive time requirements, the experimental plan of Table 8. 1 was slightly modified in the interests of expediency. Nevertheless, the runs carried out covered a wide range of conditions as intended.