1. Introduction
Pumpkin belongs to the genus
Cucurbita, family Cucurbitaceae, with
Cucurbita maxima being the species most commonly found in the Caribbean region. The most used part of the pumpkin fruit is the flesh, which is used as a vegetable in food preparations. Pumpkin seeds are rich in oil, protein and fiber and are a valuable source of minerals [
1], yet despite their high nutritional value, the seeds are usually discarded and are therefore considered a major agricultural waste of the fresh cut produce industry. The most common use of pumpkin seeds, primarily from the hull-less seed type pumpkin varieties, is as a ready-to-eat snack. The seeds are sold in the raw, dried or roasted form and can be salted or flavored with various spices. For the varieties of pumpkins that produce seeds with hulls, the seeds are first subjected to a hull-removal process.
There is a growing interest in the production of pumpkin seed powders, which are made by first drying the seeds and then grinding the dried seeds to produce a fine powder, sometimes called a “flour” due to its appearance. For hull-less seed varieties, the whole seeds may be used in the preparation of the flour or for hulled-seed varieties, the hull can be manually removed prior to drying or roasting [
2,
3,
4,
5,
6]. Due to the difficulty in removing the hulls, seeds are boiled in water for 1 h prior to removal, and in some cases, the dried seeds are subjected to a de-fatting process using n-hexane [
2,
6,
7,
8,
9].
Previous works have focused on the nutritional evaluation and functional properties of the seed powders and the use of the powder in various food applications, especially in bakery products such as cookies and biscuits and in other products such as weaning mixes [
2,
3,
4,
5,
6,
7,
10,
11,
12]. Like the seeds, pumpkin seed powders are reported to be a good source of protein, and depending on the seed type and processing method, an excellent source of fiber and oil.
While drying of seeds for the preparation of powder has been carried out using sun drying, the most common drying method is oven (tray) drying (60 °C, 4–24 h) [
2,
4,
8,
11,
13,
14]. The pioneering research works on the drying characteristics of pumpkin seeds have focused primarily on the drying of salted (25% (
w/w) NaCl, 1 h) seeds with a view to optimizing the drying process to produce a snack [
13,
15,
16]. Drying curves for salted seeds of the hull-less variety (
Cucurbita styriaca) dried at 40–60 °C (0.8 m/s) in a laboratory scale hot air dryer, as well as in the open sun and in a solar tunnel dryer have been presented [
13]. Effective diffusivity values (
Deff) and activation energy (
Ea), were determined and the Moisture Ratio (
MR) data modelled using four well-known drying models. The drying kinetics of salted, whole pumpkin seeds (
Cucurbita spp.) dried in a tray dryer and a fluidized bed dryer at 60–80 °C (0.23–0.28 m/s and 1.8 m/s) was investigated [
15]. The Moisture Ratio (
MR) data was modelled using six thin layer models and the effective diffusivity values (
Deff) were found to be similar to those presented by the previous author for de-hulled seeds, possibly due to the higher temperatures and air velocities used. A comparison of tray and fluidized-bed drying of whole, salted seeds was continued at temperatures of 70–90 °C and air velocities of 2.0–4.0 m/s [
16]. Drying conditions of 70 °C, 2 m/s was recommended for salted seeds.
Fluidized bed drying is a rapid drying method suitable for particulate (20 µm to 5mm) and heat-sensitive food pieces, including seeds [
17]. By forcing hot air through a bed of particles, the particle bed assumes a fluid-like state (hence the term fluidization). High heat and moisture transfer are achieved due to thorough mixing of the material resulting in uniform temperature distribution and increased surface area for drying. While there is potential for use of this type of drying for pumpkin seeds, too rapid drying can, however, result in seed discoloration and undesirable changes in seed texture, as reported for salted pumpkin seeds [
16].
The production of a pumpkin seed powder is an attractive option for the Caribbean Region, where pumpkin seeds are discarded. The varieties of pumpkins that are common to this Region contain seeds with hulls, and hull-removal is a tedious process. No works have been reported on the drying characteristics of whole (untreated) pumpkin seeds using fluidized bed drying method, for the purpose of producing a high protein, high fiber, pumpkin powder. The objective of this study is therefore to investigate the drying behavior of pumpkin seeds dried in a fluidized bed dryer and report on selected quality characteristics of the pumpkin seed powder.
4. Conclusions
Drying of pumpkin seeds for the purpose of making pumpkin seed powders was successfully carried out in a fluidized bed dryer at 50–80 °C. A moisture content of 4–6% (wb) could be achieved by drying seeds for 135, 110, 60 and 40 min at 50, 60, 70, and 80 °C, respectively. The drying behavior was described by the corresponding drying curves and the Alibas model was found to successfully predict the Moisture Ratio (MR) data for seeds dried at all temperatures. Increasing the temperature increased the drying rate and drying occurred in the falling-rate period only, with drying at 60–80 °C occurring in two distinct periods.
Dried seed powders were found to have high protein, fiber and fat contents. Increasing the temperature from 50 to 80 °C had an impact on the lightness of the seed powders, but no browning was observed. Seeds developed a roasted odor at temperatures above 60 °C. This flavor development is considered desirable in some applications. The seed powder could be easily ground into a powder and can therefore be suitable for incorporation into baked goods, where the particle size can be adjusted for the particular application. Further work can investigate the defatting of the dried seeds, depending on the end-use of the powder.