The Thin-layer Drying Characteristics of Rosehip

In this study, the influence of air temperature, velocity and humidity on the thin-layer drying of rosehip was investigated. A laboratory air drier was designed and used for drying experiments. The system was operated in an air temperature range of 50–80°C, air velocity range of 1·67–3·10 m s⁻¹ and air absolute humidity range of 0·005–0·08 kg [vapour] kg⁻¹ [dry air]. Six mathematical models available in the literature were fitted to the experimental data. By statistical comparison of the values for the six models, it was concluded that the logarithmic model represents drying characteristics better than the other equations.

Introduction

Dehydration is one of the oldest methods of food preservation as well as an important food processing stage (Lima et al., 2002). Dehydration of foods is aimed at producing a high-density product, which when adequately packaged has a long shelf-life, after which the food can be rapidly reconstituted without substantial loss of flavour, taste, colour and aroma (Sarsavadia et al., 1999).

Owing to the high vitamin C content and excellent taste, rosehip is a product desired by many consumers who are interested in maintaining a healthy diet. Rosehip is also high in minerals (K, P) and other vitamins as well (Yamankaradeniz, 1983). Rosehip has a higher proportion of vitamin C than any other commonly available fruit or vegetable (Demir & Ozcan, 2001). Furthermore, wild fruits have a high phenolic content (Shnyakina & Mallygina, 1975; Oszmianski & Sapis, 1988). Rosehip is well-known for its efficacy in strengthening the body's defence against infection and particularly the common cold (Shnyakina & Mallygina, 1975; Baytop, 1984). The fruit can be processed in diverse ways to produce tea, jam, juice, marmalade, etc. Although this fruit can be grown and harvested seasonally, processing may not keep pace with harvesting, or may be done far from the production site. Therefore, some means of stabilising the fruit by avoiding microbial growth in storage must be provided, and drying is certainly one of them.

Drying of agricultural products under direct sunlight is the traditional way of preservation of a large number of fruits and vegetables. In most producing areas, weather conditions during harvest are not favourable for complete sun drying of fruits and vegetables to safe storage conditions. In addition to this, sun drying has many disadvantages such as dust, insects, likelihood of microbial cross-contamination, etc. (Mathioulakis et al., 1998). All these factors call for artificial drying of agricultural products after harvesting to allow safe storage and retail distribution. Hot air drying is one of the usual unit operations in food processing which decreases drying time and is able to substantially preserve quality of the dried product (Maskan et al., 2002). No preservation operation is known to ‘improve’ fresh fruit quality. At most, drying may help preserve dry matter and some of the fruit's original organoleptic and nutritional properties. Modern methods for designing air drying operations require the mathematical description of food moisture movement during the process, known as drying kinetics (Hernandez et al., 2000).

Among the wide range of mathematical models, thin-layer drying models have found wide application due to their ease of use. They do not require evaluation of many model parameters as is common in more complex representations (Madamba et al., 1996).

Although there are a lot of studies on drying of fruits and vegetables (Madamba et al., 1996; Nieto et al., 1998; Lin et al., 1998; Sabarez & Price 1999; Sarsavadia et al., 1999; Ozdemir & Devres, 1999; Ade Omowaye et al., 2001; Maskan et al., 2002; Togrul & Pehlivan, 2002), no detailed studies were found in the literature which relate to the influence of the drying conditions and to the mathematical modelling of rosehip.

The objectives of this study were: (1) to determine the influence of air temperature, air velocity and air humidity on the drying of rosehip and to obtain drying curves and (2) to investigate a suitable thin-layer drying model for describing the drying process.