Pertanika 8(1), 79 - 88 (1985) Heat transfer to Liquid Nitrogen Droplets during Cyrogenic Freezing of Foods MOHD. ALI HASSAN, DZULKIFLY MAT HASHIM and RUSSLY ABDUL RAHMAN Department of Food Technology, Faculty of Food Science and Technology, Universiti Pertanian Malaysia, Serdang, Selangor, Malaysia. Key words: Heat transfer; liquid nitrogen droplets; cyrogenic freezing of foods. ABSTRAK Kajian ini dilakukan untuk menyelidiki fenomena pemindahan haba yang terjadi apabila titik-titik cecair nitrogen jatuh ke atas permukaanmakanan. Kepingan gelatin digunakan sebagai bahan makanan. Nitrogen cair dijatuhkan ke atasnya daripada sistem penjatuh/ bakungan. Suhu ke- pingan serta masa penyejatan titik terse but dzukur bagi suatu jangkamasa tertentu. Danpada nz"lai- nz"lai inz; koefisien pemindahan haba dz·kira. Ia dibandingkan dengan n£lai ramalan teori dan didapati nilai yf!-ng dzperolehz" adalah tiga hingga empat kalz" "lebih tinggi. Walau bagaimanapun dengan mengambz"lk£ra bahawa tit£k terse but menjadi semakin ke.c£[ semasa penyejatan, nilai purata kofilisien pemindahan haba di dalam julat terse but menjadi lebz"h hampir kepada nilai teon; ia£tu kuiang danpada dua kali ganda lebz"h tinggi. ABSTRACT This work investigates the heat transfer phenomena that occurs when lz"quid nitrogen droplets fall onto a food surface. A gelatin slab was used as the food material. Liquid nitrogen was dropped on it from a dropper/reservoir system. The temperature of the slab and the droplet evaporation time were measured over a period of time. The heat transfer coeff£c£ents were calculated from these values. These are compared with the theoretical predicted values. It is seen that the experimental values are three to four times hz"gher than the predicted values. However, if the average value of heat transfer coeff£c£ent is taken over the whole size range as the droplet evaporates, a closer agreement is obtained - the experimental values being less than twice hz"gher. INTRODUCTION Industrially, liquid nitrogen is used to cyrogenically freeze foods at it gives very high freezing rates due to its low boiling point of . -195.8°C at atmospheric pressure. An added advantage is that it gives individually quick frozen items. Liquid nitrogen is 0lle of the few refrigerants which can come into intimate con- tact with foodstuffs without causing adverse effects - it is tasteless, chemically inert, sterile and has a very low liquid viscosity. Liquid nitrogen is sprayed onto the foods to provide the cooling effect. Freezing by immer- sion of foodstuffs in liquid nitrogen is generally regarded as an inefficient method despite a high heat transfer coefficient in the nucleate boiling region. This is due to the fact that the spheroidal heat transfer coefficient is a hundred· fold lower, i.e. the coefficient of heat transfer when a solid sphere or food material -is in contact with the liquid refrigerant causing vigorous boiling as a result of the large temperature difference (Anon, 1969).