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Deep-fat frying was a process of cooking and drying through contact with hot oil and it involves simultaneous heat and mass transfer. The aims of this research was to study the misture loss and fat uptake phenomenon during sweet potatoes frying using deep-fat frying method. Sweet potatoes were shaped in bar-strip measuring 1x1x5 cm3 and fried with varying temperature of 160, 170 and 180ºC. The moisture loss and oil uptake phenomenon can be analogicially modeled referring to Newton's Law of Cooling, by equation (R2=0,997), (R2=0,997) and (R2=0,997) for moisture loss and (R2=0,95), (R2=0,94) and (R2=0,98) for fat uptake on 160,170, and 180ºC frying temperature, respectively. The activation energies were 2,72 kJ/mol for moisture loss and 4,19 kJ/mol for oil uptake. Moisture and fat content had linier relationship for 170 and 180ºC and non-linier relationship for 160ºC.ABSTRAKDeep-fat frying merupakan proses pemasakan dan pengeringan yang terjadi melalui kontak dengan minyak panas dan ini meliputi perpindahan panas dan massa secara simultan. Penelitian ini bertujuan untuk mengkaji phenomena penguapan air dan penyerapan minyak yang terjadi selama proses penggorengan ubi jalar dengan metode penggorengan deep-fat frying pada berbagai suhu penggorengan. Ubijalar dibentuk irisan balok dengan ukuran 1x1x5 cm3, kemudian digoreng pada suhu 160,170, and 180ºC. Model analogi Hukum Pendinginan Newton dapat menjelaskan dengan baik laju penguapan air dan penyerapan minyak pada penggorengan ubi jalar menggunakan metode deep-fat frying dengan persamaan MR= e-0,0052.t (R2=0,997), MR= e-0,0054.t (R2=0,997) dan MR= e-0,0064.t (R2=0,997) untuk penguapan air dan OR= e-0,0015.t (R2=0,95), OR= e-0,0018.t (R2=0,94) dan OR= e-0,0023.t (R2=0,98) untuk penyerapan minyak pada suhu 160,170 dan 180ºC. Energi aktivasi untuk penguapan air adalah 2,72 kJ.mol dan untuk penyerapan minyak adalah 4,19 kJ/mol. Kadar air dan minyak ubi jalar selama penggorengan berhubungan secara linier pada 170 dan 180ºC dan nonlinier pada 160ºC.

Submitted by Teresa Monteiro (tlago@porto.ucp.pt) on 2011-09-22T11:40:14Z No. of bitstreams: 1 trab-int_2008_ESB_1294_Silva_Cristina_189.pdf.pdf: 1575014 bytes, checksum: 04b3eb618eaf5c5d1ceaf320a2e5d6a7 (MD5) Made available in DSpace on 2011-09-26T14:49:57Z (GMT). No. of bitstreams: 1 trab-int_2008_ESB_1294_Silva_Cristina_189.pdf.pdf: 1575014 bytes, checksum: 04b3eb618eaf5c5d1ceaf320a2e5d6a7 (MD5) Previous issue date: 2008

The influence of pre-frying and final frying time on the crispness of French fries was evaluated by simultaneous analysis of the fracture and acoustic properties during instrumental simulation of human chewing. The analysis of the frequency distribution of the force and sound events corresponding to the crust of the French fry reflected an effect of both pre-frying and final frying time on instrumental crispness. Up to 60 s pre-frying, an increase in the number and magnitude of force and sound events was found, which reflect an increase in crispness. The increase in final frying time increased crispness, but it cannot counterbalance a lack of pre-frying. The moisture content of the whole French fries cannot explained the effect of pre-frying on crispness. Conversely, the effect of pre-frying can be explained when considering only the moisture content of the crust. Pre-frying may allow an easier loss of water from the crust during final frying, which will enhance its crispness.

An experimental design was used to study the effect of pre-drying (to 10, 15 and 20% weight loss) and par-frying conditions (160, 170 and 180 °C) on the crispness of French fries. Par-frying time was adjusted with a software program to obtain equal moisture content and internal texture for all samples. Crispness was evaluated with a sensory panel. Furthermore, samples were analysed with a texture analyser and with confocal scanning laser microscopy (CSLM). Par-frying at 180 °C resulted in a crispier product than at 160 and 170 °C. Pre-drying to 20% weight loss lead to blisters and reduced crispness in comparison with pre-drying to 10 and 15% weight loss. Instrumental texture measurements showed a good correlation with sensory crispness. Large differences in cell structure, such as blisters, could be observed with CSLM. CSLM was useful to explain results from the instrumental and sensory texture evaluation.