Freezing time prediction for partially dried papaya puree with infinite cylinder geometry
Introduction
Freezing as an important means of food preservation has attracted the attention of many researchers. The design of a freezer is an important task to be fulfilled by the process engineer. Establishment of general, dependable and, if possible, simple models for the prediction of freezing times of foods is essential for the design of freezers. Models which have been proposed in the past for the prediction of freezing times have been reviewed by numerous researchers (Delgado and Sun, 2001, Hung, 1990, Ramaswamy and Tung, 1984). These models may be classified as empirical or numerical models. The majority of empirical models are modifications and extensions of the Plank equation (1941). Lopez-Leiva and Hallström (2003) published a thorough survey on the Plank equation, its modifications and extensions. The Plank equation isUsing and and defining Biot number, one may obtainThis equation only predicts the freezing time for a food at its freezing point. Subcooling is also not considered.
Foods like fruits or vegetables are of cellular structure. During freezing, the ice crystals formed can cause physical rupture and separation of cells. Dehydrofreezing which involves the drying of foods to an intermediate content and subsequent freezing improves texture due to reduction in ice crystal formation. Furthermore, dehydrofreezing reduces the shipping costs due to reduced weight. Ohnishi and Mitawaki (2005) reported that osmotic dehydrofreezing protects the cell structure, in particular the cell plasma membrane, against freezing injury to give reduced softening after freezing–thawing.
Dehydrofreezing has been applied as a tool in kiwifruit preservation mainly due to reduction in the freezable water content (Chiralt et al., 2001, Moraga et al., 2006, Robbers et al., 1997). Dermesonlouoglou et al. (2007) have shown that the quality of frozen tomato can significantly be improved by osmotic dehydration as a prefreezing treatment. Bunger et al. (2004) have considered osmotic dehydration of apple cubes prior to freezing. Dehydrofrozen apple cubes obtained high scores from potential customers. They have also observed that fast freezing was the best process to preserve texture and color.
Papaya is an important tropical fruit. However, its fragility limits transportation to countries in temperate regions. Significant post harvest loss makes the preservation of papayas an interesting field of study (Fernandes et al., 2006).
The majority of the existing empirical and numerical models and the experimental data in literature are based on or has been obtained from test materials which had not undergone partial drying. For partially dried foods the estimation of the physical properties becomes a challenging task. Ilıcalı and Icier (2002) reported numerical data for the freezing time prediction of partially dried papaya infinite slabs with moisture contents 61–87%. No experimental data was reported in their work. Ilıcalı et al. (2007) have developed a numerical model for the freezing time prediction of papaya infinite slabs and tested the model against experimental data. Good agreement was observed between experimental and numerical temperature histories. Ilıcalı et al. (2009) extended their numerical model to predicting the temperature histories of papaya puree with infinite cylinder shape. Satisfactory agreement was observed between experimental and numerical profiles. Numerical models, if they are formulated and implemented correctly, are generally considered to be the most accurate, reliable and versatile freezing and thawing time prediction methods (Cleland et al., 1987). However, it will be more practical to develop simple, accurate empirical models which can be used for freezing time prediction of partially dried products. Therefore, it was considered worthwhile to assess the suitability of some of the existing empirical models in literature in predicting the freezing times of partially dried papaya infinite cylinders by comparing their predictions with experimental data (Ilıcalı et al., 2009). Best performing model was recommended.
Section snippets
Experimental data used and empirical models
Ilıcalı et al. (2009) recently presented experimental data for the freezing time of partially dried papaya infinite cylinders. Papayas purchased locally were peeled and the seeds were removed. The remaining flesh was turned into puree form using an ordinary blender. Only the infinite cylinder shape was considered. Infinite cylinder geometry was realized by insulating the two flat ends of a copper sample holder. The purees obtained were partially dried by using a SHARP Microwave Oven R9H11. The
Results and discussion
Ilıcalı et al. (2009) recently reported experimental data for the freezing time of partially dried papaya infinite cylinders. These data (Table 1) was used to assess different approximate methods for freezing time prediction. As may be observed from Table 1, the initial freezing points decrease sharply with the decrease in the initial moisture content. The accuracy of the thermocouples was ±0.5 °C. This could lead to relatively large errors in the initial freezing points, especially for the
Conclusions
The majority of the existing empirical and numerical models and the experimental data in literature are based on or has been obtained from test materials which had not undergone partial drying. Development of simple, accurate analytical models which can be used for freezing time prediction of partially dried products will be useful.
In this study, some of the approximate freezing time prediction methods developed for fresh produce was used to predict the freezing times of partially dried papaya
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