Food Preservation, Packaging and Storage

Food packaging technologies

Packaging extends shelf life by controlling the environment around the food, and each packaging technology acts on a different factor. Aseptic packaging sterilizes the food and the packaging material separately, then fills and seals the product under sterile conditions, so the product stays free of microorganisms without a heat treatment after sealing.

Vacuum packaging removes air so that the film collapses onto the product and forms a skin-tight wall, which excludes oxygen.

Active packaging goes a step further by incorporating components that play a non-passive role in product development, such as oxygen scavengers, moisture regulators, ethylene absorbers and antimicrobial agents. Ethylene absorbers for fruits and vegetables work either by oxidation, with potassium permanganate as the most common example, or by adsorption on high-surface-area materials such as activated carbon, zeolites and silica gel. Calcium hydroxide is a basic absorbent that removes carbon dioxide and acidic gases, and it does not remove ethylene.

Controlled atmosphere (CA) storage and modified atmosphere packaging (MAP) both extend the life of fresh produce by slowing microbial growth and biochemical activities such as respiration and ripening, but they differ in how the gas is controlled. CA storage actively maintains a set level of oxygen and carbon dioxide, which equipment continuously monitors and regulates, so it achieves greater precision. MAP is passive: the gas composition changes as the produce respires and as gases permeate through the film, and it is not actively regulated after sealing.

In MAP the target gas mixture depends on the product. Cured meat is packed with no oxygen, because O_2 oxidises the nitrosylmyoglobin colour pigment and promotes lipid rancidity. A high carbon dioxide fraction inhibits aerobic spoilage bacteria and moulds, and nitrogen acts as an inert filler that prevents the pack from collapsing. A mixture of 0% O_2, 50% CO_2 and 50% N_2 therefore satisfies these requirements.

Flexible laminates for dry foods are selected for moisture barrier, stiffness, heat sealability and cost. Biaxially oriented polypropylene (BOPP) gives a good moisture barrier and stiffness at low cost, and an LDPE inner layer provides the heat seal, so BOPP/LDPE is a practical and economical choice. Polyethylene terephthalate (PET) and nylon give higher stiffness or oxygen barrier but at greater cost, and polystyrene is brittle with a poor moisture barrier, so it is unsuitable for flexible pouches.

Moisture sorption isotherms

A moisture sorption isotherm is the relationship between the equilibrium moisture content of a food and its water activity a_w at a fixed temperature. Adsorption isotherms are obtained by exposing dry food to increasing humidity, and desorption isotherms by drying a moist food. The two curves generally do not coincide, and the difference between them at a given water activity is called hysteresis.

At a given water activity, the desorption curve lies above the adsorption curve, so a food approached from the moist side retains more water than one approached from the dry side. Hysteresis is commonly attributed to capillary condensation in pores, irreversible changes in pore structure, and swelling or collapse of matrix components.

Temperature also affects the isotherm. At constant moisture content, the change of water activity with temperature follows a Clausius-Clapeyron type relation, \ln\left(\frac{a_{w2}}{a_{w1}}\right) = \frac{H_b}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right), where H_b is the average moisture binding energy, R is the gas constant and T is absolute temperature. Because the bracket is positive when T_2 > T_1, water activity rises with temperature at the same moisture content.

For example, take H_b = 2400 cal mol^{-1} and a_{w1} = 0.40 at 25 ^\circC, with R = 1.987 cal mol^{-1} K^{-1}. Here 1/298.15 - 1/313.15 \approx 1.607 \times 10^{-4} K^{-1} and H_b/R \approx 1208 K, so the logarithm is about 0.194. Then a_{w2} \approx 0.40 \times e^{0.194} \approx 0.49 at 40 ^\circC.

The Guggenheim-Anderson-de Boer (GAB) equation is a multilayer sorption model that describes the isotherm over a wide range of water activity. It accounts for monolayer water, multilayer water and bulk water, which is why it is widely used for food moisture data.