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Failure criteria based on consumers’ rejection to determine the sensory shelf life of minimally processed lettuce
Postharvest Biology and Technology 49 (2008) 255–259
Contents lists available at ScienceDirect
Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Failure criteria based on consumers’ rejection to determine the sensory shelf life of minimally processed lettuce ´ Ares a,b,∗ , Ines ´ Mart´ınez b , Claudia Lareo b , Patricia Lema b Gaston a Secci´ on Evaluaci´ on Sensorial, C´ atedra de Ciencia y Tecnolog´ıa de Alimentos, Facultad de Qu´ımica, Universidad de la Rep´ ublica, Avda. Gral. Flores 2124, C.P. 11800 Montevideo, Uruguay b Instituto de Ingenier´ıa Qu´ımica, Facultad de Ingenier´ıa, Universidad de la Rep´ ublica, Julio Herrera y Reissig 565, C.P. 11300 Montevideo, Uruguay
a r t i c l e
i n f o
Article history: Received 25 April 2007 Accepted 18 February 2008 Keywords: Butterhead lettuce Sensory shelf life Fresh-cut vegetables Failure criteria Consumer studies
a b s t r a c t The aims of the present work were to determine failure criteria based on consumers’ rejection to purchase for shelf life estimation of minimally processed lettuce, and to compare criteria for whole and cut lettuce. A trained sensory panel and a consumer panel evaluated samples of whole and cut lettuce leaves packaged in passive modified atmosphere. In order to determine failure criteria to estimate sensory shelf lives, sensory attribute intensities corresponding to 25% consumers’ rejection to purchase percentage were calculated using logistic regressions. Failure criteria values were lower for cut lettuce than for whole leaves for all the evaluated attributes, suggesting that consumers reacted differently towards whole and cut lettuce leaves, being stricter towards cut lettuce than towards whole lettuce leaves. These results indicate that sensory limits depended on the product considered and therefore a unique criterion should not be used to estimate the shelf life of both cut lettuce and whole lettuce leaves. Twenty-five percent of the consumers would refuse to purchase cut lettuce if the intensity of the evaluated defects was over 10% of the measuring scale, whereas scores of 25% of the scale were needed to achieve a 25% of consumer rejection in the case of whole leaves. These failure criteria were stricter than those traditionally used for sensory shelf life estimation of minimally processed lettuce, which might assure the products’ quality at the end of its shelf life. Results of the present study showed the importance of performing consumer studies in order to establish proper criteria to estimate the shelf life of fresh vegetables. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, the demand for fresh-like minimally processed fruits and vegetables has increased, mainly due to nowadays consumers’ concern about health and convenience (Odumeru et al., 2002; Zhou et al., 2004). This fact has led to an increase in the quality and variety of products available to the consumers (Francis et al., 1999). Thus, efforts must be taken to ensure the quality of minimally processed products (Piagentini et al., 2005; Ragaert et al., 2004). Minimally processed fruits or vegetables could be defined as fresh fruits or vegetables that have been processed to increase their functionality without greatly changing their fresh-like properties (Salunkhe et al., 1991). The most used processes are washing, cutting, mixing and packaging. These processes induce mechanical injury in the tissue changing its physiology, accelerating deterioration during transport and retailing, and consequently shortening
∗ Corresponding author. Tel.: +598 2 9245735; fax: +598 2 9241906. E-mail address: [email protected] (G. Ares). 0925-5214/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2008.02.006
their shelf life (Delaquis et al., 1999). The shelf life of minimally processed vegetables is generaly limited by changes in their sensory properties and not by microbial growth (King et al., 1991; Jacxsens et al., 1999, 2002). Colour and appearance are critical quality aspects for shoppers when selecting fresh fruits and vegetables (IFT, 1990; Ragaert et al., 2004). Therefore, the shelf life of lettuce can be defined as the length of time for which lettuce can maintain an appearance that appeals to the consumer (Zhou et al., 2004). Sensory shelf life of minimal processed lettuce could be determine as the time required for a certain sensory attribute to a certain predetermine intensity, or failure criteria (Hough et al., 2002). Traditionally, the sensory shelf life of minimally processed lettuce has been estimated considering as failure criteria 50% of the scale used to measure a sensory attribute (Barriga et al., 1991; Piagentini et al., 1997, 2004, 2005; Li et al., 2001; Jacxsens et al., 2002; Zhou et al., 2004). This criterion has been arbitrarily selected, and no studies have been found supporting the validity of using this failure criterion or correlating these sensory limits with consumers’ perception.
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However, food products do not have sensory shelf lives of their own; shelf life depends on the interaction of the food with the consumer. Consumers are the ones that decide if they would consume a food product after a certain storage time. For this reason consumers are the most appropriate tool for determining food product sensory shelf life (Hough et al., 2003). Although a consumer panel would seemingly be the most appropriate tool to determine the shelf life and quality of a food product, to repeatedly assemble consumer panels for multiple measurements would be both impractical and expensive. Although a sensory panel is more appropriate for repeated assessments, its results would be more analytical and not necessarily representative of consumer responses (Hough et al., 2002). By correlating data from a consumer panel with those obtained from a trained panel, these analytical measures can be used to determine the shelf life or quality of a food product. This would enable the determination of more objective failure criteria to estimate the shelf life of minimally processed vegetables, assuring products’ sensory at the end of its shelf life. The aims of the present work were to determine failure criteria based on consumers’ rejection to purchase for shelf life estimation of minimally processed lettuce, and to compare criteria for whole and cut lettuce.
A balanced complete block design was carried out for duplicate evaluation of the samples. Testing was carried out in a sensory laboratory that was designed in accordance with ISO 8589 (1988). Evaluations were performed under artificial daylight type illumination, temperature control (between 22 and 24 ◦ C) and air circulation.
2. Materials and methods
2.4.1. Analysis of variance For experimental data for whole and cut lettuce leaves, an analysis of variance was performed considering storage time as variation factor. Honestly significant differences were calculated using Tukey’s test. Differences were considered significant when p ≤ 0.05. This analysis was performed using Genstat Discovery Edition 2 (VSN International, Oxford, UK).
2.1. Plant material, packaging and storage conditions Butterhead lettuces (Lactuca sativa L., cv Wang) were obtained from a local farm near Montevideo (Uruguay). Within 24 h after harvest, lettuce was transported to the School of Engineering in Montevideo, Uruguay. Outer damaged and yellowed leaves and stems were removed. The remaining leaves were washed with cold chlorinated water (200 mg kg−1 total chlorine) for 10 min, and then rinsed with water. Lettuces were dried by centrifugation in a manual kitchen basket type centrifuge for 2 min. One half of the leaves were cut in pieces of approximately 3 cm × 2 cm using a sharp knife. Approximately 70 ± 5 g whole lettuce leaves or cut lettuce leaves were selected at random and placed under air in 30 cm × 40 cm bioriented polypropylene (BOPP; two layers of 20 m thickness) bags. BOPP films were provided by a local manufacturer. According to the supplier the BOPP gas transmission rates were 0.23–0.34 nL O2 m−2 s−1 Pa−1 , 0.69–0.80 nL CO2 m−2 s−1 Pa−1 (both at 23 ◦ C and 101 kPa) and 17–35 g H2 O m−2 s−1 (at 37 ◦ C and 90% RH). Bags were sealed using a Supervac GK105/1 (Wien, Austria) packaging machine with air injection, and stored at 5 ± 0.5 ◦ C and 90% relative humidity. Whole lettuce leaves were evaluated after 0, 10, 21, 28, 35, 42 and 49 d of storage; whereas evaluations for cut lettuce leaves were performed after 0, 3, 6, 8, 10, 13 and 17 d.
2.3. Consumer panel Consumers were recruited among students and workers from the Chemistry Faculty in Montevideo, Uruguay. The study was carried out using 40 people who consumed lettuce. Their ages ranged between 18 and 50 and they were approximately 50% female and 50% male. At each storage time, each consumer received one leaf of lettuce, for each storage condition, in closed odourless plastic container, labelled with three digit random numbers. For each sample, consumers had to evaluate its appearance and respond “yes” or “no” to the question “Imagine you are in a supermarket. You want to buy a minimally processed lettuce, and you find a package of lettuce with leaves like these, would you normally buy it?”. 2.4. Data analysis
2.4.2. Sensory attribute development modelling In order to model the development of the evaluated sensory attributes as a function of storage time, and to estimate the activation energy of these attributes, the following equations that assume zero (Eq. (1)) and first-order (Eq. (2)) reaction rate were used: X = X0 + kT t
(1)
X = X0 exp(kT t)
(2)
where X = value for the sensory attribute X at time t; X0 = value for the sensory attribute X at time t = 0; kT = reaction rate constant at storage temperature T; t = storage time; In order to estimate the equation’s parameters, linear and nonlinear regression facilities of Genstat Discovery Edition 2 (VSN International, Oxford, UK) were used. 2.5. Regression analysis
2.2. Trained sensory panel Samples were evaluated by a panel of six assessors, who had previous experience in the evaluation of fresh vegetables. At each storage time, a lettuce whole leave or approximately 20 g of randomly selected cut lettuce was presented to the assessors in a closed odourless plastic container labelled with three digit random numbers, at room temperature. The assessors had to evaluate the following attributes: off-odour, wilting appearance, presence of dark and necrotic stains on the leaf surface, presence of browning on the midribs. They also evaluated browning of the cut edges for cut lettuce samples. For scoring, 10 cm unstructured scales anchored with “nil” and “high” were used.
A logistic regression was carried out considering percentage of rejection (calculated as the proportion of consumers who answered “no” when asked if they would normally buy a package of lettuce containing cut or whole lettuce leaves like the ones they saw) as dependent variable, and the evaluated sensory attributes as ´ explanatory variable (Gambaro et al., 2006). The following equation was used: Logistic : Rejection percentage = a +
b 1 + e−c(X−d)
(3)
where X is each of the evaluated sensory attributes, and a, b, c and d are the regression constants. This analysis was carried out using Genstat Discovery Edition 2 (VSN International, Oxford, UK).
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Table 1 Models’ parameters (c.f. Eq. (2)) (value ± standard error) for the development of sensory attributes as a function of storage time, for whole lettuce leaves in passive modified atmosphere packages stored at 5 ◦ C
Table 2 Models’ parameters (c.f. Eq. (1)) (value ± standard error) for the development of sensory attributes as a function of storage time, for cut lettuce leaves in passive modified atmosphere packages stored at 5 ◦ C
Sensory attribute
Sensory attribute
Parameter X0
Off-odour Wilting appearance Presence of dark stains Browning on the midribs
(2.0 (1.8 (5.2 (1.1
R2
kT ± ± ± ±
0.1) × 10−3 0.1) × 10−5 0.2) × 10−2 0.1) × 10−2
0.16 0.26 0.10 0.13
± ± ± ±
0.1 0.2 0.1 0.1
0.99 0.96 0.99 0.98
Presence of dark stains Browning on the midribs Browning on the cut surfaces
Parameter X0
kT
R2
0.076 ± 0.009 0.085 ± 0.007 0.039 ± 0.005
0.14 ± 0.2 0.22 ± 0.1 0.26 ± 0.1
0.79 0.84 0.86
and wilting appearance was not modelled for cut lettuce due to the low scores reached at the end of the storage period considered.
3. Results and discussion 3.1. Sensory analysis
3.2. Consumer study All the evaluated sensory attributes were significantly (p < 0.001) affected by storage time for both whole and cut lettuce leaves, supporting the validity of the chosen descriptors as indicators of lettuce deterioration. Cutting caused a highly significant (p < 0.001) increase in the intensity of all the evaluated sensory attributes. This could be explained by the fact that cutting caused an acceleration of deterioration processes of lettuce due to tissue damage, loss of cell integrity and decompartimentalization (Degl’ I`nnocenti et al., 2005). Appearance attributes were the defects that occurred first and therefore were mainly responsible for the decrease in sensory quality with storage time, suggesting that sensory shelf life of minimally processed lettuce might be determined by changes in its appearance. A significant increase in off-odour was found only at the end of the evaluated storage period (after 17 d for cut lettuce and after 42 d for whole leaves). The evaluated appearance attributes showed different relative importance for whole and cut lettuce leaves, suggesting that deterioration might not follow the same pattern in both products. The development of wilting appearance was not of great importance for cut lettuce, as only two tenths of the scale were used by the sensory panel, which is consistent with the little weight loss registered (5.1% after 17 d). Presence of dark and necrotic stains on the leaf surface, browning on the midribs and on the cut surfaces were the attributes that showed a higher intensity (higher than 50% of the scale) at the end of the evaluated storage period. These attributes might determine the end of sensory shelf life of cut lettuce. On the other hand, in the case of whole lettuce leaves wilting appearance, presence of dark and necrotic stains on the leaf surface and browning on the midribs were the attributes responsible of limiting the shelf life of fresh-cut lettuce. Sensory shelf life could be determined as the time required for a sensory attribute to reach a certain predetermined intensity. Thus, it is necessary to model the development of the evaluated sensory attributes. As shown in Table 1, for whole lettuce leaves a relatively good fit was obtained for the development of all the evaluated sensory attributes with time considering first-order reaction rate, in agreement with published data for cut lettuce (Vankerschaver et al., 1996; Piagentini et al., 2005). However, for cut lettuce the best fit for the development presence of stains on the leaf surface and browning on the midribs and cut surfaces was obtained considering zero-order reaction rate (Table 2). The development of off-odour
Consumer studies are a useful tool to determine proper failure criteria for shelf life studies. One of the first steps is to determine which attributes are taken into account by consumers when deciding to accept or reject a certain product for its purchase. As mentioned above, the presence of stains on the leaf surface and browning of the midribs and the cut surfaces were the attributes that significantly increased first and were the ones that reached higher scores at the end of the evaluated storage period. Therefore, these attributes might have been considered by consumers and for this reason they were selected for further analysis. In order to determine if the selected attributes were considered by consumers, the development of rejection percentage was modelled considering selected sensory attributes as explanatory variables. A relative good fit was obtained for logistic regression of consumers’ rejection to purchase versus the selected sensory attributes (Tables 3 and 4), suggesting that these attributes were considered by consumers when deciding to purchase a package of cut or whole lettuce leaves. Using consumers’ data, sensory shelf life could be estimated as the storage time necessary to reach a certain consumers’ rejection to purchase or consume percentage. Sensory shelf life of foods’ products has been usually determined considering 25% rejection to ´ purchase or to consume (Hough et al., 2003; Gambaro et al., 2006; ´ Ares et al., 2006, 2008; Gimenez et al., 2007). This percentage means that if a consumer sees the product at the end of its shelf life, there is a 25% probability that he will reject to buy it. Considering that few consumers will see the product near the end of its shelf life, and that of the few that do 75% will still find the product acceptable, this value is considered reasonable from a practical point of view. Regressions shown in Tables 3 and 4 were used to estimate the attribute intensity that corresponds to a 25% consumer rejection to purchase. Attribute intensity corresponding to 25% consumer rejection to purchase were lower for cut lettuce than for whole leaves, suggesting that that consumers reacted differently towards whole and cut lettuce leaves, being stricter towards cut lettuce. This means that consumers were harsher when considering cutlettuce leaves and rejected to buy less deteriorated samples than when considering whole lettuce leaves. This could be attributed to the fact that as processing increases, consumers seemed to pay more attention to the product’s appearance and to be harsher when
Table 3 Models’ parameters (c.f. Eq. (3)) for the logistic regression of consumer rejection percentage versus presence of dark and necrotic stains on the leaf surface and browning on the midribs, and attribute intensity corresponding to 25% consumer rejection to purchase, for whole lettuce leaves in passive modified atmosphere packages Sensory attribute
a
b
c
d
R2
Attribute intensity corresponding to 25% consumer rejection to purchase
Presence of dark and necrotic stains Browning on the midribs
0.6 −7.3
99.9 10.0
1.7 0.8
3.6 3.4
0.99 0.99
2.4 2.3
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Table 4 Models’ parameters (c.f. Eq. (3)) for the logistic regression of consumer rejection percentage versus presence of dark and necrotic stains on the leaf surface and browning on the midribs, and attribute intensity corresponding to 25% consumer rejection to purchase, for cut lettuce in passive modified atmosphere packages Sensory attribute
a
b
c
d
R2
Attribute intensity corresponding to 25% consumer rejection to purchase
Presence of dark and necrotic stains Browning on the midribs Browning on the cut surface
−6.9 −2.0 −4.7
104.9 96.9 100.2
3.3 4.2 2.4
0.8 0.9 1.3
0.96 0.99 0.99
0.5 0.7 0.9
Table 5 Shelf lives (days) for whole and cut lettuce leaves in passive modified atmosphere packages stored at 5 ◦ C, estimated considering attribute intensity corresponding to 25% consumers’ rejection to purchase Storage condition
Whole leaves Cut leaves
Shelf life (days) considering the following sensory attributes Dark and necrotic stains on the leaf surface
Browning on the midribs
Browning on the cut surfaces
39 3.1
40 2.8
– 3.0
deciding whether to buy it or not. Thus, sensory limits depended on the product considered and therefore a unique criterion could not be used to estimate the shelf life of both cut lettuce and whole lettuce leaves. Moreover, different failure criteria might be necessary for different types of products. As shown in Tables 3 and 4, 25% of the consumers would refuse to purchase cut lettuce if the intensity of the evaluated defects was over 10% of the measuring scale, whereas scores of 25% of the scale were needed to achieve a 25% of consumer rejection in the case of whole leaves. In most studies the sensory shelf life of minimally processed vegetables has been estimated considering the time necessary to reach an arbitrary score of 50% of the scale used to evaluate a certain sensory attribute (Piagentini et al., 2005; Li et al., 2001; Zhou et al., 2004). Comparing the results of the present from the consumer study, this criterion seems not strict enough as to assure the products’ quality at the end of its shelf life. Therefore, if shelf life of minimally processed lettuce is determined considering 50% of the sensory scale, consumers might reject to purchase the product at supermarkets before its shelf life date. This could cause important economic losses at both production and commercialization stage, as some of these products might not be bought by consumers at supermarkets. If shelf life of whole lettuce leaves is set considering 50% of presence of brown stains, at the end of its shelf life the product would be bought by less than 8% of the consumers. This percentage is reduced to 2% if cut lettuce is considered. Therefore, if sensory shelf life is estimated considering 50% of the scale, products would not be buy by more than 90% of the consumers before estimated shelf life. These results showed the importance of performing consumer studies in order to establish proper criteria to estimate the sensory shelf life of fresh vegetables 3.3. Shelf life estimation As the development of the evaluated sensory attributes with storage time was modelled (Tables 1 and 2), shelf life of cut and whole lettuce was estimated as the time necessary to reach a certain failure criterion. Attribute intensities that corresponded to a 25% consumers’ rejection to purchase were considered as failure criterion (c.f. Table 5). As expected, sensory shelf life decreased with cutting, in agreement with trained assessors’ data. As consumers were much stricter when evaluating cut lettuce, its sensory shelf life was reduced to only 3 days, whereas shelf life for whole leaves was 39–40 d. As shown in Table 5, sensory shelf lives calculated considering the different evaluated attributes were in agreement, probably because they were all affected in a similar way by storage time. Thus, they could all be used to estimate the sensory shelf life of whole and cut lettuce.
Shelf lives estimated considering 50% of the scale for presence of dark stains as failure criteria corresponded to 46 and 13 d for whole and cut lettuce, respectively. These shelf lives were significantly higher than those estimated considering failure criteria from consumer studies. The difference in shelf life considering both failure criteria was greater for cut lettuce than for whole leaves, probably because consumers were harsher when evaluating the former product. 4. Conclusions According to Ragaert et al. (2004) consumers confer great importance to labelled shelf life during the buying stage. Therefore, proper shelf life labelling of minimally processed fruits and vegetables is an important issue to ensure the quality of these products and to satisfy consumers increasing requirements. Failure criteria were calculated using consumers’ data, which contributes an improvement over more arbitrary criteria used in most studies. Attribute intensities corresponding to 25% consumer rejection to purchase were lower for cut lettuce than for whole leaves, suggesting that consumers reacted differently towards whole and cut lettuce leaves, being stricter towards cut lettuce than towards whole lettuce leaves. Therefore, a unique criterion could not be used to estimate the shelf life of both cut lettuce and whole lettuce leaves. These results suggest the importance of performing consumer studies in order to establish proper criteria to estimate the shelf life of fresh vegetables. Considering results from the present paper, arbitrary selected failure criteria might not be strict enough to estimate sensory shelf lives, which could cause important economic losses at both production and commercialization stage, as some of these products might not be bought by consumers at supermarkets. Further research is needed to study consumers’ reaction to different minimally processed products, and to get more insight in consumer decision making process in order to for further investigating the quality of this type of products. Acknowledgment The authors are indebted to PDT (Programa de Desarrollo Tec´ nologico, Uruguay) for financial support. References ´ ´ Ares, G., Gimenez, A., Gambaro, 2008. Sensory shelf life estimation of minimally processed lettuce considering two stages of consumers’ decision-making process. Appetite 50, 529–535, doi:10.1016/j.appet.2007.11.002.
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Contents lists available at ScienceDirect
Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Failure criteria based on consumers’ rejection to determine the sensory shelf life of minimally processed lettuce ´ Ares a,b,∗ , Ines ´ Mart´ınez b , Claudia Lareo b , Patricia Lema b Gaston a Secci´ on Evaluaci´ on Sensorial, C´ atedra de Ciencia y Tecnolog´ıa de Alimentos, Facultad de Qu´ımica, Universidad de la Rep´ ublica, Avda. Gral. Flores 2124, C.P. 11800 Montevideo, Uruguay b Instituto de Ingenier´ıa Qu´ımica, Facultad de Ingenier´ıa, Universidad de la Rep´ ublica, Julio Herrera y Reissig 565, C.P. 11300 Montevideo, Uruguay
a r t i c l e
i n f o
Article history: Received 25 April 2007 Accepted 18 February 2008 Keywords: Butterhead lettuce Sensory shelf life Fresh-cut vegetables Failure criteria Consumer studies
a b s t r a c t The aims of the present work were to determine failure criteria based on consumers’ rejection to purchase for shelf life estimation of minimally processed lettuce, and to compare criteria for whole and cut lettuce. A trained sensory panel and a consumer panel evaluated samples of whole and cut lettuce leaves packaged in passive modified atmosphere. In order to determine failure criteria to estimate sensory shelf lives, sensory attribute intensities corresponding to 25% consumers’ rejection to purchase percentage were calculated using logistic regressions. Failure criteria values were lower for cut lettuce than for whole leaves for all the evaluated attributes, suggesting that consumers reacted differently towards whole and cut lettuce leaves, being stricter towards cut lettuce than towards whole lettuce leaves. These results indicate that sensory limits depended on the product considered and therefore a unique criterion should not be used to estimate the shelf life of both cut lettuce and whole lettuce leaves. Twenty-five percent of the consumers would refuse to purchase cut lettuce if the intensity of the evaluated defects was over 10% of the measuring scale, whereas scores of 25% of the scale were needed to achieve a 25% of consumer rejection in the case of whole leaves. These failure criteria were stricter than those traditionally used for sensory shelf life estimation of minimally processed lettuce, which might assure the products’ quality at the end of its shelf life. Results of the present study showed the importance of performing consumer studies in order to establish proper criteria to estimate the shelf life of fresh vegetables. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, the demand for fresh-like minimally processed fruits and vegetables has increased, mainly due to nowadays consumers’ concern about health and convenience (Odumeru et al., 2002; Zhou et al., 2004). This fact has led to an increase in the quality and variety of products available to the consumers (Francis et al., 1999). Thus, efforts must be taken to ensure the quality of minimally processed products (Piagentini et al., 2005; Ragaert et al., 2004). Minimally processed fruits or vegetables could be defined as fresh fruits or vegetables that have been processed to increase their functionality without greatly changing their fresh-like properties (Salunkhe et al., 1991). The most used processes are washing, cutting, mixing and packaging. These processes induce mechanical injury in the tissue changing its physiology, accelerating deterioration during transport and retailing, and consequently shortening
∗ Corresponding author. Tel.: +598 2 9245735; fax: +598 2 9241906. E-mail address: [email protected] (G. Ares). 0925-5214/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2008.02.006
their shelf life (Delaquis et al., 1999). The shelf life of minimally processed vegetables is generaly limited by changes in their sensory properties and not by microbial growth (King et al., 1991; Jacxsens et al., 1999, 2002). Colour and appearance are critical quality aspects for shoppers when selecting fresh fruits and vegetables (IFT, 1990; Ragaert et al., 2004). Therefore, the shelf life of lettuce can be defined as the length of time for which lettuce can maintain an appearance that appeals to the consumer (Zhou et al., 2004). Sensory shelf life of minimal processed lettuce could be determine as the time required for a certain sensory attribute to a certain predetermine intensity, or failure criteria (Hough et al., 2002). Traditionally, the sensory shelf life of minimally processed lettuce has been estimated considering as failure criteria 50% of the scale used to measure a sensory attribute (Barriga et al., 1991; Piagentini et al., 1997, 2004, 2005; Li et al., 2001; Jacxsens et al., 2002; Zhou et al., 2004). This criterion has been arbitrarily selected, and no studies have been found supporting the validity of using this failure criterion or correlating these sensory limits with consumers’ perception.
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However, food products do not have sensory shelf lives of their own; shelf life depends on the interaction of the food with the consumer. Consumers are the ones that decide if they would consume a food product after a certain storage time. For this reason consumers are the most appropriate tool for determining food product sensory shelf life (Hough et al., 2003). Although a consumer panel would seemingly be the most appropriate tool to determine the shelf life and quality of a food product, to repeatedly assemble consumer panels for multiple measurements would be both impractical and expensive. Although a sensory panel is more appropriate for repeated assessments, its results would be more analytical and not necessarily representative of consumer responses (Hough et al., 2002). By correlating data from a consumer panel with those obtained from a trained panel, these analytical measures can be used to determine the shelf life or quality of a food product. This would enable the determination of more objective failure criteria to estimate the shelf life of minimally processed vegetables, assuring products’ sensory at the end of its shelf life. The aims of the present work were to determine failure criteria based on consumers’ rejection to purchase for shelf life estimation of minimally processed lettuce, and to compare criteria for whole and cut lettuce.
A balanced complete block design was carried out for duplicate evaluation of the samples. Testing was carried out in a sensory laboratory that was designed in accordance with ISO 8589 (1988). Evaluations were performed under artificial daylight type illumination, temperature control (between 22 and 24 ◦ C) and air circulation.
2. Materials and methods
2.4.1. Analysis of variance For experimental data for whole and cut lettuce leaves, an analysis of variance was performed considering storage time as variation factor. Honestly significant differences were calculated using Tukey’s test. Differences were considered significant when p ≤ 0.05. This analysis was performed using Genstat Discovery Edition 2 (VSN International, Oxford, UK).
2.1. Plant material, packaging and storage conditions Butterhead lettuces (Lactuca sativa L., cv Wang) were obtained from a local farm near Montevideo (Uruguay). Within 24 h after harvest, lettuce was transported to the School of Engineering in Montevideo, Uruguay. Outer damaged and yellowed leaves and stems were removed. The remaining leaves were washed with cold chlorinated water (200 mg kg−1 total chlorine) for 10 min, and then rinsed with water. Lettuces were dried by centrifugation in a manual kitchen basket type centrifuge for 2 min. One half of the leaves were cut in pieces of approximately 3 cm × 2 cm using a sharp knife. Approximately 70 ± 5 g whole lettuce leaves or cut lettuce leaves were selected at random and placed under air in 30 cm × 40 cm bioriented polypropylene (BOPP; two layers of 20 m thickness) bags. BOPP films were provided by a local manufacturer. According to the supplier the BOPP gas transmission rates were 0.23–0.34 nL O2 m−2 s−1 Pa−1 , 0.69–0.80 nL CO2 m−2 s−1 Pa−1 (both at 23 ◦ C and 101 kPa) and 17–35 g H2 O m−2 s−1 (at 37 ◦ C and 90% RH). Bags were sealed using a Supervac GK105/1 (Wien, Austria) packaging machine with air injection, and stored at 5 ± 0.5 ◦ C and 90% relative humidity. Whole lettuce leaves were evaluated after 0, 10, 21, 28, 35, 42 and 49 d of storage; whereas evaluations for cut lettuce leaves were performed after 0, 3, 6, 8, 10, 13 and 17 d.
2.3. Consumer panel Consumers were recruited among students and workers from the Chemistry Faculty in Montevideo, Uruguay. The study was carried out using 40 people who consumed lettuce. Their ages ranged between 18 and 50 and they were approximately 50% female and 50% male. At each storage time, each consumer received one leaf of lettuce, for each storage condition, in closed odourless plastic container, labelled with three digit random numbers. For each sample, consumers had to evaluate its appearance and respond “yes” or “no” to the question “Imagine you are in a supermarket. You want to buy a minimally processed lettuce, and you find a package of lettuce with leaves like these, would you normally buy it?”. 2.4. Data analysis
2.4.2. Sensory attribute development modelling In order to model the development of the evaluated sensory attributes as a function of storage time, and to estimate the activation energy of these attributes, the following equations that assume zero (Eq. (1)) and first-order (Eq. (2)) reaction rate were used: X = X0 + kT t
(1)
X = X0 exp(kT t)
(2)
where X = value for the sensory attribute X at time t; X0 = value for the sensory attribute X at time t = 0; kT = reaction rate constant at storage temperature T; t = storage time; In order to estimate the equation’s parameters, linear and nonlinear regression facilities of Genstat Discovery Edition 2 (VSN International, Oxford, UK) were used. 2.5. Regression analysis
2.2. Trained sensory panel Samples were evaluated by a panel of six assessors, who had previous experience in the evaluation of fresh vegetables. At each storage time, a lettuce whole leave or approximately 20 g of randomly selected cut lettuce was presented to the assessors in a closed odourless plastic container labelled with three digit random numbers, at room temperature. The assessors had to evaluate the following attributes: off-odour, wilting appearance, presence of dark and necrotic stains on the leaf surface, presence of browning on the midribs. They also evaluated browning of the cut edges for cut lettuce samples. For scoring, 10 cm unstructured scales anchored with “nil” and “high” were used.
A logistic regression was carried out considering percentage of rejection (calculated as the proportion of consumers who answered “no” when asked if they would normally buy a package of lettuce containing cut or whole lettuce leaves like the ones they saw) as dependent variable, and the evaluated sensory attributes as ´ explanatory variable (Gambaro et al., 2006). The following equation was used: Logistic : Rejection percentage = a +
b 1 + e−c(X−d)
(3)
where X is each of the evaluated sensory attributes, and a, b, c and d are the regression constants. This analysis was carried out using Genstat Discovery Edition 2 (VSN International, Oxford, UK).
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Table 1 Models’ parameters (c.f. Eq. (2)) (value ± standard error) for the development of sensory attributes as a function of storage time, for whole lettuce leaves in passive modified atmosphere packages stored at 5 ◦ C
Table 2 Models’ parameters (c.f. Eq. (1)) (value ± standard error) for the development of sensory attributes as a function of storage time, for cut lettuce leaves in passive modified atmosphere packages stored at 5 ◦ C
Sensory attribute
Sensory attribute
Parameter X0
Off-odour Wilting appearance Presence of dark stains Browning on the midribs
(2.0 (1.8 (5.2 (1.1
R2
kT ± ± ± ±
0.1) × 10−3 0.1) × 10−5 0.2) × 10−2 0.1) × 10−2
0.16 0.26 0.10 0.13
± ± ± ±
0.1 0.2 0.1 0.1
0.99 0.96 0.99 0.98
Presence of dark stains Browning on the midribs Browning on the cut surfaces
Parameter X0
kT
R2
0.076 ± 0.009 0.085 ± 0.007 0.039 ± 0.005
0.14 ± 0.2 0.22 ± 0.1 0.26 ± 0.1
0.79 0.84 0.86
and wilting appearance was not modelled for cut lettuce due to the low scores reached at the end of the storage period considered.
3. Results and discussion 3.1. Sensory analysis
3.2. Consumer study All the evaluated sensory attributes were significantly (p < 0.001) affected by storage time for both whole and cut lettuce leaves, supporting the validity of the chosen descriptors as indicators of lettuce deterioration. Cutting caused a highly significant (p < 0.001) increase in the intensity of all the evaluated sensory attributes. This could be explained by the fact that cutting caused an acceleration of deterioration processes of lettuce due to tissue damage, loss of cell integrity and decompartimentalization (Degl’ I`nnocenti et al., 2005). Appearance attributes were the defects that occurred first and therefore were mainly responsible for the decrease in sensory quality with storage time, suggesting that sensory shelf life of minimally processed lettuce might be determined by changes in its appearance. A significant increase in off-odour was found only at the end of the evaluated storage period (after 17 d for cut lettuce and after 42 d for whole leaves). The evaluated appearance attributes showed different relative importance for whole and cut lettuce leaves, suggesting that deterioration might not follow the same pattern in both products. The development of wilting appearance was not of great importance for cut lettuce, as only two tenths of the scale were used by the sensory panel, which is consistent with the little weight loss registered (5.1% after 17 d). Presence of dark and necrotic stains on the leaf surface, browning on the midribs and on the cut surfaces were the attributes that showed a higher intensity (higher than 50% of the scale) at the end of the evaluated storage period. These attributes might determine the end of sensory shelf life of cut lettuce. On the other hand, in the case of whole lettuce leaves wilting appearance, presence of dark and necrotic stains on the leaf surface and browning on the midribs were the attributes responsible of limiting the shelf life of fresh-cut lettuce. Sensory shelf life could be determined as the time required for a sensory attribute to reach a certain predetermined intensity. Thus, it is necessary to model the development of the evaluated sensory attributes. As shown in Table 1, for whole lettuce leaves a relatively good fit was obtained for the development of all the evaluated sensory attributes with time considering first-order reaction rate, in agreement with published data for cut lettuce (Vankerschaver et al., 1996; Piagentini et al., 2005). However, for cut lettuce the best fit for the development presence of stains on the leaf surface and browning on the midribs and cut surfaces was obtained considering zero-order reaction rate (Table 2). The development of off-odour
Consumer studies are a useful tool to determine proper failure criteria for shelf life studies. One of the first steps is to determine which attributes are taken into account by consumers when deciding to accept or reject a certain product for its purchase. As mentioned above, the presence of stains on the leaf surface and browning of the midribs and the cut surfaces were the attributes that significantly increased first and were the ones that reached higher scores at the end of the evaluated storage period. Therefore, these attributes might have been considered by consumers and for this reason they were selected for further analysis. In order to determine if the selected attributes were considered by consumers, the development of rejection percentage was modelled considering selected sensory attributes as explanatory variables. A relative good fit was obtained for logistic regression of consumers’ rejection to purchase versus the selected sensory attributes (Tables 3 and 4), suggesting that these attributes were considered by consumers when deciding to purchase a package of cut or whole lettuce leaves. Using consumers’ data, sensory shelf life could be estimated as the storage time necessary to reach a certain consumers’ rejection to purchase or consume percentage. Sensory shelf life of foods’ products has been usually determined considering 25% rejection to ´ purchase or to consume (Hough et al., 2003; Gambaro et al., 2006; ´ Ares et al., 2006, 2008; Gimenez et al., 2007). This percentage means that if a consumer sees the product at the end of its shelf life, there is a 25% probability that he will reject to buy it. Considering that few consumers will see the product near the end of its shelf life, and that of the few that do 75% will still find the product acceptable, this value is considered reasonable from a practical point of view. Regressions shown in Tables 3 and 4 were used to estimate the attribute intensity that corresponds to a 25% consumer rejection to purchase. Attribute intensity corresponding to 25% consumer rejection to purchase were lower for cut lettuce than for whole leaves, suggesting that that consumers reacted differently towards whole and cut lettuce leaves, being stricter towards cut lettuce. This means that consumers were harsher when considering cutlettuce leaves and rejected to buy less deteriorated samples than when considering whole lettuce leaves. This could be attributed to the fact that as processing increases, consumers seemed to pay more attention to the product’s appearance and to be harsher when
Table 3 Models’ parameters (c.f. Eq. (3)) for the logistic regression of consumer rejection percentage versus presence of dark and necrotic stains on the leaf surface and browning on the midribs, and attribute intensity corresponding to 25% consumer rejection to purchase, for whole lettuce leaves in passive modified atmosphere packages Sensory attribute
a
b
c
d
R2
Attribute intensity corresponding to 25% consumer rejection to purchase
Presence of dark and necrotic stains Browning on the midribs
0.6 −7.3
99.9 10.0
1.7 0.8
3.6 3.4
0.99 0.99
2.4 2.3
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Table 4 Models’ parameters (c.f. Eq. (3)) for the logistic regression of consumer rejection percentage versus presence of dark and necrotic stains on the leaf surface and browning on the midribs, and attribute intensity corresponding to 25% consumer rejection to purchase, for cut lettuce in passive modified atmosphere packages Sensory attribute
a
b
c
d
R2
Attribute intensity corresponding to 25% consumer rejection to purchase
Presence of dark and necrotic stains Browning on the midribs Browning on the cut surface
−6.9 −2.0 −4.7
104.9 96.9 100.2
3.3 4.2 2.4
0.8 0.9 1.3
0.96 0.99 0.99
0.5 0.7 0.9
Table 5 Shelf lives (days) for whole and cut lettuce leaves in passive modified atmosphere packages stored at 5 ◦ C, estimated considering attribute intensity corresponding to 25% consumers’ rejection to purchase Storage condition
Whole leaves Cut leaves
Shelf life (days) considering the following sensory attributes Dark and necrotic stains on the leaf surface
Browning on the midribs
Browning on the cut surfaces
39 3.1
40 2.8
– 3.0
deciding whether to buy it or not. Thus, sensory limits depended on the product considered and therefore a unique criterion could not be used to estimate the shelf life of both cut lettuce and whole lettuce leaves. Moreover, different failure criteria might be necessary for different types of products. As shown in Tables 3 and 4, 25% of the consumers would refuse to purchase cut lettuce if the intensity of the evaluated defects was over 10% of the measuring scale, whereas scores of 25% of the scale were needed to achieve a 25% of consumer rejection in the case of whole leaves. In most studies the sensory shelf life of minimally processed vegetables has been estimated considering the time necessary to reach an arbitrary score of 50% of the scale used to evaluate a certain sensory attribute (Piagentini et al., 2005; Li et al., 2001; Zhou et al., 2004). Comparing the results of the present from the consumer study, this criterion seems not strict enough as to assure the products’ quality at the end of its shelf life. Therefore, if shelf life of minimally processed lettuce is determined considering 50% of the sensory scale, consumers might reject to purchase the product at supermarkets before its shelf life date. This could cause important economic losses at both production and commercialization stage, as some of these products might not be bought by consumers at supermarkets. If shelf life of whole lettuce leaves is set considering 50% of presence of brown stains, at the end of its shelf life the product would be bought by less than 8% of the consumers. This percentage is reduced to 2% if cut lettuce is considered. Therefore, if sensory shelf life is estimated considering 50% of the scale, products would not be buy by more than 90% of the consumers before estimated shelf life. These results showed the importance of performing consumer studies in order to establish proper criteria to estimate the sensory shelf life of fresh vegetables 3.3. Shelf life estimation As the development of the evaluated sensory attributes with storage time was modelled (Tables 1 and 2), shelf life of cut and whole lettuce was estimated as the time necessary to reach a certain failure criterion. Attribute intensities that corresponded to a 25% consumers’ rejection to purchase were considered as failure criterion (c.f. Table 5). As expected, sensory shelf life decreased with cutting, in agreement with trained assessors’ data. As consumers were much stricter when evaluating cut lettuce, its sensory shelf life was reduced to only 3 days, whereas shelf life for whole leaves was 39–40 d. As shown in Table 5, sensory shelf lives calculated considering the different evaluated attributes were in agreement, probably because they were all affected in a similar way by storage time. Thus, they could all be used to estimate the sensory shelf life of whole and cut lettuce.
Shelf lives estimated considering 50% of the scale for presence of dark stains as failure criteria corresponded to 46 and 13 d for whole and cut lettuce, respectively. These shelf lives were significantly higher than those estimated considering failure criteria from consumer studies. The difference in shelf life considering both failure criteria was greater for cut lettuce than for whole leaves, probably because consumers were harsher when evaluating the former product. 4. Conclusions According to Ragaert et al. (2004) consumers confer great importance to labelled shelf life during the buying stage. Therefore, proper shelf life labelling of minimally processed fruits and vegetables is an important issue to ensure the quality of these products and to satisfy consumers increasing requirements. Failure criteria were calculated using consumers’ data, which contributes an improvement over more arbitrary criteria used in most studies. Attribute intensities corresponding to 25% consumer rejection to purchase were lower for cut lettuce than for whole leaves, suggesting that consumers reacted differently towards whole and cut lettuce leaves, being stricter towards cut lettuce than towards whole lettuce leaves. Therefore, a unique criterion could not be used to estimate the shelf life of both cut lettuce and whole lettuce leaves. These results suggest the importance of performing consumer studies in order to establish proper criteria to estimate the shelf life of fresh vegetables. Considering results from the present paper, arbitrary selected failure criteria might not be strict enough to estimate sensory shelf lives, which could cause important economic losses at both production and commercialization stage, as some of these products might not be bought by consumers at supermarkets. Further research is needed to study consumers’ reaction to different minimally processed products, and to get more insight in consumer decision making process in order to for further investigating the quality of this type of products. Acknowledgment The authors are indebted to PDT (Programa de Desarrollo Tec´ nologico, Uruguay) for financial support. References ´ ´ Ares, G., Gimenez, A., Gambaro, 2008. Sensory shelf life estimation of minimally processed lettuce considering two stages of consumers’ decision-making process. Appetite 50, 529–535, doi:10.1016/j.appet.2007.11.002.
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