Light & Food Preservation

Consumers demand high-quality processed foods with minimal changes in nutritional and sensory properties. Nonthermal methods are considered to keep food quality attributes better than traditional thermal processing. Pulsed light (PL) is an emerging non-thermal technology for decontamination of food surfaces and food packages, consisting of short time high-peak pulses of broad-spectrum white light. It is considered an alternative to continuous ultraviolet light treatments for solid and liquid foods. This paper provides a general review of the principles, mechanisms of microbial inactivation, and PL treatments applications on foods. Critical process parameters that are needed to be optimized for better efficiency of PL treatments are also discussed. PL has considerable potential to be implemented in the food industry. However, technological problems need to be solved to avoid food overheating and achieve better penetration and treatment homogeneity. Besides, more extensive research is needed to understand how PL affects quality food attributes.

Nonthermal technologies

Nonthermal technologies are being applied in food processing as a viable alternative to thermal processing. Traditionally, most foods are thermally processed by subjecting them to temperatures between 60 °C for a few minutes and 100 °C for a few seconds. During this period, a large amount of energy is transferred to the food, which may trigger reactions that lead to undesirable changes or by-products formation. During nonthermal processing, food temperature is held below that achieved in thermal treatments. Thus, vitamins, essential nutrients, and flavors are expected to undergo minimal or no changes.

Pulsed light (PL) is used for the rapid inactivation of microorganisms on food surfaces, equipment, and food packaging materials. The terms high-intensity broad spectrum pulsed light and pulsed white light are synonymous with PL.

Inert-gas flash lamps generate intense and short pulses of ultraviolet (UV) light for microbial inactivation started during the late 1970s in Japan. In 1988, extensive experimentation carried out by PurePulse Technologies Inc. provided a pulsed light process called PureBright® to sterilize pharmaceuticals, medical devices, packaging, and water. The process's efficacy was tested against a broad range of microorganisms, including bacteria (vegetative cells and spores), fungi, viruses, and protozoa. However, the food industry adopted the technology only in 1996, when the Food and Drug Administration approved the use of PL technology for the production, processing, and handling of foods.

Description of PL

PL involves the use of intense pulses of short duration and a broad spectrum to ensure microbial inactivation on the surface of either foods or packaging materials. Electromagnetic energy is accumulated in a capacitor during fractions of a second and then released in the form of light within a short time (nanoseconds to milliseconds), resulting in an amplification of power a minimum of additional energy consumption (Dunn et al. 1995). Typically, the equipment used to produce PL comprises one or more adjustable xenon lamp units, a power unit, and a high-voltage connection that allows the transfer of a high current electrical pulse. As the current passes through the lamp unit's gas chamber, a short, intense burst of light is emitted. The light produced by the lamp includes broad-spectrum wavelengths from UV to near-infrared. The wavelength distribution ranges from 100 to 1,100 nm: UV (100–400 nm), visible light (400–700 nm), and infrared (700–1,100 nm). Pulses of light used for food processing applications typically emit 1 to 20 flashes per second at an energy density in the range of about 0.01 to 50 J cm−2 at the surface (Barbosa-Canovas et al. 1998).

Liquid Foods

Many fluids, such as water, have a high degree of transparency to a broad range of wavelengths, including visible and UV light. Other liquids, such as sugar solutions and wines, exhibit more limited transparency. Increasing the number of solids will diminish the intensity of UV radiation's penetration. In an aqueous solution, the lower the transparency, the less effective the PL treatment. Liquids with high UV absorbance must be treated as a thin layer to reduce the liquid's radiation absorption. In this manner, the liquid's UV absorption is low, and bacteria are more likely to be subjected to lethal doses. The absorbance of clarified fresh juices and juices containing pulp varies considerably. Clarified apple juice has a low absorbance, with absorption coefficients about 11 cm−1, whereas absorbance of orange juice can achieve values close to 50 cm−1. A positive correlation between vitamin C content and the absorption coefficient of clear apple juices was observed.

Conclusions

PL is a novel non-thermal technology to inactivate pathogenic and spoilage microorganisms on foods. The significant microbial reductions in short treatment times, the limited energy cost of PL, the lack of residual compounds, and its great flexibility are some of the technique's major benefits. This method is clearly efficient in inactivating microorganisms in vitro, but its potential for real foods is still under investigation. Further studies need to be conducted to assess PL treatments' effects on food properties beyond safety and spoilage. There is a need for optimizing the critical process factors to achieve the target inactivation level for specific food applications without affecting quality. PL equipment with good penetration and short treatment times need to be designed for commercial purposes. Also, the applicability of PL treatments on an industrial scale needs to be compared with other nonthermal or conventional thermal processes.

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