Keywords: Ageing effect on respiration, fresh-cut produce, modified atmosphere packaging, respiratory quotient, wounding effect on respiration. Abstract.
Modelling the Influence of Storage Time on the Respiration Rate of Shredded Carrots at Different Temperatures under Ambient Atmosphere T. Iqbal, F.A.R Oliveira and P.V. Mahajan Department of Process and Chemical Engineering, University College Cork Ireland J.P. Kerry Department of Food and Nutritional Sciences, University College Cork, Ireland
L. Gil Post-Harvest Research Sub-Unit, CECAICETA, University of Porto, Portugal M.C. Manso and L.M. Cunha Post-Harvest Research Sub-Unit, CECAICETA, University of Porto, and University Fernando Pessoa, Porto Portugal
Keywords: Ageing effect on respiration, fresh-cut produce, modified atmosphere packaging, respiratory quotient, wounding effect on respiration Abstract Measurement and modelling of the respiration rate of fresh-cut produce is vital for the engineering design of modified atmosphere packaging systems. In this work the closed system methodology was used to measure the respiration rate (O2 consumption and CO2 production) of shredded carrots as a function of time and temperature. The experiments were carried out at 4, 8, 12, 16 and 20°C for 144, 108, 90, 72 and 54 hours, respectively, during storage under ambient atmosphere. The effect of temperature on respiration rate was well described by an Arrhenius type equation. The respiratory quotient was approximately constant over time, but it showed a polynomial increase with temperature, ranging from 0.51 to 0.83. Under all the experimental conditions tested, respiration rate went through an initial lag phase before increasing sharply as storage time progressed. This effect was enhanced with increasing temperature and was well described by a Weibull model with a shape constant independent of temperature and with a time constant decreasing with temperature according to an Arrhenius type equation. The rate of O2 consumption at time zero and at equilibrium, at a reference temperature of 12°C, were estimated to be 24±1 and 118±7 mL.kg-1.h-1, respectively. The time constant at the same temperature was 52±5 h and the shape constant was 1.6± 0.1. The activation energy, 79±1 kJ.mol-1, was the same for both the dependence of the time constant and of respiration rate on temperature, and was within the normal range for fresh produce stored in ambient air. INTRODUCTION Measurement and mathematical modelling of respiration rate of fresh produce is essential for the engineering design of modified atmosphere packages (MAP). Minimally processed fresh produce is becoming increasingly important because of consumer demand for convenience and freshness, added value to processors and weight reduction for transport (Budu et al., 2001). Fresh-cut produce is different from intact fruit and vegetables in terms of their physiology, handling and storage requirements. Its processing results in tissue and cell integrity disruption, with a concomitant increase in enzymatic, respiratory and microbiological activity and therefore, reduced shelf life (Lamikanra, 2002). This effect might be minimised by the use of adequate temperature management and MAP. Respiration is a metabolic process that provides energy for plant biochemical processes (Kays, 1991). It is the oxidative breakdown of the more complex substrates normally present in the cells, such as starch, sugars and organic acids, into CO2 and water, with the simultaneous production of energy and other molecules, which can be used by the cell for synthetic reactions (Kader, 1987). Respiration rate can be expressed as
Proc. 3rd IS on Model IT Eds.: M.L.A.T.M. Hertog and B.M. Nicolaï Acta Hort. 674, ISHS 2005
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production of CO2 ( R CO 2 ) and consumption of O2 ( R O2 ). The respiratory quotient (RQ) is the ratio of CO2 produced to O2 consumed by the product and its normal range in aerobic respiration is 0.7 to 1.3. For carbohydrate, lipids and organic acids substrates, the RQ is expected to be 1, below 1 and above 1, respectively (Kays, 1991). Respiration plays a major role in the postharvest life of fresh fruits and vegetables. Loss of substrates in respiration can result in loss of food reserves in the tissue, loss of taste quality (sweetness) and food value to the consumer. Most of the studies reported in the literature consider the respiration rate to be independent of time. Some authors have however reported that this is not the case, particularly for fresh-cut produce, and these changes may have a major impact in the gas composition achieved in MAP (Fonseca et al., 2002b). The objective of this work was to investigate the changes of respiration rate of shredded carrots stored under ambient air over time at different temperatures, and to develop a predictive mathematical model accounting for the effect of both time and temperature. MATERIAL AND METHODS Sample Preparation and Experimental Conditions Fresh carrots (Daucus carota) were bought from a local shop in Cork, Ireland. Carrots were allowed to equilibrate to the test temperatures, peeled, washed, towel-dried and then shredded in a food processor. Shredded carrots were centrifuged in a salad spinner, to remove excess water, weighed (approximately 150g) with a balance and placed into glass jars of about 1.9 L. Sixteen to 18 jars were used for each set of experiments at a given temperature. The open jars were stored up to 144, 108, 90, 72 and 54 hours at 4, 8, 12, 16 and 20°C, respectively. Determination of respiration rate was made after specified time intervals (8, 6, 5, 4 and 3 hours at 4, 8, 12, 16 and 20°C, respectively). All the experiments were replicated. Respiration Rate Measurement The closed system methodology was used for measurement of the rates of production of CO2 ( R CO 2 ) and consumption of O2 ( R O 2 ) (Fonseca et al., 2002a; Hong and Kim, 2001; Jacxsens et al., 2000; Lencki et al., 2004; Ratti et al., 1996). After the specified storage time the jars were closed with a lid containing a rubber septum in the middle, to take the gas samples from the jar. To ensure a hermetic seal, vaseline was incorporated into the gap between lid and jar for all glass jars. The gas composition (O2 and CO2 volumetric fraction) was monitored over time with a PBI Dansensor (Checkmate, 9900, Denmark). Samples were taken at constant intervals (every 15 minutes for 20 and 16°C and every 30 minutes for 12, 8 and 4°C) with the help of a needle connected to the Dansensor, through the septum on the centre of lid. The needle sampled about 5-6 mL of the atmosphere inside the jar. O2 concentration decreased while CO2 concentration increased with time inside the sealed container. R CO 2 and R O 2 were determined by fitting equations 1 and 2 to the experimental data by linear regression in the period where the respiration rate was constant: R O 2 .W yO 2 = yO ,o − t (1) 2 Vf R CO 2 .W y CO 2 = y CO , o + t (2) 2 Vf where y O 2 and y CO 2 are, respectively, the O2 and CO2 volumetric fractions in the gas
mix at time t, y O 2 , o and y CO 2 , o are, respectively, the O2 and CO2 volumetric fractions
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in the gas mix at time zero, W is the mass of the product and Vf is the free volume inside the container calculated as 1.76 L using equation 3. Vf = V −
W
(3) ρ where V is the volume of the jar and ρ is the product density (1.068 kg.L-1). Model Constants Estimation The constants of the model developed to describe the influence of time and temperature on respiration rate were estimated by fitting this model to the experimental data by non-linear regression using the Statistica software (Statsoft, Tulsa, OK, USA). Analysis of variance was also performed with the same software. RESULTS AND DISCUSSION Effect of Time and Temperature on the Respiration Rate of Shredded Carrots Respiration rate showed a lag time and then increased over time eventually levelling off. Figure 1 shows the results obtained at the two extreme temperatures (4 and 20°C) and at the middle temperature (12°C) tested. This effect was enhanced with increasing temperature: at 4°C respiration rate increased by approximately 3-fold whereas at 20°C it increased by 6-fold. Varoquaux and Wiley (1994) had suggested similarly that the respiration rate of fresh cut produce can increase up to 7-fold or more depending on product, cutting grade and temperature. The respiratory quotient (RQ= R CO 2 / R O 2 ) did not change significantly over time (P < 0.05) but increased with temperature from 0.51 to 0.83 (see figure 2), which shows that the respiration pattern changes with temperature. The increase in RQ was depicted by a second order polynomial model (R2= 97.6%):
(4) RQ = (−1.4732 × 10 −3 ± 0.0005).T 2 + (0.85992 ± 0.28).T − (124.65 ± 39.49) where T is the temperature in K. The effect of both time and temperature on respiration rate was then analysed using for each sampling time the average rate (R) of O2 consumption and CO2 produced, converted to the equivalent value of R O 2 , that is: R=
R O2 +
R CO 2
RQ
(5)
2
Modelling the Effect of Time and Temperature on the Respiration Rate of Shredded Carrots Different sigmoidal models were fitted to the experimental data at each temperature and it was found that the best fits were obtained with the Weibull model:
R − R eq
t − e τ
β
= (6) R o − R eq where Req and R,o are the average respiration rate (equation 5) at time zero and at equilibrium, respectively, τ is a time constant and β is a shape constant. The shape constant was independent of temperature and the time constant decreased with temperature according to an Arrhenius type equation (Hong and Kim, 2001; McLaughin and Berine, 1999; Ratti et al., 1996; Van De Velde and Hendrickx, 2001): τ = τ ref × e
Eτ 1 1 × − R T T ref c
(7)
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where τ is the time constant at temperature T (K), τref is the time constant at a reference temperature, Tref, Eτ is the activation energy (kJ.mol-1) and Rc is the universal gas constant (0.008314 kJ.mol-1.K-1). It was further observed that the initial and the equilibrium respiration rate increased with temperature according to an Arrhenius-type equation: R o = R o, ref × e
E Ro − R c
1 1 × − T T ref
E R eq − Rc × e
1 1 × − T Tref
(8)
R eq = R eq, ref (9) where Ro,ref and Req are the initial and the equilibrium respiration rate, respectively, at the reference temperature and E R o and E R eq are the activation energies regarding the initial
and the equilibrium respiration rate, respectively. The average temperature in the range tested, 285.15 K, was used as reference temperature for equations 7, 8 and 9. The activation energies on equations 7 to 9 were not statistically different (P
