As. J. Food Ag-Ind. 2012, 5(05), 38

As. J. Food Ag-Ind. 2012, 5(05), 388-394


Asian Journal of Food and Agro-Industry
ISSN 1906-3040 Available online at www.ajofai.info


Research Article


Effect of freeze drying on quality of desalted tusk fish

Minh Van Nguyen1,2,3*, Asbjorn Jonsson2, Sigurjon Arason2,3

1Faculty of Food Technology, Nha Trang University, 02 Nguyen Dinh Chieu, Nha Trang, Vietnam.

2Matis ohf./Icelandic Food and Biotech R&D, Vinlandsleid 12, 113 Reykjavik, Iceland.

3Faculty of Food Science and Nutrition, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland.


Email: [e-mail]


Abstract


In this study, influence of freeze-drying on the quality of desalted tusk was investigated. The changes in colour, water content and rehydration ratio were determined and mass balance of the process was also calculated. The results showed that the lightness of desalted tusk was the same with salted tusk. However, the lightness increased significantly after freeze-drying. The rehydration ratio of freeze-dried tusk was high, mainly due to the porous texture of the samples and unchanged structure of protein. The water content of freeze-dried desalted tusk was 1.4%, meaning that about 98% of water in the fish muscle was removed. The process yield was about 21.7%.


Keywords: Brosme brosme, rehydration ratio, colour, microstructure, seafood, Vietnam, Iceland.


Introduction


Drying is one of the most important methods for food preservation and has been shown to be an energy intensive unit operation. It has been used to preserve fish and fish products for a long time in most parts of the world. On the other hand, global demand for dried fish and fish products has also been driven by the colour and flavour of the products. In recent years, much attention has been paid to the quality of food during drying. Both drying methods and quality changes occurring in food muscle during drying affect the quality of dried products [1].


Conventional air drying is one of the most commonly used methods for food dehydration. However, this method has some disadvantages such as long drying time and high drying temperature, resulting in a low quality of the final product. Significant deterioration in quality occurs during the drying process, and the final product has low porosity, low sorption capacity and high apparent density [2]. Currently, there are some alternative drying methods such as heat pump drying, microwave drying and freeze-drying, that have been applied to dry food. These drying methods have different effects on microstructure and the quality of dehydrated products. Different drying

As. J. Food Ag-Ind. 2012, 5(05), 388-394 389


methods produce amorphous materials with various physical properties, for example, particle size, structure, surface area and porosity may differ [3].


Freeze-drying is an innovative drying process and is based on sublimation of ice as its main drying mechanism; this differentiates the process from the conventional drying methods that rely on the capillary motion and evaporation of liquid water for drying. In freeze-drying, water in products is removed by sublimation at low temperature and pressure below the triple point of water. Firstly, water in products is frozen in the pre-freezing stage and it is then removed by sublimation of ice in the primary drying stage. The residual water after the primary drying stage is reduced by desorption in the secondary drying stage [4]. Free-drying provides dried products with porous structure, small or negligible shrinkage, superior flavour and aroma retention and improved rehydration capacity compared to products of other drying methods [2, 3]. It is the best method for preserving materials which are unstable in solution and to dehydrate high quality and heat sensitive products such as quality food, pharmaceuticals and biomedical products [2]. Furthermore, as much as 98% of the water can be removed, the food is lightweight, which reduces the cost of transporting and storage. However, it should be kept in mind that disadvantages of freeze-drying are its high costs, both for capital and running costs and long processing time [5]. These costs are usually passed on to the consumers, which makes freeze-dried food expensive when compared to food preserved by some other methods such as canning or freezing. In spite of this, freeze-drying has been applied in a variety of products from food ingredients and whole ready meals to biological and pharmaceutical products because of excellent quality. In the food processing field, freeze-drying has been applied in many products such as fruit [6], bulbs [7], vegetables and yoghourt [8]. However, in the seafood domain, there are only a few examples of research work that has been conducted, such as freeze-drying of tuna [9] and freeze-drying of cod fish [10].


The objective of this study was to investigate the effect of freeze-drying on the quality of desalted tusk fish. Better understanding about freeze-drying of desalted tusk could be applied to other fish products.


Materials and Methods


Sampling and freeze-drying process
The salted tusk was received from Visir hf., Djúpivogur, Iceland. The fish was desalted at 2oC in an

open plastic container by immersing in tap water without stirring. The tusk was desalted with water at ratio of 1:5 (fish:water) for 24 hours and then at ratio of 1:6 (fish:water) for 72 hours. The desalted fish was freeze-dried in a freeze dryer (VirTis Genessis 25 SQ EL, Gardiner, N.Y., USA) for about 120 hours in 8 different steps with different temperature and pressure in the compartment. The colour, rehydration and water content of final products were determined. The mass-balance for the process was also calculated.


The changes in chamber temperature, condenser temperature, probe temperature of the samples and chamber pressure during the drying process are shown in Figure 1.


Colour measurement
The intensity of the flesh colour was measured by using the Minolta CR-300 chromameter (Minolta Camera Co., Ltd; Osaka, Japan) in Lab* system with CIE IlluminantC. The instrument records the L* (lightness), a* (redness) and b* (yellowness) values on CIELAB colour scale. The a* value describes the intensity in green colour (negative) and in red colour (positive). The b* value describes intensity in blue colour (negative) and in yellow colour (positive).

As. J. Food Ag-Ind. 2012, 5(05), 388-394 390


Three positions (close to the head, in the middle and close to the tail) above the lateral line of the fillets (n=5) were measured. The average L*, a* and b* values of three measurements for each fillet were used to calculate the mean and standard deviation.





















Figure 1. Temperatures and pressure profile during freeze-drying of desalted tusk.


Rehydration determination
The rehydration potential of freeze-dried samples was determined by the method of Nathakaranakule and co-workers [11]. About 10 g samples were immersed in 100°C water. After 2, 4, 6, 8 and 10 minutes of rehydration the samples were taken out and drained for 5 minutes at room temperature. The samples were weighed again and rehydration ratio was defined as the ratio of the weight of rehydrated samples to the weight of freeze-dried samples.


Water content
Water content was determined as weight loss after drying according to ISO 6496 [12].


Statistical analysis
Microsoft Excel 2007 (Microsoft Corp., San Leandro, Calif., U.S.A.) was used to calculate the means and standard deviations for all multiple measurements and to generate graphs. The data sets obtained were compared by multiple comparisons ANOVA by using all pair wise comparisons by SigmaStat 3.5 (Jandel Scientific Software, Ontario, Canada). Significance of difference was defined at p

Kết quả (Việt) 1:

Thực phẩm as. J. Ag-IND 2012, 5(05), 388-394Các tạp chí Châu á thực phẩm và nông-công nghiệpISSN 1906-3040 có sẵn trực tuyến tại www.ajofai.infoNghiên cứu bài viếtTác dụng của làm khô trên chất lượng của desalted tusk cá đóng băngMinh Van Nguyen1, 2, 3 *, Asbjorn Jonsson2, Sigurjon Arason2, 31Faculty công nghệ thực phẩm, trường đại học Nha Trang, 02 Nguyễn Đình Chiểu, Nha Trang, Việt Nam.2Matis ohf. / Iceland thực phẩm và công nghệ sinh học R & D, Vinlandsleid 12, 113 Reykjavik, Iceland.3Faculty của khoa học thực phẩm và dinh dưỡng, trường đại học của Iceland, Dunhagi 5, 107 Reykjavik, Iceland.Thư điện tử: [e-mail]óm tắtTrong nghiên cứu này, ảnh hưởng của đông khô vào chất lượng của desalted tusk được điều tra. Những thay đổi trong màu sắc, nước nội dung và thức tỷ lệ được xác định và cân bằng khối lượng của quá trình này cũng đã được tính toán. Kết quả cho thấy rằng nhẹ nhàng của desalted tusk đã cùng với muối tusk. Tuy nhiên, nhẹ nhàng tăng lên đáng kể sau khi đông khô. Tỷ lệ rehydration của đông khô tusk đã cao, chủ yếu là do kết cấu xốp của các mẫu và không thay đổi cấu trúc của protein. Hàm lượng nước của đông khô desalted tusk là 1,4%, có nghĩa là rằng khoảng 98% nước trong cơ bắp cá đã được gỡ bỏ. Quá trình đã khoảng 21,7%.Từ khoá: Brosme brosme, tỷ lệ rehydration, màu sắc, microstructure, hải sản, Việt Nam, Iceland.Giới thiệuSấy khô là một trong những phương pháp quan trọng nhất để bảo quản thực phẩm và đã được chứng minh là một hoạt động chuyên sâu đơn vị năng lượng. Nó đã được sử dụng để bảo tồn cá và sản phẩm cá trong một thời gian dài trong hầu hết các vùng của thế giới. Mặt khác, các nhu cầu toàn cầu cho cá khô và sản phẩm cá cũng đã được thúc đẩy bởi màu sắc và hương vị của các sản phẩm. Những năm gần đây, nhiều sự chú ý đã được trả tiền cho chất lượng của thực phẩm trong quá trình sấy khô. Cả hai phương pháp sấy và chất lượng thay đổi xảy ra trong thực phẩm cơ bắp trong quá trình làm khô ảnh hưởng đến chất lượng sản phẩm khô [1].Thông thường máy sấy là một trong những phương pháp phổ biến nhất được sử dụng để thực phẩm mất nước. Tuy nhiên, phương pháp này có một số nhược điểm như lâu sấy thời gian và cao khô nhiệt độ, kết quả là một chất lượng thấp của sản phẩm cuối cùng. Sự suy giảm đáng kể chất lượng xảy ra trong quá trình làm khô, và cuối cùng sản phẩm có độ xốp thấp, công suất thấp sorption và mật độ rõ ràng cao [2]. Hiện nay, có là một số thay thế làm khô các phương pháp như máy bơm nhiệt sấy, lò vi sóng làm khô và đông khô, mà đã được áp dụng để thực phẩm khô. Các phương pháp sấy có các hiệu ứng khác nhau trên microstructure và chất lượng của sản phẩm mất nước. Khác nhau làm khô Thực phẩm as. J. Ag-IND 2012, 5(05), 388-394 389phương pháp sản xuất các vật liệu vô định hình với nhiều tính chất vật lý, ví dụ, kích thước hạt, cấu trúc, diện tích bề mặt và độ xốp có thể khác biệt [3].Freeze-drying is an innovative drying process and is based on sublimation of ice as its main drying mechanism; this differentiates the process from the conventional drying methods that rely on the capillary motion and evaporation of liquid water for drying. In freeze-drying, water in products is removed by sublimation at low temperature and pressure below the triple point of water. Firstly, water in products is frozen in the pre-freezing stage and it is then removed by sublimation of ice in the primary drying stage. The residual water after the primary drying stage is reduced by desorption in the secondary drying stage [4]. Free-drying provides dried products with porous structure, small or negligible shrinkage, superior flavour and aroma retention and improved rehydration capacity compared to products of other drying methods [2, 3]. It is the best method for preserving materials which are unstable in solution and to dehydrate high quality and heat sensitive products such as quality food, pharmaceuticals and biomedical products [2]. Furthermore, as much as 98% of the water can be removed, the food is lightweight, which reduces the cost of transporting and storage. However, it should be kept in mind that disadvantages of freeze-drying are its high costs, both for capital and running costs and long processing time [5]. These costs are usually passed on to the consumers, which makes freeze-dried food expensive when compared to food preserved by some other methods such as canning or freezing. In spite of this, freeze-drying has been applied in a variety of products from food ingredients and whole ready meals to biological and pharmaceutical products because of excellent quality. In the food processing field, freeze-drying has been applied in many products such as fruit [6], bulbs [7], vegetables and yoghourt [8]. However, in the seafood domain, there are only a few examples of research work that has been conducted, such as freeze-drying of tuna [9] and freeze-drying of cod fish [10].The objective of this study was to investigate the effect of freeze-drying on the quality of desalted tusk fish. Better understanding about freeze-drying of desalted tusk could be applied to other fish products.Materials and MethodsSampling and freeze-drying processThe salted tusk was received from Visir hf., Djúpivogur, Iceland. The fish was desalted at 2oC in anopen plastic container by immersing in tap water without stirring. The tusk was desalted with water at ratio of 1:5 (fish:water) for 24 hours and then at ratio of 1:6 (fish:water) for 72 hours. The desalted fish was freeze-dried in a freeze dryer (VirTis Genessis 25 SQ EL, Gardiner, N.Y., USA) for about 120 hours in 8 different steps with different temperature and pressure in the compartment. The colour, rehydration and water content of final products were determined. The mass-balance for the process was also calculated.The changes in chamber temperature, condenser temperature, probe temperature of the samples and chamber pressure during the drying process are shown in Figure 1.Colour measurementThe intensity of the flesh colour was measured by using the Minolta CR-300 chromameter (Minolta Camera Co., Ltd; Osaka, Japan) in Lab* system with CIE IlluminantC. The instrument records the L* (lightness), a* (redness) and b* (yellowness) values on CIELAB colour scale. The a* value describes the intensity in green colour (negative) and in red colour (positive). The b* value describes intensity in blue colour (negative) and in yellow colour (positive). As. J. Food Ag-Ind. 2012, 5(05), 388-394 390Three positions (close to the head, in the middle and close to the tail) above the lateral line of the fillets (n=5) were measured. The average L*, a* and b* values of three measurements for each fillet were used to calculate the mean and standard deviation.Figure 1. Temperatures and pressure profile during freeze-drying of desalted tusk.Rehydration determinationThe rehydration potential of freeze-dried samples was determined by the method of Nathakaranakule and co-workers [11]. About 10 g samples were immersed in 100°C water. After 2, 4, 6, 8 and 10 minutes of rehydration the samples were taken out and drained for 5 minutes at room temperature. The samples were weighed again and rehydration ratio was defined as the ratio of the weight of rehydrated samples to the weight of freeze-dried samples.Water contentWater content was determined as weight loss after drying according to ISO 6496 [12].Statistical analysisMicrosoft Excel 2007 (Microsoft Corp., San Leandro, Calif., U.S.A.) was used to calculate the means and standard deviations for all multiple measurements and to generate graphs. The data sets obtained were compared by multiple comparisons ANOVA by using all pair wise comparisons by SigmaStat 3.5 (Jandel Scientific Software, Ontario, Canada). Significance of difference was defined at p<0.05.Results and DiscussionMass balance and changes in water content during the processThe changes in mass and water content of the tusk during the process are depicted in Figures 2a and b, respectively. The mass of tusk varied from different processing steps. In desalting step, fish absorbed water, resulting in a significant increase (p<0.05) in mass (31.1%) (Figure 2a). This result was in accordance with the result of water content. The water content of tusk increased from 61.5% to 81.6% (Figure 2b). The skin removal proportion was 15.8% of desalted tusk. After freeze-drying step, about 98% of water was removed, leading to a significant (p<0.05) decrease in water content (from 81.6% to 1.4%). The process yield was 21.7% compared to the initial weight of the salted tusk.


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