Food Process Design: Overview

Transcription

1 2 Food Process Design: Overview Mohammad Shafiur Rahman and Jasim Ahmed Introduction Generally, a process is defined as a sequence of events directed to a definite end. Processes are termed chemical or food when one or more essential steps involves a chemical reaction or conversion of food from one form to another. Process design refers to the organization of food processes and manufacturing methods, while plant design refers to the configuration of the whole processing plant, including the processing/control equipment, the utilities, the plant buildings, and the waste treatment units (Saravacos and ostaropoulos, 2002). The two terms are used interchangeably in the technical literature. Food processing involves industrial processes in which raw materials are converted or separated into desired products. The food engineer develops, designs, and engineers both the complete process and the equipment involved; chooses the appropriate raw materials; operates the plants efficiently and economically; and finally ensures product safety and quality. No process occurs without many accompanying mechanical steps, such as pumping and conveying, size reduction of particles, removal of water from wet food, evaporation and distillation with attendant boiling and condensation, absorption, extraction, membrane separations, and mixing. Various branches of engineering (e.g. mechanical, chemical, instrumentation, electrical, computer) are relevant to food conversion (product and process principles), preservation, storage and distribution, and food Handbook of Food Process Design, First Edition. Edited by Jasim Ahmed, M. Shafiur Rahman Blackwell Publishing Ltd. Published 2011 by Blackwell Publishing Ltd. Ahmed_0113_c02_main.indd 18

2 Food Process Design: Overview 19 product development and assessment. The engineering components deal with design (estimation of processing time, energy requirements, etc.), fabrication, installation, operation, and maintenance of the equipment used and resources needed in each step. Design is a creative activity whereby an innovative solution for a problem is conceived. The design does not exist at the outset, but the designer starts with an objective in mind and, by gradually developing and evaluating possible designs, arrives at the best way of achieving the target. During the design process, the designer faces various constraints, with economic considerations clearly a major constraint. Process engineering constitutes the specification, optimization, realization, and adjustment of the process as applied to the manufacture of bulk products or discrete products. Simple equations, graphs and standard tables are used to estimate the design parameters of unit operations. Empirical data and rules of thumb are used to facilitate the various design calculations, simplified equations and short-cuts. Advanced levels of process design are required for complicated systems. In more complex systems, computer-aided process/plant design is used. Food process design is somewhat different from other design processes. The main focus is not solely on process improvement but has a strong relation to product development, quality, and sensory attributes. Methods such as quality function deployment and chain information models are specially focused on product design, but are only weakly coupled to the actual processing system. The real challenge for process synthesis in food process design is to develop processes that can consider both the changes in consumer behavior and market demand (Brenner et al., 2005). This requires a high degree of flexibility but mostly the ability to understand and control the crucial product transformations that occur during the process (Hadiyanto, 1997). Components of Food Process Design The design process for a food processing system includes a number of basic considerations. 1. What is the main purpose of each unit operation (i.e. what does the process perform)? This includes preservation, safety, and/or conservation of sensory and/or functional properties. 2. What are the products to be used by the process? 3. Development of process flow sheet, material and energy balances, and preliminary sizing of process equipment. 4. Development of a schematic diagram and the components is the first step in designing unit operations. The design and operation components are as follows. 1. Exploration and selection of appropriate types of equipment. 2. Capital and operating cost for different sizes of the equipment. Ahmed_0113_c02_main.indd 19

3 20 Handbook of Food Process Design 3. Processing parameters and desired product characteristics or parameters. 4. Basic theoretical principles and mode of operation (batch or continuous). 5. Process control, operations, and maintenance of the unit operations. 6. Hygienic design, cleaning and sanitation methods. Unit Operations and Complete Process Because of the variety and complexity of modern processes, it is not possible for one person to remember very single process involved. Each process is divided into a series of steps, called operations, which follow each other sequentially (McCabe et al., 2001). Further, individual operations utilize common techniques based on similar scientific principles. Thus a food processing system can be divided into a number of unit operations in order to develop a product with desired quality and safety attributes. A number of steps are involved in the production of a product from raw materials, and all the steps comprise the complete process. The unit operations describe the details of each step or operation in the complete process. In food or agricultural processing operations, the raw materials used as feedstock usually comprise whole or parts of living plants or animals. These may be converted to yield pure compounds, such as the production of pure sucrose from sugar cane or sugar beet. Alternatively, a portion of a plant or animal may be washed and trimmed (or the inedible tissue discarded) and preserved using heating or cooling, for example seafood, poultry, or various common fruits or vegetables preserved by refrigeration, canning, or freezing. The composition of plant and animal feedstocks may be highly variable. The first question that should be asked is: What do we want to process and what do we want to achieve? For example, what is the temperature at the center of an apple dipped in cold water for 1 hour or how long will it take to reach a temperature of 5 C? Using this example, we need to determine some of the variables, such as time, temperature, and possible changes (quality, chemical reaction). The process variables comprise two types, one due to the material (intrinsic), the other due to the system (extrinsic). In the example described, the selected variables can be listed as mass of apple, surface area and shape, diameter of apple, presence of skin, porosity of apple, viscosity of surrounding fluid, agitation of water (conduction or convection), temperature and pressure of the surrounding fluid, time, initial temperature of apple, initial temperature of medium, and mass of water. In many cases we need to know processing time and conditions, t = f (variables), mainly in order to decide how long to process the product. Process Flow Diagram The purpose of the flow diagram is to visualize the complete process more easily. The level of pictorial information on a flow diagram can be detailed. After developing the Ahmed_0113_c02_main.indd 20

4 Food Process Design: Overview 21 Raw Fish Dried Fish Harvesting Cooling Cutting Pre-treatment Drying Packaging Figure 2.1 Flow diagram for drying fish. flow diagram the process should be analyzed in order to understand its dynamics so that appropriate design rules or prediction procedures can be developed. It can be empirical or based on the fundamentals. For example, Figure 2.1 shows the process flow diagram for preparing dried fish from harvested fish. In this flow diagram, harvesting is one unit operation within the complete process of fish drying. Other unit operations are cooling, washing/cutting, pretreatments (such as salting), drying, and packaging. However, the main unit operation is drying. Different methods can be used to dry fish. Drying in the sun is the simplest method, but the main disadvantage is contamination and product loss by insects and birds. When the climate is not particularly suitable for air drying, or when better quality is desired, mechanical air drying can be used. Other types of drying include in-store drying, convection air drying, explosion puff-drying, spray drying, fluidized bed drying, spouted bed drying, ball drying, rotary drum drying, drum drying, vacuum drying, freeze drying, and modified atmosphere and heat pump drying. Drying operations pose a number of questions. How long should drying continue? How much heat should be supplied? Which type of dryer should be used? What are the operating conditions (e.g. temperature, pressure, air humidity, air velocity)? What are the capital and processing costs? What are the environmental effects of the processing (i.e. pollution)? In developing a product, consideration should be given to the properties of the ingredients and the characteristics of the final product that need to be developed or generated. In generating the desired characteristics of the final product, the processing or operating conditions may need to be varied (Figure 2.2). Codes, Standards, and Recommended Practices Codes and standards allow food process design to be universal, although different countries have different codes and standards. The terms code and standard are Ahmed_0113_c02_main.indd 21

5 22 Handbook of Food Process Design Processing Variables Material Variables [Input] Process Materia l Variables [Output] Figure 2.2 Diagram showing variability of materials and processing variables. used interchangeably, although code is mostly reserved for a code of practice (e.g. a recommended design or operating procedure) while standard is preferred for sizes (e.g. 1-inch nominal size steel pipe schedule number) and compositions. Numerous rules have been developed to ensure safe and economical design, fabrication, installation, and testing of equipment, structure and materials. These rules have been codified by the national standards organizations of each country in consultation with professional societies, trade groups, and government agencies. For example, the American Society for Testing and Materials (ASTM) provides standards related to materials testing including packaging materials, while the International Organization for Standardization (ISO) provides various standards for different food products and processes. Process Severity, Quality and Safety Safety factors must be judiciously applied and should not be compromised by careless design. Process design should be optimal with regard to processing conditions, should be economically justifiable, and safety should be established after careful consideration of all the factors. For example, in newly designed pasteurizer, the residual microbial load after pasteurization should be assessed and compared with the standard when considering if it is a safe and efficient process. In many cases the severity of processing is related to safety and quality (Figure 2.3). For example, the severity of processing results in higher nutritional loss and generally poorer product quality (Figure 2.3a). It is therefore obvious that an optimum combination of processing time and product quality produces the desired food characteristics, with first safety and then other quality attributes secured. In many cases, the correct level of processing severity is required to develop the appropriate flavor and texture. Deep-fat frying is one of the most important unit operations in the catering and food processing industries. Although many aspects of the frying process are still poorly understood, the processing time can be optimized based on the product characteristics (Blumenthal and Stier, 1991). Figure 2.3b shows different degrees of food quality as a function of frying time while Table 2.1 provides more details of the characteristics. Ahmed_0113_c02_main.indd 22 9/28/2011 7:01:36 PM

6 Food Process Design: Overview 23 (a) (b) ndex r Quality In Safety or Optimum Safety Nutrition Sensory Food Quali ity A Optimum B C D E Process Severity Heating Time Figure 2.3 (a) Process severity, safety and quality diagram. (b) The food quality curve as a function of frying time (quality in the identified region is discussed in Table 2.1). (From Blumenthal and Stier, 1991, courtesy of Elsevier.) Table 2.1 Effects of oil quality on the characteristics of fried potato strips. Region a Oil quality Characteristics of fried potato strips A Break-in White, raw interior, no rich potato odor, surface not crisp B Fresh Slight darkening of surface, some crust formation, interior not fully cooked C Optimum Golden brown, oily surface, fully cooked center (ringing gel), rich potato odor D Degraded Oily surface, darkening and spotting of surface, surface hardening, excess oil, center not fully cooked E Runaway Excessively oily (greasy), dark-hardened surface, walls beginning to collapse, center hollow a Regions are shown in Figure 2.3b. Source: Blumenthal and Stier (1991). References Blumenthal, M.M. and Stier, R.F. (1991) Optimization of deep-fat frying operations. Trends in Food Science and Technology 2: Brenner, M. (2005) The chain information model: a systematic approach for food product development. PhD thesis, Wageningen University, The Netherlands. Hadiyanto (2007) Product quality driven food process design. PhD thesis, Wageningen University, The Netherlands. McCabe, W.I., Smith, J.C. and Harriot, P. (2001) Unit Operation of Chemical Engineering. McGraw-Hill, New York, pp Saravacos, G.D. and ostaropoulos, A.E. (2002) Handbook of Food Processing Equipment. luwer Academic/Plenum Publishers, New York, pp Ahmed_0113_c02_main.indd 23 9/28/2011 7:01:36 PM