Food Properties Estimation
– Food Packaging Systems
DEFINITION OF FOOD PROPERTY
A property of a system or material is any observable attribute or characteristic of that system or material.
A food property is a particular measure of the foodâ€™s behavior as a matter, its behavior with respect to energy, its interaction with the human senses, or its efficacy in promoting human health and well-being
Thermal properties are related to heat transfer in food, and
thermodynamic properties are related to the characteristics indicating phase or state changes in food.
Mass transfer properties are related to the transport or flow of components in food.
Electromagnetic properties are related to the foodâ€™s behavior with the interaction of electromagnetic energy (e.g., dielectric constant, dielectric loss, and electrical resistance).
Kinetic properties are kinetic constants characterizing the rates of changes in foods.
A sensory property can be defined as the human physiologicalâ€“psychological perception of a number of physical and other properties of food and their interactions.
Health properties relate to the efficacy of foods in promoting human health and well-being.
APPLICATIONS OF FOOD PROPERTIES
An understanding of food properties is essential for scientists and engineers to solve the problems in food preservation, processing, storage, marketing, consumption, and even after consumption.
It would be very difficult to find a branch of food science and engineering that does not need the knowledge of food properties.
Process Design and Simulation
Processing causes many changes in the biological, chemical, and physical properties of foods. A basic understanding of these properties of food ingredients, products, processes, and packages is ssential for the design of efficient processes and minimization of undesirable changes due to
Food properties are used in the engineering design, installation, optimization, and operation of food processing equipments including a complete plant. For example, during canning, foods need to be heated for sterilization. The duration and the temperature at which heating needs to be carried out can be based on quality, safety, nutrition content, and process efficiency. In this case, thermal
properties such as thermal conductivity and diffusivity as well as microbial lethality and nutrition loss are required for all heat transfer calculations and to predict the end point of heating process
Density: Density is the mass of a material per unit volume.
a. True density (Ït): It is the density of a pure substance or a composite material calculated from its componentsâ€™ densities considering conservation of mass and volume.
b. Material density (Ïm): It is the density measured when a material has been thoroughly broken into pieces small enough to guarantee no closed pores remain. It is also known as substance density.
c. Particle density (Ïp): It is the density of a particle that has not been structurally modified and includes the volume of all closed pores but not the externally connected pores.
d. Apparent density (Ïa): It is the density of a substance including all pores remaining in the material.
e. Bulk density (Ïb): It is the density of a material when packed or stacked in bulk and is defined as the mass of the material per the total volume it occupies. The bulk volume of packed materials depends on the geometry, size, and surface properties of the individual particles
Bulk density that takes under consideration all pores inside and outside the individual particles when a material is packed or stacked in bulk can be determined by placing a mass of particles into a container (e.g., measuring cylinder) of known volume
The density of food materials depends on temperature and the temperature dependence of densities of major food components [pure water, carbohydrate (CHO), protein, fat, ash and ice] has been presented
Example 1.4. Calculate the true density of spinach at 20â—¦C having the composition given in Table E.1.4.1.
Taking the total mass of the spinach as 100 g, the mass fraction of each component in spinach is found and shown in Table E.1.4.2.
Specific heat is defined as the amount of heat (J) needed to increase the temperature of unit mass (kg) of a material by unit degree (K). The unit of specific heat, therefore, becomes J/kg-K.
Since most of the food processing operations are either at or in a close range of atmospheric pressure, the specific heat of food products is usually presented at constant pressures. it should be noted that the water content of foods greatly influences their specific heats since water has a much higher specific heat than the other major constituents.
Effect of temperature, on the other hand, is negligible in the unfrozen temperature range while a dramatic effect is observed in the frozen temperature range.
Empirical equations to predict specific heat are based on the composition of the food materials. The earliest equation to predict the specific heat of foods is Siebelâ€™s equation where the data reported for specific heat of food materials are based on the following equations: Equations 16.1 and 16.2 are reported for values above and below freezing, respectively, where Xw is the moisture content of the material in wet basis (fraction) and cp is the specific heat in J/kg-K. Siebelâ€™s equation was only a function of the moisture content
Heldman and Singh suggested the following equation to determine the specific heat of food materials using the carbohydrate, protein, fat, and ash content of a food at 20 oC Gupta suggested the following equation to determine the specific heat of food products as a function of temperature and water content in a temperature range of 303â€“336 K and in a moisture content range of 0.1%â€“80%:
A more comprehensive model including the effect of temperature, in addition to the composition of the food materials, was published by Choi and Okos . Their model is as follows:
The thermal conductivity of a food is an important thermophysical property that is commonly used in calculations involving rate of conductive heat transfer. The rate of heat flow through a material by conduction can be predicted by Fourierâ€™s law as
Miles et al. (1983) compiled the thermophysical properties prediction models of foods and gave
Ramaswamy and Tung, 1981 proposed a correlation to predict the thermal conductivity of fruit juices, sugar solutions, and milk: Salvadori and Mascheroni (1991) proposed a general correlation for meat products. When heat transfer is parallel to fiber,
Thermal diffusivity (Î±) is a physical property associated with transient heat flow. It is a derived property. The unit of thermal diffusivity is m2/s in the SI system.
It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy. Materials with large thermal diffusivity will respond quickly to changes in their thermal environment while materials of small thermal diffusivity will respond more slowly, taking longer to reach a new equilibrium condition
THERMAL DIFFUSIVITY PREDICTION OF FOODS
Thermal diffusivity of a food material is affected by water content and temperature, as well as composition and porosity.
Moisture contents in food and temperature change considerably during most of the food processing operations. Thus, prediction of thermal diffusivity by moisture content and temperature can be used in thermal analysis of food processing operations.
Again the proximate composition of food varies with the type of food product. Therefore, the effect of proximate composition and porosity on thermal diffusivity should be included in the prediction models.
Moreover, the nonhomogeneity of the food materials may cause te diffusivity to vary at different locations in the food materials
Martens performed multiple regression analysis on 246 published values on thermal diffusivity of a variety of food products and obtained the following regression equation:
The heat transfer coefficient h (W/m2K) at a solid/fluid interface is given by
where qlA is the heat flux (W/m2) and /IT is the temperature different (Â°C or K).
The heat transfer coefficient h at a given interface can be determined experimentally by various methods.
In the constant heating (steady state) method, the heat flux q/A is measured (e.g. by electrical measurement) at a given temperature difference AT, and the coefficient h is calculated from Eq above.
In the quasi-steady state method, the heat transfer coefficient is determined from the slope of the heating line of a high conductivity solid, which is assumed to heat uniformly. The heat transfer coefficient can be estimated from the analytical or numerical solution of the heat conduction (Fourier) equation:
General Correlations of the Heat Transfer Coefficient
Correlations of heat transfer data are useful for estimating the heat transfer coefficient h in various processing equipment and operating conditions. These correlations contain, in general, dimensionless numbers, characteristic of the heat transfer mechanism, the flow conditions, and the thermophysical and transport properties of the fluids. Table 9.3 lists the most important dimensionless numbers
used in both heat and mass transfer operations.
Table 9.4 shows some heat transfer correlations of general applications. For natural convection, the parameters a and m characterize the various shapes of the equipment and the conditions of the fluid.
The ratio of tube diameter to tube length D/L is important in the laminar
flow (Re < 2100), but it becomes negligible in the tubular flow in long tubes (L/D> 60). For shorter tubes, the ratio D/L should be included in the correlation.
The viscosity ratio Î·] Î·w refers to the different viscosity in the bulk of the fluid Î· and at the tube wall Î·w. This ratio becomes important in highly viscous fluids, like oils, in which the viscosity drops sharply at the high wall temperatures, increasing the heat transfer coefficients.
Perry’s Chemical Engineers’ Handbook
The resistance to flow is mathematically defined as the shear stress divided by the rate of shear strain. Shear stress is the force acting in the plane of the fluid, and shear rate is the velocity gradient of the fluid between the plates.
Foods exhibit different types of flow. In Newtonian materials, viscosity is not affected by changes in shear rate and remains constant. However, changes in shear rate do affect the viscosity of non- Newtonian materials. Most foods fall into this category and exhibit specific behaviors:
Pseudoplastic: as the shear rate increases, the viscosity decreases.
Plastic (aka viscoplastic): a yield point must be reached before flow begins and the fluid exhibits psuedoplastic behavior with decreasing viscosity and increasing shear.
Dilatant: As the shear rate increases, the viscosity increases and time dependent or directional viscosity characteristics occur.
Thixotropic: Viscosity decreases at a constant shear rate over time.
Rheopectic: Viscosity increases with a constant shear rate over time.
Bingham plastic: Shear stress (i.e., yield point) must be applied to initiate flow and the flow is not affected by changes in shear rate
where Âµ is the Newtonian viscosity (PaÂ·s), t is the shear stress (Pa) and Ğ
is the shear rate (1/s);
And power law model (Ostwald-de-Waale model)
where K is the consistency coefficient (PaÂ·sn), and n is the flow behavior index (dimensionless).
Primary Packaging refers to the package that holds the product, and has the most direct contact with the actual contents. For example, in a 12-pack box of canned soda, the cans are the primary packaging.
Secondary Packaging is the container that holds the primary packaging. For the 12-pack of soda, the box that stores and carries the cans would be the secondary packaging. Kits that bundle together different products are also an example of secondary packaging. Secondary packaging is often ready retail-shelf ready.
Tertiary packaging is used for shipping, bulk handling and warehouse storage, and is not commonly displayed on the retail shelf. Pallets used to transport products are considered tertiary packaging. In addition, stretch wrappers used to unitize pallet loads for easier, more stable transport are also a type of tertiary packaging.
The purpose of packaging
The aims of packaging include:
prevent physical damage, e.g. from knocking, shaking or crushing;
prevent contamination from micro-organisms, pollution or vermin;
protect against dehydration or dampness;
protect the productâ€™s nutritional and sensory characteristics;
keep the product in peak condition;
help to increase a productsâ€™ shelf life.
When designing packaging it is important to consider the following:
Is it easy to handle and open?
Is it a convenient shape, so it is easy to stack?
Which colours will be used on the packaging?
What size of print should be used? (Can consumers read it easily?)
Will it be economical to produce?
What about environmental considerations? (Will it be recyclable or does it make minimum use of natural resources?)
Increased rates of production during the last few decades have made it necessary to use different methods and materials to pack and protect food products.
However, other factors also determine the choice of materials used, especially in relation to food hygiene and safety. For example, the material must be suitable for the food, as some chemicals present in the food or packaging may react together.
Characteristics of a Good food package
Be low cost & non toxic.
Be fat & water proof.
Provide barriers against light, gasses & water vapours.
Provide sanitary protection to the food.
Be of light weight, easily filled, poured & dispose off.
Be sealable, printable, & temper proof.
Have reasonable impact resistance & mechanical strength.
Exhibit adequate transparency where required.
Provide information for nutrition value, nature of product, method of preservation, storage conditions, expected shelf life etc.
Factors considered in selecting a packaging unit
This deal with how best the mechanical & handling properties of the package material cut down the adverse effects of spoilage & deterioration agents of food nutrients with time at convenient & low cost to the consumers.
Spoilage & Deterioration agents
Product & its Nutrients
Rigid & Flexible Metals
Flexible & Rigid cellulosic & Plastics
Flexible & Rigid Paper Products
Laminates & Multilayer Materials
Protective Packaging in Tropical Environment
Rigid & Flexible Metals
Rigid metal containers—-Tin cans
Good appearance & relative low cost.
High chemical inertness & lack of toxicity.
Good mechanical strength & sealing properties.
Good resistance to thermal shock.
Rigid & Flexible Metals
Steps in manufacturing of three piece tin can:
Manufacturing of steel plate
Manufacturing of tin can
Cutting body blank
Notching of body blanks
Forming can body
Sealing the ends
Flexible metal containers:
Aluminum is a metal of low tensile strength but at thickness of 2.5-5mm, it has been successfully made into small cans with sufficient strength to withstand heat processingâ€
Alternative to aluminum:
Manganese & magnesium
Cans were traditionally made from tin plate sheet, but now more commonly aluminium is used (for drinks).
The inside of the can is often sheet coated with lacquers to prevent the cans rusting and reacting with the contents, especially acidic foods.
Reasons for popularity of glass containers:
They are inert.
Adaptive to high speed filling.
Are comparatively cheap & reusable.
Can resist high internal pressure & vertical load.
Deformation can only occur destructively.
Manufacturing of glass containers
Manufacturing by heating at about—1500oC
Ferrous & Manganese—Traces
Poured into moulds in which compressed air is passed from the neck side of container—–curing in oven.
Glass container seal
For normal pressure:
Ordinary stoppers, aluminum caps, metal screw caps.
For high pressure:
Crown corks, external screw caps
Flexible & rigid or semi rigid cellulosic & plastics
â€œThey are formed from basic organic molecules based on cellulose in the case of cellulosic & other organic compounds in case of plasticsâ€
Cellophane, cellulose acetate etc
Flexible & rigid or semi rigid cellulosic & plastics
Polyethylene vinyl acetate
Propylene vinyl chloride
polythene low density is used as a film wrapping, resistant to water. High density is used for boil-in-the-bagâ€™ products;
polyamide (nylon) â€“ provides a very good barrier to oxygen, so used for vacuum packaging, especially for foods containing fat (which can be susceptible to oxidation).
*polyethlene terephthalate (PET) rigid plastic bottles, light-weight,
little risk of breakage and keep the fizz in carbonated drinks;
polystyrene â€“ expanded polythene used for trays and insulated containers to keep food products cold, e.g. ice cream and sorbets or hot, e.g. coffee, soup and burgers.
Packaging film made from LDPE for frozen foods, with a density of about 910 kg/m3 is typically soft, flexible, and readily stretched. It has good clearness and provides a good barrier to moisture but a poor barrier to oxygen. It gives no off-odors or flavors to foods and is readily heat-sealed to
itself. These desirable features, with its very low cost per unit area, have made LDPE one of the most widely used plastic packaging materials. It also shows excellent cold resistance, withstanding extreme low temperature of -70 oC
HDPE has a slightly higher density of about 940 kg/m3 than LDPE, with very little long-chain branching and a greater level of crystallinity.
As a result, it is stronger in tension, stiffer, harder, and more gas-impermeable than LDPE; however, it has reduced clarity and impact resistance resulting from its greater crystallinity.
Strength, perhaps its most important property, is a function of molecular weight.
HDPE is used for packaging films and for applications such as bottles, jars, and vials because of the ease of converting HDPE to blow- or injection-molded containers where it is needed for greater strength, stiffness, and lower clarity.
PET (POLYETHYLENE TEREPHTHALATE ) is a commercially very important food packaging material because at elevated temperatures, it has excellent mechanical properties with inertness to food for reheated frozen foods. Its excellent high-temperature properties led its early use to boil-in-bag packaging and packaging for readymade meals where products are warmed up for consumption without removing them.
Especially oriented PET has very excellent strength and toughness, and possesses better oxygen barrier property, especially for fatty foods, and better CO2 barrier property than any of the common polyolefins such as PE and PP
Flexible & rigid or semi rigid cellulosic & plastics
Flexible & rigid paper products
They are important due to,
Low cost, Low weight
Wrapping papers, bags, pouches etc
Basic material cellulose from,
Wood pulp, straw, jute etc
Paper, board and foil are commonly used to package foods. Board used for food packaging is often coated with a wax of polythene to prevent interaction with contents.
Flexible & rigid paper products
Laminates & multilayer materials
It is made of concentric layers of two or more basic packaging material i.e. as paper, foil, film glued togetherâ€
Produce from flexible laminate made of 2-4 layers which include polyester, aluminum foil, etc.
2-ply: 12um nylon or polyester/70um polyolefin
3-ply: 12um polyester/12umaluminium/70umpololefin
Specific functions of layers
It provide strength, printability & scuff resistance.
Serve as barrier to light, moisture & gas.
Excellent water vapour, gas & light barrier.
Heat transfer characteristics
Provide heat seal integrity, compatibility with food product & strength.
Advantages of using retort pouches
Shorter process time.
Improve product quality due to less overcooking.
No interaction of product & container.
Processing of unique products due to facility of vacuum sealing.
Savings in storage & transportation due to reduce wt & space compared to rigid containers.
Conveniences in cooking & ripening.
Labelling information consists of,
The name of the food
List of ingredients
Net contents and drained weight
Name and address of the manufacturer
Date marking and storage instructions
Instruction for use
Freshnessâ€ indicator labels
Example: Modified atmosphere packaging
Modified atmosphere packaging (MAP) is a technique used to lengthen the shelf-life of food products of minimally processed or fresh foods.
The air surrounding the food in the package is changed to reduce the activity of microorganisms.
Meat, fish, fruits and vegetables often use the method during packaging.
Equilibrium modified atmosphere packaging (EMAP) is most commonly used for cut fresh-cut produce.
Equilibrium modified atmosphere packaging (EMAP)
Micro-perforation adjusts the permeability of the film to the respiration rate of the individual product. Fresh oxygen is drawn in to replace the oxygen consumed by respiration, while excess carbon dioxide can escape.
Types of PlastÄ±c MaterÄ±als Used Ä±n FÄ±lms
Polyethylene Terephthalate (PET)
Polyvinyl Chloride (PVC)
Polystyrene (PS) and Derivatives
Polyethylene Terephthalate Glycol (PETG)
Polyethylene Naphthalate (PEN)
Polyamide (PA, Nylon)
Ethylene Vinyl Acetate (EVA)
Cyclo-Olefin Copolymers (COC)
Polyvinyl Butyral (PVB)
Ethylene Vinyl Alcohol (EVOH)
Polyvinyl Alcohol (PVOH)
Polyvinylidene Chloride (PVDC)
Liquid Crystal Polymers (LCP)
Polyarylamide MXD6 (PA MXD6)
CARTONS FOR LIQUID PRODUCTS
The packaging requirements about barrier properties and hygiene are stringent when packing liquid food products. Seal integrity and seal area contamination are very important.
There are two types of machines:
form a reel, form the package, and fill it in a continuous operation, and
A premanufactured blank
In the Tetra-Pak system, the containers are continuously formed from a roll of packaging material, aseptically filled, sealed, and formed into bricks. The packaging material is a multilayered sheet comprised of printed cardboard, aluminum foil, and plastic layers. An outer polyethylene layer protects the ink layer and enables the flaps to be sealed in the formation of the final brick form. Next, there is a bleached cardboard layer for the graphics and printed material, which provides
mechanical rigidity. A laminated polyethylene layer follows that binds the next aluminum layer to the cardboard. The very thin aluminum foil layer provides the required barrier to gases and light. This is followed by two inner polyethylene layers that provide a liquid barrier. The outside of the package is imprinted with a design that will be exactly in line with the package dimensions. The packaging material is pretreated to facilitate the forming process
All food contact surfaces of both filling and packaging must be sterilized. Two methods are used by the Tetra-Pak to sterilize the food contact machinery surfaces: sterilization with hot (i.e., 360 oC maximum) air by spraying the surfaces with 35% hydrogen peroxide, followed by drying with sterile air. The sterilization time for either procedure is 30 min. The packaging material is continuously sterilized during production by the application of hydrogen peroxide and drying. In
one method, a thin film of hydrogen peroxide solution (15â€“35%) is applied to the inner surface of the packaging material by passing it over a roll wetted with the sterilant. Excess fluid is squeezed off by a roller and the wetted surface is dried under a stream of hot sterile air. In another method, the packaging material is fed through a deep bath filled with a 30â€“40% hydrogen peroxide solution
maintained at a minimum of 70 oC
A. FILLING OF LIQUIDS
Filling machines for liquids can be divided into five basic types, namely: (1) vacuum filling, (2) constant volume, (3) gravity filling, (4) pressure filling, and (5) pressure
Vacuum fillers are of three types: rotary, tray, and automatic feed. On a rotary machine every bottle is handled individually. It is centered under a filling stem, raised, and then filled as it travels around the machine, independent of all other bottles. On a tray-type machine, bottles are placed abreast in trays and rolled on conveyors under the filling head which may consist of one to eight
feeding stems. The automatic feed type will operate by means of a lever that discharges the group of filled bottles and moves the empty bottles into position under these stems
FILLING OF SOLIDS
There are four basic methods of filling solid products into containers. These are volumetric filling, vacuum filling, filling after weighing, and counting. The choice of the method depends on the product to be filled.
Fine powder products can be filled using the vacuum principle, which is basically the same as that used for liquids. A vacuum is drawn on the container and the product flows from the hopper at atmospheric pressure into the container which is less than an atmospheric pressure. If a glass, plastic, or metal container is used which will withstand a vacuum, then no additional problems occur. However, much of this type of product is marketed in a fiber board or paper cylindrical container which is porous and on which no vacuum should be pulled. To overcome this problem, a rigid shroud is placed over the container and the vacuum drawn on it Thin-wall containers that would collapse with normal vacuum are also used in this manner, for example, metal, plastic, fiber, or paper. Wide-body containers with very small necks can readily be filled by vacuum. Vacuum filling is used for fine powders