Analytical Procedures

3.2.1 Berry Sample Preparation

Frozen berries (North line variety) of 2004/05-harvest season were procured from Riverbend Plantations, Saskatoon. Prior to all experiments, frozen berries were taken out of cold storage and thawed at room temperature for 2 h, until the produce temperature was equilibrated. Initial moisture content and total soluble solids (TSS) content were 76% and 15.8⁰ brix, respectively. Prior to individual drying experiments, the whole fruit samples were taken out of cold storage and thawed for 2 h. The moisture content of each sample was measured individually.

3.2.2 Moisture Content Determination

Saskatoon berry samples of 5 g were dried in a vacuum oven at 70⁰C and 25 psi for 7 h to assess their initial moisture contents. This experiment was carried out in three replicates. The initial moisture content (MC) of the saskatoon berries was determined as 76% dry basis (d.b.) according to AOAC Standards (1990).

3.2.3 Total Soluble Solids (TSS) Measurement

Saskatoon berries were manually crushed and the fruit syrup was extracted through cheesecloth. Digital hand held pocket Refractometer (PAL-1, 0-53⁰ brix, and PAL-2,50-93⁰ brix, ATAGO Co. Ltd, Japan) was used to determine the brix level of the fruit syrup. PAL-1 had a measuring range of 0 to 53^o brix and PAL-2 from 53 to 95⁰ brix. For all measurements, 0.3 ml of the fruit extract sample was used.

3.2.4 Dielectric Properties Measurement and Sample Preparation

In order to better understand the interaction of microwave and fruit samples, the dielectric properties measurement was necessary. Also, from modeling point of view this information might be useful. Measurement of the dielectric properties was performed with an open-ended coaxial probe connected to HP 8510B Network Analyzer setup in bioprocessing laboratory. The analyzer generates a microwave signal through the coaxial probes. The material is tested by bringing in contact the flat surface of the probe with the material under test. The fields at the end of the probe “fringe” into the material and change as they come in contact with the material. The network analyzer detects the magnitude and phase shift of the reflected signal and calculates the reflection coefficient. Then graphically computer controlled software calculates the dielectric properties from these data and displays it as a function of frequency.

The coaxial probe (Figure 3.2) is a convenient and broadband technique for lossy (materials with high dielectric loss factor values) liquids and solids. It is non-destructive and little or no sample preparation is required for liquids or semi-solids. In the case of a solid material under test, the material face must be machined at least as flat as a probe face, as any air gap can be a significant source of error. It operates at frequencies between 0.045 and 26.5 GHz. The

technique assumes the material under test to be non-magnetic and uniform throughout.

The HP 8510 is a high performance microwave Vector Network Analyzer (VNA) system. It consists of a HP 8510B, a Test Set (e.g. HP8515A S-Parameter Test Set), and a Microwave Signal Source (HP 8341B). This Agilent VEE (Vector network analyzer driver) controls the HP 8510 "system" as a whole. That is, commands issued through the HP 8510 driver also control the test set and the signal source. The open–ended coaxial probe technique was used to measure dielectric properties of grain samples.

This method was selected for the following reasons:

• It allows simple sample measurement and data analysis,

• The instrumentation is commercially available, and

• Dielectric properties can be obtained over a wide frequency range in a single measurement and with adequate accuracy for thermal calculations (Engelder and Buffler, 1991).

An HP software program provided the permittivity based on the measured reflection coefficient (Engelder and Buffler, 1991). For coaxial probe measurements, the HP dielectric probe kit (HP 8510B) is utilized, which consists of the probe, related software, and calibration standards. The calibration consists of measuring three known standards with the probe (usually open air, short block, and distilled water at room temperature). The calibration process removes systematic errors from the measurement. The operating frequency range of the system was set to 0.5 MHz to 5 GHz. All the measurements were taken at room temperature (23⁰C) and constant bulk density. The menu driven software was installed on the personal computer and the control algorithm based software computes the complex permittivity of the material under test from the S-parameter information, which was relayed from the vector network analyzer (VNA).

To reduce cable flexure and probe motion errors during measurements, a fixture was developed which securely clamps the probe and its cable in a vertical position (as shown in Figure 3.2). The fixture is equipped with a small table on which the calibration standard or material under test is placed. This table is mounted to a manually operated vertical position translator, which allows the operator to raise the material under test up to the probe tip with high positional precision and with the proper contact pressure.

Figure 3.2 Open-ended coaxial probe and adjustable platform

All dielectric measurements were performed at room temperature (23⁰C).

Measurements for dielectric properties were performed using the HP Network analyzer system on fresh and frozen saskatoon berries for the following different fruit conditions:

• Whole berry,

• Cut berry,

• Berry juice / Syrup

Osmotic dehydration process that involves loss of moisture and solid gain can affect the dielectric properties of the fruit. As the drying experiments involved drying of untreated and osmotically dehydrated saskatoon berries under microwave conditions, measurement of dielectric properties before and after osmotic dehydration revealed the effect of osmotic dehydration on drying characteristics and microwave absorption characteristics of the material.