#750_Manzocco_bozza


	

Ital. J. Food Sci., vol 29, 2017 - 317 

PAPER 
 
 
 
 
 

EFFECT OF PULSED LIGHT 
ON SELECTED PROPERTIES OF CUT APPLE 

 
 
 

L. MANZOCCO, P. COMUZZO, M. SCAMPICCHIOa and M.C. NICOLI 
Dipartimento di Scienze Agro-Alimentari, Ambientali e Animali, Università degli Studi di Udine,Via 

Sondrio 2/A, 33100, Udine, Italy  
a Faculty of Science and Technology, Libera Università di Bolzano, Piazza Università 1, 39100, Bolzano, Italy 

*Corresponding author. lara.manzocco@uniud.it	
 
 
 
 
 
 

ABSTRACT 
 
The effect of pulsed light (0, 8.8 and 17.5 J cm-2) on selected properties of cut apple (thermal 
power, oxygen consumption, volatile compounds, colour and firmness) was investigated 
during storage at 30 °C for up to 6 days. Samples exposed to pulsed light showed lower 
heat production due to a decrease in tissue respiration. Pulsed light treated samples also 
showed different evolution of volatile compounds (ethanol, acetaldehyde and ethyl 
acetate), browning and tissue softening. Modification of tissue metabolisms by pulsed 
light could be exploited in the processing of fresh-cut fruit and vegetables leading to 
advantages well beyond its germicidal activity. 
 
 
 
 
 
 

Keywords: fresh-cut, isothermal microcalorimetry, light, respiration, volatile 
 



	

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1. INTRODUCTION 
 
Pulsed light processing consists in exposing food to consecutive intense flashes of a 
radiation with a spectrum similar to that of the sun, including not only ultraviolet light but 
also visible and infrared radiation (170-2600 nm) (MORARU and UESUGI, 2009; 
FALGUERA et al., 2011). Because of its germicidal effect, pulsed light has been 
investigated as an interesting technological approach to decontaminate the surface of 
fresh-cut fruits and extend their shelf life (GÒMEZ-LÒPEZ et al., 2007; OMS-OLIU et al., 
2010; RAMOS-VILLARROEL et al., 2011; GÒMEZ et al., 2012). The antimicrobial efficacy of 
pulsed light has been attributed to localized photothermal and photophysical effects but 
especially to the capability of its UV light component to modify the structure of 
biomolecules (WEKHOF, 2000; TAKESHITA et al., 2003; WANG et al., 2005; 
KRISHNAMURTHY et al., 2008). Pulsed light has actually the potential of modifying 
protein conformation and configuration, leading to significant modification in their 
biologic activity (MANZOCCO, 2015). These changes promote the death of 
microorganisms but also the inhibition of oxidative enzymes (GÒMEZ et al., 2012; 
MANZOCCO et al., 2013a; 2013b; IGNAT et al., 2014). It is thus likely that pulsed light 
could modulate the metabolic activity of vegetable tissues following adaptation 
mechanisms to the light stressor (LUCKEY, 1980). A circumstantial evidence supporting 
this hypothesis is the well-known physiological response of plant tissue to UV-C light, 
which is part of the pulsed light spectra. Common response to UV-C light in plants 
actually involves: (i) enhancement of biosynthesis of phenols toxic to pathogens; (ii) 
reduced ethylene production, and (iii) development of antisenescent putrescine exerting 
opposite physiological effects to ethylene (STEVENS et al., 1998; MAHARAJ et al., 1999). 
Physical effects could also be expected since UV-C light actually promotes moisture 
redistribution in the plant tissue due to local thermal effects deriving from electron 
excitation of food components (MANZOCCO et al., 2011a). It has been hypothesised that 
modification of water availability could further modify tissue metabolic activity 
(MANZOCCO et al., 2011b). Nevertheless, limited information is available about effects of 
pulsed light other than the germicidal one. 
The aim of the present work was to evaluate the effect of pulsed light on some 
metabolism-related properties of cut apple. In particular, Golden Delicious apple tissues 
were submitted to pulsed light in a fluence range typically applied to inactivate 
microorganisms and inhibit enzymatic browning (up to 17.5 J cm-2). Samples were then 
analysed for heat production, oxygen consumption, carbon dioxide formation, evolution 
of volatile compounds, colour and firmness during storage at 30 °C.  
 
 
2. MATERIALS AND METHODS 
 
2.1. Sample preparation 
 
Golden delicious apples were purchased at the local market. Fruits were washed with 
water and rinsed. Apples were cored using a perforated cylinder having 4 mm internal 
diameter and 50 mm length. The extremities of the cored apple tissue were discarded to 
get a 35 mm long cylinder of apple mesocarp. A cylindrical shape was chosen to guarantee 
homogeneous exposure of sample surface to pulsed light and allow sample introduction 
in the microcalorimetry vials. 
 
 
 



	

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2.2 Pulsed light treatments 
 
Pulsed light treatments were carried out at room temperature by using a pulsed light 
mobile decontamination unit (Claranor, Avignon, France) equipped with 4 xenon lamps 
with maximum emission in the range 200-1000 nm (200-400 nm: 41%; 400-700 nm: 51%; 
700-1000 nm: 8%). Lamps were positioned at each side of a quartz plaque held in the 
centre of the cube shaped chamber. Two lamps were symmetrically positioned above and 
below the apple cylinder at a distance of 1 cm. Two lamps were symmetrically positioned 
at the lateral side of the apple cylinder at 1 cm distance from the quartz plaque, which 
corresponded to 3 cm from the sample. Apple cylinders were thus individually exposed to 
increasing light fluence up to 17.50 J cm-2, by means of increasing number of light pulses. 
According to the manufacturer’s instructions, each pulse delivered to the sample a light 
fluence of 1.75 J cm-2. Pulse duration was 0.50 !s and repetition rate was 0.50 Hz.  
Treated apple cylinders were individually introduced in 1.2 mL capacity vials (Lab 
Logistic Group GmbH, Meckenheim, Germany), hermetically sealed in the presence of air 
with butyl septa and metallic caps (Lab Logistic Group GmbH, Meckenheim, Germany), 
and stored for increasing time up to 6 days at 30 °C (Climacell 222, MMM Group, 
Grӓfelfing, Germany). This temperature was chosen to emphasise metabolic activity of 
apple tissue. 
Analogous sample not exposed to pulsed light were prepared as control. 
 
2.3. Temperature 

 
Temperature was measured by a Testo 805 pyrometer (Testo, Settimo Milanese, Italy). 
Sample temperature was measured immediately after the pulsed light treatment. Interval 
time between the end of the pulsed light treatment and sample temperature measurement 
was less than 10 s.  
 
2.4. Isothermal microcalorimetry 
 
Isothermal calorimetry was performed by a multichannel microcalorimeter (TAM III Air 
isothermal calorimetry, TA instruments, New Castle, Delaware, USA) equipped with an 
oil bath thermostat (accuracy 0.0001 °C) operating through a Peltier element. The 
instrument has a sensitivity of ± 100 nW and allows accurate maintenance of temperature 
in the 25 ± 5 °C temperature range. Vials containing the apple cylinders were placed in the 
calorimeter and isothermal traces recorded at 30 °C for 5 days. The thermal profile was 
expressed as W g-1 by normalising the heat flux (W) of each sample based on the sample 
weight. 
 
2.5 Firmness 
 
Firmness was measured by Warner-Blatzler shear test using an Instron 4301 (Instron LTD. 
High Wycombe, United Kingdom). The instrumental settings and operations were 
accomplished using the software Automated Materials Testing System (version 5, Series 
IX, Instron LTD, High Wycombe, United Kingdom). The blade was lowered into the apple 
sample perpendicularly to the cylinders at a speed of 5 cm/min. Force was measured over 
time and sample firmness was taken as the force (kN) required to shear apple cylinders. 
 
 
 
 



	

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2.6 Image analysis 
 
Images of apple cylinders were acquired by using an image acquisition cabinet (Immagini 
& Computer, Bareggio, Italy) equipped with a digital camera (EOS 550D, Canon, Milan, 
Italy). In particular, the digital camera was placed on an adjustable stand positioned 60 cm 
above a black cardboard base where the apple sample was placed. Light was provided by 
4 100 W frosted photographic floodlights, in a position allowing minimum shadow and 
glare. Other camera settings were: shutter time 1/125 s, F-Number F/6.0, focal length 60 
mm. Images were saved in jpeg format. Image-Pro® Plus (ver. 6.3, Media Cybernetics, Inc., 
Bethesda, MD, USA) was used to analyzed apple browning. Brown pixels in the apple 
images presented 103 < R < 194, 76 < G < 188, 5 < B < 100. Browning was defined as the 
percentage ratio between brown pixels and pixels corresponding to the apple area. 
 
2.7 Gas chromatographic analyses 
 
A Fisons 8000 Series gas chromatogram, equipped with a thermal conductivity detector 
Fisons HWD (both from Fisons Instruments, Milan, Italy), was used for analysis of oxygen 
and carbon dioxide in the headspace of vials containing apple cylinders. Compounds were 
separated on two glass columns (2 m x 2 mm i.d.), packed with Porapaqs (80/100 mesh), 
in isothermal conditions (column temperature 70 °C). Carrier gas was nitrogen, at a flow 
rate of 27 ml min-1; injector and detector temperatures were 180 and 120 °C respectively. 
The temperature of the filament was 170 °C. Samples were equilibrated at 25 °C before 
injection; then, 200 µL of headspace was sampled by a 500 µL gastight manual syringe 
(Dynatech, Batonrouge, Lousiana, USA) and immediately injected in the GC system. 
Capillary gas chromatography (GC) and solid-phase microextraction (SPME) were used 
for the analysis of low-boiling volatile compounds in the headspace of the vials containing 
apple cylinders. The fiber used was a 1 cm 85 µm Carboxen/PDMS (Supelco, Bellefonte, 
PA, USA). SPME was run for 2 min at 25 °C; vials were preliminary equilibrated at the 
operating temperature for 15 min before microextraction. GC injection was carried out in 
split mode (split ratio 1:10) and the fiber remained in the injector for 1 min. The GC system 
was a HRGC 8560 Mega Series 2 gas chromatograph (Carlo Erba, Milan, Italy), equipped 
with a flame ionisation detector (FID); carrier gas was helium, at a linear flow rate of 35 cm 
s-1. Compounds were separated on a J&W DB-Wax capillary column (30 m x 0.25 mm i.d., 
0.25 µm film thickness), purchased from Agilent Technologies Inc. (Santa Clara, CA, USA). 
The column temperature was programmed as follows: 35 °C for 2 min, then at 4 °C min-1 
up to 44 °C; temperature was then further increased at 20 °C min-1 up to 240 °C, held for 15 
min. Injector and detector temperature were set at 250 and 240 °C respectively. The 
concentration of the volatile compounds was expressed in absolute area units. The identity 
of the peaks was confirmed by comparing their retention time with those of standard 
compounds; all the standards were from Sigma-Aldrich (St. Louis, MO, USA). 
 
2.8 Statistical analysis 
 
Analyses were performed on at least duplicated samples. Colour and firmness analyses 
were performed on at least 8 duplicated samples. Results are reported as mean value ± SD. 
Pearson correlation analysis was performed by using Statistica for Windows (ver. 5.1, 
Statsoft Inc., Tulsa, USA, 1997). A p-value >0.05 was set as a statistical threshold for 
significance. 
 
 
 



	

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3. RESULTS AND DISCUSSION 
 
Apple cylinders were submitted to pulse light treatments with increasing fluence at 
environmental temperature. After the treatment, the temperature of the apple cylinder 
surface never exceeded 30 °C. The effect of pulsed light was initially monitored by 
isothermal microcalorimetry. This technique was chosen since the evaluation of heat 
production provides a direct indication of the metabolic responses of raw materials, such 
as respiration and reaction to wounding stress (CRIDDLE et al., 1991; GÒMEZ GALINDO 
et al., 2005; WADSÖ and GÒMEZ GALINDO, 2009; ROCCULI et al., 2012). Apple tissue 
was thus exposed to increasing fluence of pulsed light and evaluated for heat production 
under isothermal conditions at 30 °C for up to 6 days (Fig. 1). 
 
 

 
 
Figure 1. Calorimetric traces at 30 °C of apple tissue exposed to pulsed light with increasing fluence. 
 
 
Independently on the fluence of the pulsed light treatment, calorimetric traces of all 
samples showed a main exothermal peak within 1-2 days of observation. Prolonging 
observation time, no further changes in calorimetric traces were detected (data not 
shown).  
According to the literature, the exothermal peak (Fig. 1) should be related to the thermal 
effects of microbial growth and/or metabolic activity of the apple tissue (RIVA et al., 2001; 
GÓMEZ GALINDO et al., 2005). However, calorimetric peaks due to microbial growth are 
generally preceded by a plateau signal corresponding to the lag phase of micro-organisms. 
As clearly shown in Figure 1, no initial plateau signal was detected, suggesting the peak to 
be mainly attributable to heat generated by the respiration process of wounded apple 
tissue. It can be inferred that the hypoxic conditions that are quickly generated inside the 
vial upon apple tissue respiration may inhibit microbial growth making its contribution 
negligible as compared to that of tissue metabolism. 
The calorimetric trace of the control untreated apple showed the occurrence of a sharp and 
narrow peak (Fig. 1). The peak shoulder could be explained considering the occurrence of 
physiologic activities with different metabolic rate depending on the storage period. After 
1 day of storage at 30 °C, the calorimetric signal reached a plateau value. The latter was 
above the starting base line and indicated that beyond this storage time, there was still a 
slight but constant residual metabolic activity. When apple samples were exposed to 

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pulsed light with increasing fluence, the exothermal peak showed a progressively lower 
intensity and appeared broader. In addition, the plateau was reached in longer times and 
its level was lower than in the control sample. These results indicate that pulsed light 
treatment modifies tissue metabolism. To this regard, the UV-C light component of pulsed 
light was reported to reduce ethylene production in fresh tomato, favouring the 
accumulation of putrescine, an anti-senescence agent with an opposite physiological effect 
with respect to ethylene and delayed the appearance of the climacteric peak (MAHARAJ et 
al., 1999). A similar effect of UV-C radiation has been also reported for other climacteric 
fruits, such as apples and peaches (LU et al., 1991).  
To verify the effect of pulsed light on apple tissue respiration, samples were stored for 
increasing time under the same temperature adopted during the calorimetric analysis (30 
°C) and evaluated for oxygen consumption and carbon dioxide formation (Fig. 2).  
 
 

 
 

 
 

Figure 2. Headspace oxygen and carbon dioxide during storage at 30 °C of apple tissue exposed to 
increasing fluence of pulsed light. 

 
 

Headspace oxygen was rapidly consumed during the first days of cut apple storage while 
carbon dioxide concomitantly accumulated following the typical gas evolution of plant 



	

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tissues performing a respiration metabolism. The respiration-driven oxygen consumption 
and formation of carbon dioxide resulted progressively less intense as the fluence of the 
pulsed light treatment was increased. This result apparently contradicts with the 
negligible effect of pulsed light on respiration of endive salad and mung bean sprouts 
(KRAMER et al., 2015). Contradictory results were also reported by RAMOS-
VILLARROEL et al. (2011) with reference to fresh-cut avocado. These authors showed that 
pulsed light treatment reduced oxygen consumption but increased respiration products 
such as carbon dioxide and ethanol. Additional information can be obtained considering 
the plant respiration effects of UV-C light, which is an important component of the pulsed 
light radiation. UV-C light was reported to reduce respiration rates of fresh cut cantaloupe 
melon and intact Fuji apples (LAMIKANRA et al., 2005; YUJUAN et al., 2015). It can be 
hypothesised that the different effects of pulsed light on respiration rate may be strongly 
dependent not only on the nature of the plant matrix and its peculiar respiration 
metabolism but also on the spectral composition of the light radiation. 
The progressive decrease of oxygen partial pressure in the sample headspace (Fig. 2) is 
well known to be associated to hypoxic conditions, leading to accumulation of ethanol and 
acetaldehyde involved in post harvest maturation and flavour development (DIXON and 
HEWETT, 2000; PESIS, 2005). Samples were thus analysed for ethanol, acetaldehyde and 
ethyl acetate (Fig. 3). 
The latter was taken as an example of volatile compound that is expected to play a role in 
the flavour of fresh-cut apple derivatives (SONG and BANGERTH, 1996; DIXON and 
HEWETT, 2000). Gas chromatographic analyses were only performed on samples stored 
up to 2 days of storage due to the minimum metabolic activity on further storage (Figure 1 
and 2). Pulsed light treated apple tissues showed significantly lower amounts of ethanol 
and higher values of acetaldehyde than the control sample (Fig. 3). In addition, when 
pulsed light was applied at the highest fluence (17.5 J cm-2), it also modified the evolution 
of ethyl acetate (Fig. 3). As reported in the literature, piruvate is converted to acetaldehyde 
and CO2 by the enzyme pyruvate decarboxilase, and acetaldehyde is reduced to ethanol by 
the enzyme alcohol dehydrogenase (MATHEWS and VAN HOLDE, 1996). Esterification 
of alcohols, although not fully understood in its biosynthetic pathway, is then responsible 
for the formation of esters contributing to apple flavour during maturation (DIXON and 
HEWETT, 2000). Data shown in Figure 3 suggest that exposure to pulsed light radiation 
could modulate the activity of the enzymes of the anaerobic biosynthetic pathway for the 
formation of acetaldehyde, ethanol and esters. To this regard, light radiation is known to 
be absorbed by enzyme proteins due to the presence of endogenous chromophores within 
their structure (DAVIES and TRUSCOTT, 2001). As a consequence, light would modify the 
structure of enzymes leading both to their activation or inactivation (MANZOCCO et al., 
2009). Results acquired in this experimentation indicate that pulsed light is able to modify 
the metabolic response of apple tissue to the wounding stress, modulating the kinetics of 
respiration and volatile formation (Figs. 2 and 3). These phenomena would be probably 
the result of a combination of different biological activities that are concomitantly 
monitored when measuring the thermal power of the sample (Fig. 1). In order to verify the 
capability of isothermal calorimetry to study the metabolic consequences of pulsed light 
treatment, correlation analysis was performed. Table 1 shows the correlation coefficients 
between thermal power and analytical parameters used to monitor respiration and volatile 
formation, during storage at 30 °C of cut apple exposed to increasing fluence of pulsed 
light.  
 
 
 



	

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Figure 3. Headspace ethanol, acetaldehyde and ethyl acetate during storage at 30 °C of apple tissue exposed 
to increasing fluence of pulsed light. 
 
 
 
 



	

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Table 1. Correlation coefficients between thermal power and indices of respiration and volatile formation of 
cut apple exposed to increasing fluence of pulsed light and stored for increasing time at 30 °C. 
 

  Thermal power 
Oxygen 0.77a 
Carbon dioxide -0.73 a 
Ethanol -0.77 a 
Acetaldehyde 0.05 
Ethyl acetate -0.32 

 
a significant at p < 0.05 
 
 
A very low correlation of thermal power with acetaldehyde and ethyl acetate formation 
was observed. This can be attributed to the fact that the evolution of these parameters is 
quite complex, being affected by a number of different concomitant and consequent 
reaction pathways. By contrast, a high correlation between changes in thermal power and 
oxygen, carbon dioxide and ethanol was detected. This result definitely indicates that the 
evolution of the calorimetric signal (Fig. 1) mainly accounts for the metabolic phenomena 
associated with the respiration process of apple tissue (GÓMEZ GALINDO et al., 2005).  
The effects of pulsed light on respiration rate could be associated to modification of the 
metabolism pathways leading to tissue browning and softening. To verify this hypothesis, 
apple samples exposed to pulsed light were evaluated for the development of browning 
and the change in firmness. The effect of increasing pulsed light fluence on the percentage 
of brown colour on the surface of cut apple during storage is shown in Fig. 4.  
 

 
Figure 4. Browning of cut apple exposed to increasing fluence of pulsed light and stored at 30 °C. 
 
 
The control sample showed a quick development of enzymatic browning during the first 
hours after cutting (SAPERS and DOUGLAS, 1987). No significant further colour changes 
were detected when the sample was maintained at 30 °C for up to 6 days. Apple samples 
exposed to pulsed light appeared browner just after their preparation (0 days). This is in 



	

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agreement with the enhancement of phenol biosynthesis as a common response of plant 
tissue to UV-C light (STEVENS et al., 1998; MAHARAJ et al., 1999). After 16 hours of 
storage, pulsed light treated samples showed a browning level analogous to that of the 
control sample. A significant decrease in browning was observed when pulsed light 
samples were stored beyond one day. This sample whitening could be attributed to the 
modification of the phenolic metabolism controlling the formation of brown polyphenols 
in apple tissue. However, it is not excluded that brown polyphenols could be further 
degraded to uncoloured compounds. A similar effect was reported as a consequence of 
pulsed light treatment of protein rich ingredients, such as egg white (MANZOCCO et al., 
2013a). In that case, sample bleaching was attributed to melanoidin electronic transitions 
following light radiation absorption. Colour changes could also be accounted for by 
changes in physical structure of apple tissue. Water distribution could actually play a role 
in determining the overall sample colour as well as changes in apple tissue firmness. To 
this regard, Fig. 5 shows the evolution of firmness during storage at 30 °C of apple 
samples exposed to increasing fluence of pulsed light.  
 

 
 
Figure 5. Firmness of cut apple exposed to increasing fluence of pulsed light and stored at 30 °C. 
 
 
All samples showed an initial increase in firmness, reaching a maximum value after 16 
hours of storage. Beyond this storage time, firmness decreased so that a maximum value 
was identified, followed by a plateau. The increase in firmness could be attributed to 
sample dehydration while the following decrease could account for progressive pectolityc 
activity (OMS-OLIU et al., 2010). The maximum value of firmness resulted lower in the 
case of the samples exposed to pulsed light. These result is in agreement with the fact that 
radiation can promote dehydration of a thin surface layer of the wounded plant tissue, 
begetting a protective film that hinders moisture migration from the inside to the 
environment (MANZOCCO and NICOLI, 2015).  
 
 
4. CONCLUSIONS 
 
Pulsed light has been investigated as an interesting technological approach to 
decontaminate the surface of fresh-cut fruits and extend their shelf life. Results reported in 
this work demonstrate that pulsed light exerts additional complex effects of different 



	

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metabolism-related properties of fresh-cut vegetables. In the case of apple, exposure to 
pulsed light modified the kinetics of respiration, volatile formation, browning and tissue 
softening. The possibility of pulsed light implementation in fresh-cut processing will be 
strictly dependent on the availability of detailed information about its consequences on 
tissue metabolisms, especially in relation to different temperature and atmosphere 
conditions during storage. These data should be merged with those relevant to the 
antimicrobial activity of pulsed light in order to select optimal fluence to be adopted 
during fruit treatment. In this context, isothermal calorimetry could represent a very 
useful methodological tool since potentially allowing to concomitantly quantify both 
metabolic and microbial activity.  
 
 
ACKNOWLEDGEMENTS 
 
This research was supported by Ministero dell’Istruzione, dell’Università e della Ricerca (Prot. 957/ric, 28/12/2012), 
through the Project 2012ZN3KJL “Long Life, High Sustainability”. 
 
 
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Paper Received January 12, 2016  Accepted January 25, 2017