Paper No.: 02
Paper Title: PRINCIPLES OF THE FOOD PROCESSING AND PRESERVATION
Module – 31: Preservation of foods by pulsed light technology.
Paper Coordinator: Dr. P. Narender Raju, Scientist, ICAR-NDRI, Karnal
Content Writer: Dr. Shaik Abdul Hussain¹, Ramesh Pothuraju², Yogesh Khetra¹, Dr.
Vidhu Yadav³
¹Scientist, Dairy Technology Division, ICAR-NDRI, Karnal
²Research Scholar, Animal Biochemistry Division, ICAR-NDRI, Karnal
³Research Scholar, Dairy Technology Division, ICAR-NDRI, Karnal
Preservation of foods by pulsed light
technology
Introduction: Pulsed light
¨ Pulsed light (PL) technology is a non-thermal food processing technique used to inactivate surface micro-organisms on foods, packaging material and equipments
¨ It involves the discharge of high voltage electric pulses into the food product placed between two electrodes for few seconds
¨ PL is also called as pulsed ultraviolet light, high intensity broad- spectrum pulsed light and pulsed white light
¨ PL spectrum wavelength distribution ranges from 100 to 1100 nm:
UV (100–400 nm), visible light (400–700 nm), and infrared (700–1100 nm)
Principle of the pulsed
light system
Principle of the pulsed light system
¨ PL technology involves accumulation of electromagnetic energy in a capacitor during fractions of a second followed by its release in the form of light within a short time (nanoseconds to milliseconds)
¨ This process results in amplification of power with a minimum of additional energy consumption
¨ This amplified high radiation energy is focused on treatment area in the form of pulses to achieve decontamination
Pulsed light system
Fig. 1 Schematic representation of Pulsed light system
Microbial inactivation by Pulsed light
technology
Microbial inactivation by PL
¨ Microbicidal effect of PL may be attributed: rich broad spectrum ultraviolet content, high peak power and the ability to regulate the pulse duration and frequency output
¨ Mechanisms involved in microbial inactivation by PL technology include: photochemical mechanism, photothermal mechanism and physical mechanism
a. Photochemical mechanism
¨ It states that the UV portion PL causes photochemical transformation of pyrimidine bases in the DNA of bacteria, viruses and other pathogens to form dimers
¨ Formation of these dimers prevents DNA unzipping for replication and the organism becomes incapable of reproduction thus leading to death
¨ Shorter wave length (100-280 nm) UV region is mainly responsible for photochemical effects
b. Photothermal mechanism
¨ Photothermal mechanism states that during PL treatment when energy exceeds 0.5 Joule/cm², bacteria can be ruptured by temporary overheating caused by absorption of all ultraviolet light from a flash lamp
¨ Relatively longer wave length UV region and to some extent infrared region of PL spectrum are responsible for photothermal effects
c. Physical mechanism
¨ Physical mechanisms by which microbial inactivation achieved through PL processing include collapse of cell structure, enlargement of vacuoles, increased cell membrane permeability and depolarization of cell membrane
Factors affecting the microbial inactivation
by pulsed light
a. Type of microorganism
¨ Optical properties of cells viz. degree of scattering and absorption of light influence the effect of PL
¨ The nature of the cells i.e. Gram positive or Gram negative etc.
also influence the microbicidal effect of PL
¨ Some microorganisms with efficient DNA repair mechanisms are resistant to pulsed light
b. Interaction between light and the substrate
¨ The composition of the food sample and its surface integrity (rough, smooth) effect the absorption of the PL thereby its effect on microbes
¨ PL absorption on food surfaces reduces its microbicidal effect
¨ For translucent materials, some part of the incident light interacts with the internal structures and causes multiple internal reflections, redirections which decrease the PL effect
c. The distance from the light source
¨ As the distance from light source and depth of the substrate increases microbial inactivation effects of PL diminishes
d. Design of pulsed light system
¨ Proper PL system design with minimal energy losses and maximum efficiency is necessary to achieve effective microbial inactivation
Applications of PL
technology
a. Application of PL technology to Liquid foods
¨ Increasing the amount of solids in liquid foods diminish the intensity of penetration of the PL
¨ Liquid foods with high UV absorbance must be treated as a thin layer in order to increase PL effectiveness
¨ With increase in the number of UV absorbing components in the liquid foods, the effectiveness of PL decreases
¨ The effectiveness of PL in liquid mediums could be increased by minimizing the sample depth, reducing distance between PL source and the sample and/or increasing the treatment time
b. Application of PL technology to Solid foods
¨ Surface topography of solid foods influences the PL effects
¨ PL treatment of solid foods with relatively simple surfaces leads to high levels of microbial inactivation
¨ Part of the radiations of pulsed light spectrum may be absorbed by food constituents on rough surfaces leading to reduction in effectiveness of PL
¨ On complex surfaces like meat etc. PL treatment is not much effective
c. Application of PL technology to food powders
¨ Composition and the colour of the powders influence the microbicidal effects of PL treatment
¨ Coloured powders will absorb more PL and this will reduce its effectiveness
¨ However, thermal effects occurring due to absorption of PL may result in microbial reduction in powders
d. Application of PL technology to
decontaminate food process equipment
¨ Pulsed light treatment is highly suitable for decontamination of the food process equipment surfaces
¨ Surface topography of food contact surfaces affects the microbicidal effects of PL treatments
¨ Smoothest finish surfaces are not suitable for PF treatment because of their highly hydrophobic and reflective nature that leads to cell clustering and thus reduces the PL effect
Merits and limitations of PL
technology
Merits
¨ Inactivation of microbes by PL is a very fast process
¨ Consumption of energy is very less in this process
¨ No toxic chemicals or harmful residues are obtained during PL treatment
¨ Nutritional and sensory qualities of the food are minimally affected
Limitations
¨ Low penetration power of PL which limits its microbicidal action
¨ Limited control on food heating still remains a main concern in PL technology
¨ In food powders the thermal effect of PL results in undesirable colour alterations
¨ Shadow effects due to surface topography of the food samples limit PL efficacy
¨ Foods with rough or uneven surfaces, crevices, or pores are unsuitable for PL
¨ PL is not an adequate technology for cereals, grains, and spices as they are opaque
Suggested Readings
• Koutchma, T., Forney, L. J. and Moraru, C. I. (2010). Ultraviolet light in food technology: principles and applications. CRC Press, Boca Raton.
• Elmnasser, N., Guillou, S., Leroi, F., Orange, N., Bakhrouf, A. and Federighi, M. (2007). Pulsed-light system as a novel food decontamination technology: a review. Canadian Journal of Microbiology, 53(7): 813-821.
• Oms-Oliu, G., Martín-Belloso, O. and Soliva-Fortuny, R. (2010). Pulsed light treatments for food preservation - A review. Food and Bioprocess Technology, 3(1): 13-23.
Learn more
Critical Process Factors in PL technology
• Pulsed Light characteristics - wavelength, intensity, duration and number of the pulses applied
• Packaging and food attributes viz. type, color and transparency
• For fluid food, transparency and depth of the fluid column become critical factor
• The lethality of the pulsed light increases with increasing light intensity or fluence
• PL is non-ionizing and does not penetrate opaque materials, but is transmitted through many packaging materials and therefore may be used to treat packaged products
Do you know?
• There exists reversal mechanisms for PL damage in some microorganisms which include photoreactivation, dark repair mechanism and the spore photoproduct specific repair system
• Photoreactivation means the reversal of ultraviolet damage in bacteria by illumination with visible light
• Dark repair mechanism: it does not require light as photoreactivation does to repair DNA
• The spore photoproduct specific repair system: Spores can repair themselves from the spore photoproduct by the common excision repair system
Do you know?
• Inactivation of spores: Spores may exhibit more resistance to pulsed light due to a thick protein coat. In general, pulsed light is effective in inactivating various spores.
• Inactivation of yeasts: Limited studies indicate that pulsed light can be successfully used for the inactivation of yeasts from buffer, agar surfaces, food or food contact surfaces.
• Inactivation of viruses: Pulsed light was also effective in inactivating viruses. However, the reduction decreased significantly in the presence of protein, suggesting that the efficacy of the technology depends upon the composition of the food matrix.
Glossary
• Fluence rate: Measured in Watt/meter² (W/m²) and is the energy received from the lamp by the sample per unit area per second.
• Fluence: Measured in Joule/meter² (J/m²) and is the energy received from the lamp by the sample per unit area during the treatment.
• Dose: It is used sometimes as a synonym of fluence.
• Exposure time: length in time (seconds) of the treatment.
• Pulse width: time interval (fractions of seconds) during which energy is delivered.
• Pulse-repetition-rate (prr): number of pulses per second (Hertz [Hz]) or commonly expressed as pps (pulses per second).
• Peak power: Measured in Watt (W) and is pulse energy divided by the pulse duration.
E - links
• https://www.youtube.com/watch?v=AQ4BY93Ot8A
• https://www.youtube.com/watch?v=2QmpJM14ajg
• https://www.youtube.com/watch?v=NIuQ_JCj6UM
