Acta Univ.

Sapientiae, Alimentaria, 12 (2019) 21–41

DOI: 10.2478/ausal-2019-0002

Effect of UV light on food

quality and safety

J. Csapó1,2 J. Prokisch1
e-mail: csapo.janos@gmail.hu e-mail: jprokisch@agr.unideb.hu

Cs. Albert2 P. Sipos1

e-mail: albertcsilla@cs.sapientia.ro e-mail: siposp@agr.unideb.hu

1
University of Debrecen, Faculty of Agricultural and Food Sciences and
Environmental Management, Institute of Food Technology,
HU-4032 Debrecen, Böszörményi út 138.
2
Sapientia Hungarian University of Transylvania (Cluj-Napoca, Romania),
Faculty of Economics, Socio-Human Sciences and Engineering,
Department of Food Science, RO-530104 Miercurea Ciuc, 1 Libertăţii Sq.

Abstract. The recent years have seen a great number of instances when
ultraviolet (UV) radiation was used in the preservation process of all
sorts of foods. Since the purine and pyrimidine bases of DNA and RNA
absorb well the 254 nm radiation, its application with the use of a correct
dosage can result in disinfections of various orders of magnitude. It can
be particularly effective in cases where technology does not allow a more
intensive heat treatment. When used properly, UV treatment can be a
competitive procedure in the case of foodstuffs where the large surface
area allows for UV rays to penetrate the entire volume of the substance.
Incorrectly applied UV treatment may change the composition of foods.
Free-radical as well as photochemical reactions can digest the proteins,
damage the antioxidants, oxidize the lipids, make changes to the colour

Keywords and phrases: UV light, sterilization, microorganisms, dairy products, food

composition, photodegradation
21
22 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

and substance, and produce undesirable flavourings and odorous sub-

stances. Some vitamins are particularly sensitive to UV irradiation in
the course of which losses could reach even 50%. Photosensitive water-
soluble vitamins are vitamin C, B12 , B6 , B2 and folic acid, while vitamins
A, K and E are the fat soluble sensitive to light, carotene being the only
provitamin with such properties. On the other hand, UV treatment can
be a useful tool of food safety because of the photosensitivity of fungal
toxins.

1 Introduction
The internationally accepted definition of pasteurization is as follows: “All
methods and procedures, or the combination of these, applied on foodstuffs
to reduce the number of pathogenic microorganisms relevant to human health
to a level where, under normal conditions of production, transport, and stor-
age, they cannot constitute a danger to humans.” Treatment with UV rays is
also in correspondence with the above definition provided it complies with the
conditions outlined. There is broad consensus that both traditional and novel
pasteurization procedures need to be validated, and it must be made certain
that these methods will indeed lead to the destruction of the pathogenic mi-
croorganisms most relevant in terms of human health, which are followed by
the authorities’ (in the USA: NACMCF – National Advisory Committee on
Microbiological Criteria for Foods, 2005) licensing procedures for the different
foodstuffs.
UV radiation is a non-ionizing radiation from whose spectrum (140–400 nm)
the wavelengths between 250 and 280 nm can be utilized as a germicide since
the light of this wavelength can be absorbed by both nucleic acids and most
proteins containing aromatic amino acids as well, the subsequent transforma-
tion having the potential to destroy microorganisms. UV lamps have been
widely used before for purposes of air sterilization as well as for late-winter
skin treatment of infants and young children because exposure to UV light
leads to the synthetization of vitamin D, a necessary circumstance for optimal
development. UV radiation can be used for the sterilization of air spaces and
surfaces, its applicability being, however, limited by the fact that its energy
decreases quadratically as distance from the light source grows and that it has
a low penetrating capability. Caution is recommended during its application
as it is harmful to the eyes and may cause conjunctivitis or even skin cancer
when used in large doses (Koutchma et al., 2009).
In food production, UV light is used to increase the shelf life of foods and
 Effect of UV light on food quality and safety 23

reduce the number of pathogenic microorganisms. UV treatment was applied

with good results in increasing the shelf life of fruit juices, various drinks, veg-
etables, fruits, meat, poultry and seafood products, destroying the pathogenic
microorganisms found therein and reducing maturation intensity. The ques-
tion arises, on the other hand, as to what changes can occur in the composi-
tion of foods upon UV treatment. How does photodegradation affect organic
molecules? What changes can photochemical reactions trigger that may ul-
timately have a negative impact on quality and nutritional value? Vitamins
with a high structural diversity are of particular interest, most of them being
potentially sensitive to UV light by virtue of their structure (Koutchma et al.,
2009).

2 Effects of UV-treated milk on microorganisms

Fruit juices and milk are perhaps the two food categories that permit the most
efficient studying of UV treatment effects on microorganisms as both of these
categories make laminar as well as turbulent flow possible, allowing for the
entire volume of the liquid to be exposed to UV treatment.
Upon exposure of goat’s milk to 15.8 mJ/cm2 cumulative UV radiation,
the amount of Listeria monocytogenes decreased by 5-log10 . Applying an UV
treatment of 15 kJ/litre on cow’s milk reduced the number of coliform bacteria
by three orders of magnitude, but there was no significant decrease in the case
of spores (Matak et al., 2005). Under laboratory conditions, when using tubes
permeable to UV light and applying static mixing, UV treatment was not
efficient enough against Mycobacterium avium subsp. paratuberculosis as the
rate of decrease was only half an order–one order of magnitude upon treatment
with a dose of 1 J/ml. Using a special mixing device and the same level of
irradiation improved the rate of decrease to 2.5–3.3-log10 (Altic et al., 2007).
This latter finding also draws attention to the fact that the dosage of UV
light is not the sole determining factor in reducing the number of germs, but
apparatus design is of crucial importance as well. Recently, UV reactors op-
erating in continuous current mode have been developed primarily for the
pasteurization of fruit juices, thus avoiding the formation of turbidity during
treatment. According to one technological procedure, laminar flow is applied
forming an extremely thin layer of film that reduces the path of UV light in
the substance, thus allowing the light to permeate the entire volume of the
substance (Koutchma et al., 2004). Another solution is the application of a
turbulence that allows the total amount of the substance to get in the im-
24 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

mediate proximity of the light source, likewise enabling the light to permeate
all particles of the substance. These devices are currently being tested, in
the course which flow rates, turbulence, or the level of UV irradiation are
optimized.
In order for the conditions to be normalized during UV treatment, exper-
imenters must succeed in exposing all areas of the liquid – whether it is a
laminar or a turbulent flow – to a sufficient dose of UV light that is capable
of destroying the microorganisms. A spiral tubular reactor could offer such a
solution (Koutchma et al., 2007), making possible that all of the treated liquid
gets the optimal UV dose (Forney & Pierson, 2004; Forney et al., 2004).
In the May 2011 issue of New Scientist, heat pasteurization was considered
an alternative method (Gupta, 2011). According to the report, introducing
pasteurization has significantly cut down the number of foodborne diseases
despite not destroying all bacteria. At the same time, however, it reduces the
nutritional value of milk, which is most significant in proteins and vitamins.
Since this is especially the case with colostrum, it has been tested whether
the UV treatment of colostrum would lead to the desired microbe-destroying
effect without the drastic decrease of its immunological value. The question
has been raised as to whether or not UV treatment can serve as an alternative
for pasteurization in the case of colostrum.
In their experiments, they attempted to pasteurize colostrum with UV light
on a farm keeping dairy cows, as it is widely known that immunoglobulins
in colostrum condense due to heat and become immunologically worthless to
the calf. A similar situation prevails during the pasteurization of mother’s
milk by heat treatment for the composition of mother’s milk, considering its
protein fractions, is similar to that of the bovine colostrum. In carrying out
the procedure, the basic assumption was that although the applied dose of UV
light would not destroy the bacteria completely, it would render them unable
to reproduce due to the damage caused in their DNA, while the applied energy
would not damage the immunoglobulins, which would preserve their ability to
provide passive immunity to the calf.
To serve the purposes of the experiment, a device was constructed in which
threaded tubes encircled the UV lamps, allowing the total amount of the
turbulently flowing milk to receive the UV treatment. Upon treatment, part
of the microorganisms was destroyed, but the proteins were not significantly
damaged. Nonetheless, supervisory bodies contend that there are still plenty
of experiments to be carried out in order to prove the applicability of this
procedure for the preservation of mother’s milk (Gupta, 2011).
Pereira et al. (2014) treated colostrum and milk with UV light in an at-
 Effect of UV light on food quality and safety 25

tempt to find out the degree to which bacteria would be destroyed and what
changes would occur in the nutritional value of colostrum and milk, particu-
larly in its immunoglobulin G content. Their experiments were driven by a
USDA statement that 58% of the calves in the US are given unpasteurized
colostrum and milk to drink, which carries the risk of infection. In the course
of the experiment, both the milk samples and the colostrum were exposed to a
continuous UV radiation of 45 J/cm2 . Prior to UV treatment, the colostrum
as well as the sterile milk samples were inoculated with Listeria innocua,
Mycobacterium smegmatis, Salmonella serovar typhimurium, Escherichia coli,
Staphylococcus aureus, Streptococcus agalactiae and Acinetobacter baumannii
microorganisms. The IgG content of the treated and untreated samples was
continuously determined with the ELISA method.
It has been established that UV treatment significantly reduced microbial
count in milk (log CFU/ml) in the case of Listeria monocytogenes (a de-
crease of 3.2 ± 0.3 log CFU/ml), Salmonella spp. (3.7 ± 0.2 log CFU/ml),
Escherichia coli (2.8 ± 0.2 log CFU/ml), Staph. aureus (3.4 ± 0.3 log CFU/ml),
Streptococcus spp. (3.4 ± 0.4 log CFU/ml), and A. baumannii (2.8 ± 0.2 log
CFU/ml). UV treatment did not result in a significant decrease in the case of
M. smegmatis (1.8 ± 0.5 log CFU/ml), whereas with colostrum significant de-
crease was observed for Listeria spp. (1.4 ± 0.3 log CFU/ml), Salmonella spp.
(1.0 ± 0.2 log CFU/ml), and Acinetobacter spp. (1.1 ± 0.3 log CFU/ml), but
for E. coli (0.5 ± 0.3 log CFU/ml), Strep. agalactiae (0.8 ± 0.2 log CFU/ml),
and Staph. aureus (0.4 ± 0.2 log CFU/ml) the decrease did not reach one
order of magnitude. The UV treatment of colostrum resulted in an average of
50% decrease in IgG content.
Donaghy et al. (2009) studied the destruction of the various strains of
Mycobacterium avium ssp. paratuberculosis (Map) in milk as an effect of UV
treatment. Milk treated at ultrahigh temperature was inoculated with various
strains of Mycobacterium avium ssp. Paratuberculosis and then treated with
UV light of 0–1836 mJ/ml in a 20 litre reactor. Following treatment, the mi-
croorganisms were grown in an appropriate culture medium, and then their
number was determined. It has been established that destruction took place
at an order of magnitude of 0.1–0.6 log10 . They concluded that UV radiation
treatment alone is not suitable for the destruction of pathogenic microorgan-
isms, so it is advisable to be combined with other procedures. They take the
view that milk is an inappropriate medium because UV rays can find their
way and take effect with difficulty through the opaque liquid. In one of their
studies, Donahue et al. (2012) ascertain that heat treatment significantly re-
duces total germ count in colostrum, while its IgG content barely undergoes
26 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

any change at all. First-milking colostrum was heated to 60 ◦ C and kept in

a pasteurizer tank for 60 minutes, and then the total germ count and im-
munoglobulin G content of colostrum were determined. After analysing 266
unique colostrum samples, they pointed out that due to heat treatment the
total germ count of colostrum and the number of coliforms decreased by 2.25
log10 and 2.49 log10 respectively, but this heat treatment had only a slight
impact on the IgG concentration of colostrum. They found that heat treat-
ment of colostrum at 60 ◦ C for a period of 60 minutes makes it possible to
reduce the number of microorganisms by two orders of magnitude, while the
IgG concentration of the colostrum suffers only minimal changes, and such a
heat-treated colostrum can be applied safely to provide passive immunity to
the calf.
Teixeira et al. (2013) studied the effects of heat and ultraviolet light on
colostrum and hospital milk and analysed the impact of these treatments on
calves’ health and the growth parameters. The declared aim of the study was
to examine the effects of heat and UV treatment on the total microbial count of
milk, on the immunoglobulin G and lactoferrin concentration, and on calves’
health, growth, and blood serum IgG level. Part of the colostrum samples
was heat treated at 63 ◦ C for 60 minutes, whereas another part of them was
exposed to UV radiation of 45 J/cm2 , and test results were then compared to
untreated milk samples. One part of hospital milk samples was heat treated
at high temperature (72 ◦ C) and for a short time (15 seconds), while the other
part was exposed to UV radiation of 45 J/cm2 , and test results were then
compared to untreated milk. They showed that heat treatment reduced the
number of microorganisms more efficiently than UV treatment and that both
IgG and lactoferrin concentration were significantly lower in treated milk when
compared to raw milk. A comparison of hospital milk samples demonstrated
that high-temperature heat treatment reduced the concentration of lactoferrin
as compared to raw or UV-treated milk. An analysis of the IgG concentra-
tion of calves’ blood serum showed that none of the treatment types had a
significant effect thereon.
Singh & Ghalya (2006) studied the efficiency of cheese whey sterilization by
applying 5–70 ml/min flow time in a traditional and a spiral UV reactor. Test
results in the traditional reactor failed to get close to a 100% efficiency, but
attempts made in the spiral reactor approximated this level when flow rate
ranged between 35 and 40 ml/min. In the case of both reactors, they managed
to balance the lowering of treatment temperature by increasing the flow rate.
100% efficiency could only be achieved with an extremely long (45 and 240
minutes) flow time.
 Effect of UV light on food quality and safety 27

Further developing their methodology (Singh & Ghalya, 2007), they de-
signed an UV spiral reactor for the sterilization of cheese whey, and then
compared its antimicrobial effect with that of a traditional UV reactor. Both
reactors were tested at equal volumes and at different (5, 10, 15, 20, 25, 30,
35, 40, 50, 60 and 70 ml/min) flow rates. It was found that despite the turbid
nature of whey both reactors could be used with great efficiency for steriliza-
tion. Technical problems occurring in the spiral reactor were much fewer in
number than in its traditional variant.
During the sterilization process of cheese whey, Mahmoud & Ghalya (2005)
studied – at different values of fluid thickness and after different retention
times – the obstructions formed in an UV tubular reactor as well as the com-
position of the substance responsible for the clogging. Substances precipitated
on UV lamps significantly reduced sterilization efficiency. A close correlation
was found between the degree of obstruction and the applied temperature.
63.5–77.2% of protein, 12.6–16.5% of fat, and 6.5–9.5% of minerals were mea-
sured in the dry matter content of the substance causing the blockage, which
values were about 1%, 0.5%, and 0.4%, respectively, in the case of whey. Upon
reducing the layer thickness of whey, the amount of precipitated matter in-
creased. High temperature and low pH were favourable to precipitation, whose
mechanism was explained with adsorption and direct exchange. It was estab-
lished that contact between the flowing substance and the quartz wall must
be reduced as that may also be the agent responsible for precipitation during
the UV sterilization process.

3 Effects of UV irradiation on fungal toxins

Toxins are secondary metabolites of microscopic fungi, representing serious
food and feed safety hazard for both humans and animals (Beardall & Miller,
1994) and resulting in a disease called mycotoxicosis. Although these com-
pounds are generally heat-stable molecules, for example, the aflatoxins are
decomposed only above 268–269 ◦ C (Peng et al., 2018), a specific wavelength
of light sources can result degradation. Even the sunlight is useful for de-
creasing their concentration – direct solar irradiation applied on poultry feed
for 3–30 hours resulted in a 25–60% decrease of aflatoxin B1 (Herzallah et
al., 2008), but the UV light has a stronger decomposing effect of mycotox-
ins due their photosensitivity. Murata et al. (2008) evaluated the effect of
mild and strong (0.1 and 24 mW/cm2 ) UV irradiation at a 254 nm wave-
length on toxin-contaminated feed samples and found that even a low dose of
28 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

irradiation totally decomposed the initial 30 mg/kg zearalenone (ZEN) and

deoxynivalenol (DON) concentration after 60 minutes, while a higher dose re-
sulted in a more rapid elimination. Murata et al. (2011) also evaluated the
1.5 mW/cm2 intensity UV-C treatment at the same wavelength on artificially
DON-contaminated corn silage and found a 21–22% decrease after a 30- and
60-min. treatment. Jajic et al. (2016) confirmed the DON-decreasing effect
of both UV-A and UV-C radiation on naturally contaminated maize but un-
derlined that the change is not consistent, maybe due to the uneven toxin
distribution; therefore, the results of the evaluation on artificially contami-
nated solid samples and contaminated homogeneous solutions can be taken
into account only to a limited degree on natural solid samples. They found
the UV-A treatment much more effective than UV-C. Aflatoxins have been
referred to as UV-resistant toxins for a long time because the 254-nm wave-
length irradiation was found to have no effect, but Patras et al. (2017) applied
medium-pressure UV lamp light in the 200 to 360 nm wavelength region in
different doses (from 0 to 4.88 J/cm2 ) on aflatoxins dissolved in pure water
and found that the highest dose resulted 67%, 30%, and 98% reduction for
AFG1 , AFB2 and AFB1 , respectively, and noted that aflatoxins had an ab-
sorption maximum at 320 nm. Dong et al. (2010) made similar examinations
on patulin-contaminated apple cider with 14.2 to 99.4 mJ/cm2 UV-C treat-
ment, found rapid decrease in toxin content (9.4 to 43.4% decrease within
15 s), and observed that the dose–effect connection is strongly linear.

4 Effects of UV irradiation on the quality of milk

and dairy products
It is common knowledge that milk and dairy products are highly sensitive to
UV irradiation since quality deterioration occurs very quickly if kept for a
longer period of time in a glass recipient or in a translucent polycarbonate
packaging, whereas opaque multilayer packaging protects them against deteri-
oration. Milk can very easily produce foul-smelling compounds that remind us
of burning protein, but a cabbage-like taste can also be easily formed as well
as the oxidation of fats and unsaturated phospholipids, which occurs during
the photochemical reaction and leads to the development of oxidized flavour
(Spikes, 1981).
With milk, it is well known that upon UV treatment its vitamin D con-
tent increases in the transformation of 7-dehydrocholesterol into vitamin D.
In addition to this useful transformation, a great number of valuable compo-
 Effect of UV light on food quality and safety 29

nents can deteriorate on this wavelength and foul-smelling products can be

generated. It has also been found, however, that the treatment increasing
the vitamin D content of milk does not reduce the concentration of carotene,
vitamin A, thiamine, and riboflavin.
Matak et al. (2005) attempted to reduce the population of Listeria
monocytogenes in goat’s milk by applying UV treatment. Certain types of
goat’s cheese are made of raw milk, which increases the products’ food safety
risk. Gourmets continuously look for these products in the supermarkets,
which makes risk reduction a significant issue. As the U.S. Code of Federal
Regulations and the Pasteurized Milk Ordinance strictly sets out the rules
for pasteurization, UV treatment could serve as an alternative to heat treat-
ment, while those substances responsible for the gourmets’ preference for goat’s
cheese would not be damaged. In the course of the experiment, fresh goat’s
milk was inoculated with 107 CFU/ml of Listeria monocytogenes (L-2289) and
then treated with UV light using a dosage ranging between 0 and 20 mJ/cm2 .
They managed to achieve a decrease of more than 5 log10 when the cumulative
UV dose reached 15.8 mJ/cm2 . Their experiment clearly indicated that UV
treatment is appropriate for reducing the number of Listeria monocytogenes
by several orders of magnitude.
Exposing goat’s milk to UV treatment of 15.8 mJ/cm2 , thiobarbituric acid
test was used to measure the oxidative and hydrolytic degradation processes.
They found that UV treatment increased the amount of thiobarbituric acid–
active compounds and the degree of acidity. It has been shown that not only
lipase activity but UV treatment was also responsible for the increased amount
of free fatty acids and that the amounts of pentane, hexane, and heptane also
increased owing to UV treatment; what is more, treatment performed at 254
nm caused the milk to smell like cabbage (Matak et al., 2007).
Lu et al. (2011) developed a new technology for reducing the bacterial count
of milk. In the process, milk was transferred through a quartz spiral helical
tube while being blasted with UV rays and radio frequency radiation of 2.65
MHz. It was found that decrease in microbial count is significantly affected
by flow rate, the internal diameter of the quartz tube, the UV light sources
of various quality, and the different types of bacteria. According to them,
the apparatus functioning with electrodeless UV lamp was more efficient in
destroying microorganisms compared to the traditional, low-pressure, high-
intensity mercury-vapour lamp. When the UV dose reached 21.3 mJ/cm2
at a 28.8 litre/hour flow rate and at 1.5 mm diameter, total bacteria count
decreased by 6 log10 . Upon repeating their experiment with milk and applying
a dose of 21.3 mJ/cm2 , total bacteria count decreased by 3–4 log10 orders of
30 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

magnitude in the case of microorganisms such as Salmonella and Shigella

spp., Listeria monocytogenes, Staphylococcus spp., Enterobacteriaceae, lactic
acid bacteria, or pseudomonades. It has been found that electrodeless UV
source is less energy consuming, requires less space, and its operation is much
simpler too than with heat treatment. They came to the conclusion that this
new technology is viable and can replace heat treatment methods.

5 Effects of UV irradiation on food composition

In summary, it can be said that properly applied UV treatment can be a
competitive method for decreasing the number of harmful microbes in foods if
we do not wish to use thermal processes. At the same time, UV treatment may
adversely affect food composition since a number of harmful photochemical
reactions may be activated following the formation of free radicals, which may
reduce the number of valuable food components (Lu et al., 2011). Undesirable
reactions reduce vitamin content, digest proteins, destroy antioxidants, oxidize
lipids, cause changes to substance and colour, and leave undesirable smells
(Koutchma et al., 2002; Adhikari & Koutchma, 2002).
UV treatment of citrus fruits can modify their taste and reduces β-carotene
as well as vitamin A and C content in all fruit and vegetable juices (Guerrero-
Beltrán & Barbosa-Cánovas, 2006). The aforementioned call our attention to
the fact that UV radiation treatment adopted for microbial inactivation must
have an intensity that will cause minimal changes to the nutritional value and
palatability of foods (Noci et al., 2008).
Photochemical reactions have the greatest impact on the components that
are capable to absorb UV light such as vitamin A, riboflavin, vitamin C, and
a few food colourings. Others have reported that vitamin A and β-carotene
decreased only when food was irradiated with visible light. With liquid milk,
the oxidation of ascorbic acid in the presence of riboflavin was activated by
superoxide anion (Koutchma & Shmats, 2002).
UV treatment may have significant effects in the case of foodstuffs con-
taining large amounts of unsaturated fatty acids when these, affected by free
radicals, oxidize, thus producing a rotten smell and reducing antioxidant ef-
fect. We have no data as to whether any toxic substances are produced upon
UV treatment that may present a risk to human health (Koutchma et al.,
2009).
 Effect of UV light on food quality and safety 31

6 Measuring the effects of UV treatment

Effects of UV treatment to food quality can be measured by drawing conclu-
sions from the organoleptic properties (appearance, colour, fragrance, smell,
texture, and flavour) and by determining the colour, pH, and chemical com-
position, including first of all vitamins, which are perhaps the most sensitive
to UV treatment. Our examination must also comply with the sample type
we intend to evaluate as different foods are not equally sensitive to UV light
(Heiss & Radtke, 1968). Sour cream, whipped cream, dried vegetable soup
powder, butter, margarine, and mayonnaise proved to be the most sensitive to
sunlight and fluorescent light, and so these gave off an unpleasant smell after
a short period of UV treatment. 1–3 days of irradiation was necessary for
sugar, chocolate, cheese, bacon, raw sausage, green beans, and salty peanuts
to undergo visible changes. Rice and potato chips making up the third group
showed significant changes only after 4–7 days, while 10–30 days were neces-
sary for pastries, almond, and split peas to show such changes. These changes
are, of course, affected by the wavelength of UV light to which the given food
shows sensitivity, the transparency of packaging, and treatment temperature
(Koutchma et al., 2002).

7 The photodegradation of organic molecules

In direct photochemical reactions, light energy gets directly absorbed, after
which the chemical reaction as well as the changes in food composition take
place. The chemical reaction is dependent on the photon’s energy and light
exposure time. The energy of a 254 nm UV photon corresponds to 472 Joules,
which can be suitable both for loosening the bond between the O-H, C-C, C-H,
C-N, H-N and S-S and for the activation of various chemical reactions. In the
case of indirect photochemical reactions, there are one or two components in
the system that are sensitive to light exposure, which then launch a series of
reactions that can yield several kinds of products.
Direct photochemical reactions depend on the wavelength of the adopted
light for that will determine the photon’s energy and the wavelength of light
which the molecule in question will be able to absorb. Once the photon has
been absorbed, the molecule enters an excited state and undergoes a pho-
tochemical change during which it may dissociate into radicals, may isomer-
ize, dimerize, or form ions. Free radicals and ions are particularly reactive
intermediates that can enter into fast, additional reactions with other food
components, in which end-products are created.
32 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

Photo-oxidation is one of the most sensitive reactions to light. In the course

of this, photosensitive intermediate products get from ground state to a short-
lived excited state and then transform into long-lived intermediates. In the
following, these will transform into end-products either in free-radical reac-
tions by way of hydrogen/electron transfer or through energy transfer. In
this process, hydrogen peroxide or superoxide anion is produced, for instance,
which are able to react with many kinds of food components (Koutchma et
al., 2009).
Nucleic acids are especially great absorbers of 254 nm UV light. Only purine
and pyrimidine bases absorb in DNA and RNA, while the structural framework
of nucleic acids, the phosphodiester bonds, do not absorb at this wavelength.
Those components are sensitive to 254 nm that contain conjugated bonds
such as compounds containing aromatic and double rings, while disulphide
bridges are also sensitive absorbents. Proteins are sensitive to this wavelength
only if they contain amino acids with aromatic rings (phenylalanine, tyro-
sine, tryptophan) or if they have a disulphide (S-S) bridge. Vitamins A, B12
(cyanocobalamin) and D, folic acid, vitamins B2 (riboflavin or lactoflavin)
and E (tocopherols), the aforementioned tryptophan, unsaturated fatty acids
in oils and fats, and the unsaturated fatty acids of phospholipids are all ex-
tremely sensitive to UV light. Literature data suggest that the structure of
vitamin D can change upon exposure to UV light, and intermediate superoxide
radicals can also enter into reaction with vitamin K (Spikes, 1981). Visible
light does not affect ascorbic acid, but it strongly absorbs UV light at 254 nm;
at this wavelength, plant pigments too are highly light absorbent.
In assessing the effects of UV light, a problematic issue may be that the
absorption of the various light-sensitive compounds was studied in clear solu-
tions, but there are very limited data on transformations in complex matri-
ces such as foods. Generally speaking, organic molecules containing unsatu-
rated bonds are strong UV absorbents. The longer the system of conjugated
bonds is, the higher the wavelength will be at which maximum absorption
takes place. Heterocyclic aromatic compounds such as purine and pyrimidine
bases and, e.g., aromatic side-chain amino acids show strong absorption at
254 nm, with maximum absorption values sometimes reaching above 300 nm
(Spikes, 1981).
Carbohydrates are not particularly sensitive to light, but some carbohy-
drate derivatives, such as sugar alcohols or saccharic acids, can be sensitive
to light, and upon its absorption the fragmentation of polysaccharides can
take place, thus changing, for instance, the properties of fruits and vegetables.
Research indicate that UV light accelerated the oxidation of fats and oils.
 Effect of UV light on food quality and safety 33

Of the essential amino acids, histidine, phenylalanine and tryptophan showed

significant decomposition levels when exposed to UV light as a result of which
their protein structures underwent certain changes that modified the solubil-
ity, thermal sensitivity, mechanic properties and enzymatic digestibility of the
protein; what is more, in the process of treating milk, for instance, unwanted
odorous substances appeared in significant amount.
Thanks to the special pigments, foods take on a characteristic colour which
can substantially change upon UV treatment, although it is precisely UV light
that promotes the formation of certain pigments.

8 Effects of UV light on vitamin content

UV treatment of vitamins gives us cause for concern since many of the vitamins
are sensitive to light, especially to UV radiation. Photosensitive water-soluble
vitamins include vitamins C, B12 , B6 , B2 and folic acid, while vitamins A,
K and E are among the photosensitive fat-soluble vitamins, carotene being
the only provitamin with such properties. Most experiments were performed
in the 290–700 nm wavelength range, and very few were carried out in the
240–260 nm range so crucial for disinfection (Ye, 2007). Upon examination
of the vitamin C content of apple juice before and after UV treatment (254
nm, 25W), they found that UV treatment caused 50% of the original vitamin
C content to decompose at the slowest flow rate (Ye et al., 2007). Vitamin C
shows maximum absorption at around 260 nm, which is why vitamin C content
has significant influence on the absorption of UV rays at this wavelength.
Therefore, higher-energy UV rays must be applied in the pasteurization of
products enriched with vitamin C. Another experiment showed that vitamin
C decomposition takes place according to zero-order reaction and that the
death rate for E. coli was two and a half times bigger with samples receiving
vitamin C supplementation. In the case of apple juice, a correlation was
found between vitamin C decomposition, the applied energy and the adopted
technology (e.g. flow rate).
In fresh fruit juices, vitamin A plays an important role as well, contributing
with 25% to the daily vitamin A requirement. Irradiating apple juice with an
UV dose of 200 mJ/cm2 caused its vitamin A content to decrease to around
50% of the original value, which calls attention to the vitamin-A-damaging
effect of UV treatment (Adhikari & Koutchma, 2002). Considering that vita-
mins A and C are the two most essential vitamins in fruit juices, this raises
awareness of the fact that UV treatment may cause substantial vitamin loss.
34 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

Besides these two vitamins, a 50% decrease was observed with riboflavin and
β-carotene content as well, while others reported on a much slighter decrease
of 11–16% upon the UV treatment of vitamins C, B6 and A. The irradiation of
a similar dose caused vitamin C to undergo a more significant decomposition
than β-carotene (California Day-Fresh Food Inc., 1999).
Summarizing the data obtained, it can be established that UV treatment
resulted a decrease of about 30–40% and 18–25% in the vitamin C content
of apple juice and carrot juice respectively. In the above cases, the applied
irradiation dose was 600 mJ/cm2 and 1450 J/s respectively (Koutchma &
Shmalts, 2002).

9 Shelf life and changes in quality due to UV treat-

ment
Effects of UV treatment have been primarily studied in the nowadays very
popular fruit juices. The bulk of the dry matter content of fruit juices is car-
bohydrate, wherefore UV light has no particular effect on these types of foods.
UV treatment is first of all applied to extend shelf life – in this process, they
analysed how UV light affects aroma, colour and nutrient content (Tandon et
al., 2003). Applying UV light of various energy and wavelengths for various
durations and at varying temperatures yielded no significant changes in the
organoleptic properties of the treated and untreated fruit juices and led to no
significant differences between pasteurization with UV light and heat. Follow-
ing the treatment of orange juice with UV light of 100 mJ/cm2 , the loss of
vitamin C was around 17%, just as if pasteurization were performed with heat
treatment (Tran & Farid, 2004). The amount of total phenolic components
in the apple juice significantly decreased due to UV treatment, but this was a
slighter decrease compared to a heat treatment of similar efficiency (Tran &
Farid, 2004).
UV treatment makes changes to enzymatic activity as well. In mango nectar,
polyphenol oxidase activity decreased to 19%, and the product kept its bright
fresh colour over a long period of time (Guerrero-Beltrán & Barbosa-Cánovas,
2006). Another experiment focused around the UV treatment of apple juice
and found that total polyphenol amount was significantly reduced, but this
decrease was smaller than in the case of heat treatment. UV light did not
affect total antioxidant capacity and did not reduce either polyphenol oxidase
or peroxidase activity as compared to the corresponding heat treatment. They
concluded that UV treatment in no respect caused more negative changes in
 Effect of UV light on food quality and safety 35

composition than the equally efficient mild heat treatment (Noci et al., 2008).

10 Application of continuous and pulsed UV light

during food production
Application of UV treatment aimed at extending shelf life and destroying
pathogenic microorganisms does not have an unequivocal reception due to the
component-damaging and -changing effects of ultraviolet light. In 2000, the
U.S. Food and Drug Administration claimed that UV treatment was com-
pletely safe and destroyed human pathogenic microorganisms in fruit juices
(US, FDA, 2000a). It has also been established that UV irradiation may
cause the decomposition of some components and the creation of others, but
these are not dangerous to human health. Fruit juices treated with UV rays
were claimed to be at least as safe as the commercially available products
not treated with UV rays. However, it has also been made clear that UV
treatment may be considered as microbiologically safe only if the number of
human pathogenic microorganisms decreases by five orders of magnitude when
compared to the control sample. This degree of reduction must be ensured
and verified at all times whenever it comes to applications for human use (US,
FDA, 2000b).
In most cases, UV rays are created with low-pressure mercury-vapour lamps,
and the liquid is transferred through tubes that permit the full passage of
UV rays. As most fruit juices absorb UV rays to the maximum, the greater
part of the energy emitted gets absorbed within a few millimetres from the
radiation source and does not reach other parts of the food, wherefore the light
energy inside the tube will not be enough to destroy the human pathogenic
microorganisms. Therefore, it is recommended that there be a turbulent flow
in the light-transmitting tube allowing the bulk of the liquid to be in contact
with the tube wall, where it can get the radiation dose necessary for the
destruction of microorganisms. Compliance with the above conditions helps
in destroying pathogenic microorganisms (US, FDA, 2001).
The intensity of UV radiation necessary for the destruction of human patho-
genic microorganisms varies according to the type of liquid and fruit juice, the
initial microbial count, the design of the applied apparatus, flow rate, number
of lamps and time of irradiation. In view of the aforementioned, authorities
do not prescribe a maximum and minimum radiation dose but recommend
that maximum safety be achieved in the various applications, i.e. pathogenic
microorganisms be destroyed in sufficient amount at all times. It must also be
36 J. Csapó, J. Prokisch, Cs. Albert, P. Sipos

taken into consideration that the production of UV radiation is also capital

intensive, wherefore using a dose that is higher than the optimum can be
uneconomical and may contribute to the adverse transformation of certain
components.
In 2005, authorities approved the use of pulsed UV treatment in food pro-
duction, processing and treatment (US, FDA, 2005). Pulsed UV treatment is
safe in the producing, processing and treating of foods if xenon lamps emitting
radiation in the range of 200–1.000 nm are used with a pulsing frequency not
greater than 2 milliseconds, if the treatment aims at the destruction of the
microorganisms on the surface, if the effects of the pulsed UV-light treatment
are appropriate, and if total radiation dosage does not exceed 12.0 J/cm2 (US,
FDA, 2005).
Krishnamurthy et al. (2004) used pulsed UV radiation for the inactivation of
Staphylococcus aureus. The energy of the pulsed light was 5.6 J/cm2 , and puls-
ing duration increased up to 30 seconds. This technology yielded a decrease
of 7–8 log10 when UV radiation time reached at least five seconds. Sample
thickness, exposure time, and treatment method significantly influenced the
bactericidal effect. Pulsed UV radiation is considered a potential solution in
the destruction of pathogenic microorganisms.

11 How safe is treatment with UV light?

A Canadian institute specialized in the safety of new food products examined
how efficient an apparatus suitable for UV treatment is in reducing the mi-
crobial count of apple juice as well as of cider. They studied the changes in
the composition and organoleptic properties of apple juice and cider upon UV
treatment as compared to the control sample and if there was a possibility
for the formation of toxic substances during UV treatment (Health Canada,
2004). They found that UV treatment had no harmful effect whatsoever on
human organism, and it could be used efficiently for reducing the number of
microorganisms in both cider and apple juice. Nevertheless, they have also
established that UV treatment was not sufficient to completely destroy the
microorganisms, particularly when the initial total microbial count was ex-
tremely high.
 Effect of UV light on food quality and safety 37

12 Regulating the use of UV light

In the European Union, there is no specific legislation regarding the usage of
UV light in food production but only decisions with reference to the irradiation
of foods, varying between Member States. Food products for which radiation
can be used during their production or storage are now under discussion in the
Member States. Radiation is allowed only if it is absolutely necessary in the
technological process, if it is useful for the consumers, and if it does not aim
at replacing either hygiene and health protection rules or the best practices
used in production and agriculture (Koutchma et al., 2009).

Acknowledgement
The work is supported by the EFOP-3.6.3-VEKOP-16-2017-00008 project.
The project is co-financed by the European Union and the European So-
cial Fund. We wish to express our gratitude for the support of Sapientia
Hungarian University of Transylvania (Cluj-Napoca), Faculty of Economics,
Socio-Human Sciences and Engineering, Food Science Department (Miercurea
Ciuc).

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