Changes of physicochemical and sensory characteristics of packed ripe table olives from Spanish cultivars during shelf life
Pedro García-García*,Antonio Higinio Sánchez-Gómez and Antonio Garrido-Fernández
Department of Food Biotechnology, Instituto de la Grasa (AECSIC)
Avda. Padre García Tejero 4, 41012 Sevilla (Spain)
Short title: Ripe olive shelf-life
*Corresponding author:
Pedro García-García
Department of Food Biotechnology, Instituto de la Grasa (AECSIC)
Avda. Padre García Tejero 4, 41012 Sevilla (Spain)
Tlf: +34 954690850
Fax: +34 954691262
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Summary
Changes in the physicochemical and sensory characteristics of commercial plain (whole) and pittedripe olives of the Gordal, Manzanilla,Hojiblancaand Cacereñacultivars were studied during a three-year period in conditions that mimic those found in real life during the storage of such products. No spoilage developed during this period.Throughout the shelf-life, a marked valley decrease in the pH of cover brine at the beginning of storage followed by a progressive decrease was observed,the surface colour (measured instrumentally) and firmness of the olives degraded in accordance with a first-orderkinetic, iron and calcium addition reduced colour and firmness degradation respectively. Also, a slight browning of the cover brines at the beginning of the storage was observed. No significant changes in most of the sensory characteristics were observed by the panel test during shelf-lifeexcept for alimited change in olive surface colour. At the end of the shelf life, most of the samples were classified as “extra” category according to the IOC sensory evaluation method and only plain Gordal presentation was classified as “first, choice or select”.
Key words: olives, sensory analysis, shelf-life
Introduction
Ripe olive processing was introduced in California (USA) at the beginning of the 20th century. Nowadays, this style is wide spread in all table olive producing countries. According to the latest data published by the International Olive Council (IOC), ripe olives may account for around 30% of the world’s table olive production, which was about 2,400,000 tons in the 2011/2012 season (IOC, 2013); this means that 630,000 tons/year are prepared as ripe olives.
The style is described as “olives darkened by oxidation” in the Trade Standard for Table Olives issued by the International Olive Council (IOC, 2004). However, they are commonly known by their original American name: ripe olives.
Usually, fruits for producing this style are previously stored in an aqueous solution (brine or acidic water) and darkened throughout the year according to demand (de Castro et al 2007).The darkening process consists of successive treatments of the fruits with a dilute solution of NaOH (lye);during the intervals between lye treatments the fruits are suspended in water through which air is bubbled (Sánchez-Gómez et al 2006). Throughout this operation the fruits darken progressively (Breneset al 1992). The colour formed is not stable and fades progressively after oxidation. To preventthis deterioration, fruits are immersed in ferrous lactate or gluconate for several hours (García et al 2001). The product has a final pH above 4.6 and its preservation is only achieved by sterilization (CODEX/COI 1990). The most common commercial presentationsin retail are whole and pitted olives.
The UK Institute of Food Science and Technology (IFST) defined shelf-life as ‘‘the period of time during which the food product will remain safe, retain its desired sensory, chemical and microbiological characteristics, and comply with any label declaration of nutrition data” (IFST 1993).
According to the European Union Directive (2000) relating to the labelling, presentation and advertising of foodstuffs, "date of minimum durability of a foodstuffs is the date until which the food retains its specific properties when properly stored”. However, after that date, the food may still be in satisfactory condition and quality, with a high probability of retaining those circumstances for a further additional period of time.
The shelf-life is different from the expiration date, which refers to food safety, while shelf-life has to do with quality of food. Basically, the appropriate shelf-life depends on the manufacturer who makes the product. To be confident of its statement, the company should have done the necessary work in order to determine the correct shelf-life.
In Spanish style green olives, a first approach to the shelf-life was carried out by Sánchez et al(1997) but was limited to only Manzanilla cultivar packed under laboratory conditions stored at 20 ºC but, recently, a research with different cultivars and commercial preparations under real conditions of storage has been published (Sánchez-Gómez etal 2013). Additionally, the losses in fruit firmness and colour with time in untreated green olives of the Conservolea cultivar packed in vacuum or modified atmospheres were reported by Panagou (2004).
Inripeolives, the evolution ofparameters suchas the pHof the cover brine, surface colourand fruit firmness of different cultivars (Hojiblanca, ManzanillaandCacereña)has been monitored, usingan accelerated shelf-life test (ASLT) (García-García et al 2008). However, changes in sensory characteristics were not monitoreddue the reduced time period of study and the strong effect of temperature (up 60ºC) on these attributes.
The aims of the present work were to study the evolution of the physico-chemical and sensory characteristics of commercial ripe olives of different Spanish cultivars (Gordal, Manzanilla,Hojiblanca and Cacereña), according to presentations (plain and pitted) and stored under real preservation conditions during the period of time (3 years) currently adopted for the "best before" recommendation on the label.
Materials and Methods
Olives and storage
The study was carried out with fruits from the most common cultivars devoted to ripe olive trade preparations in Spain (Gordal, Manzanilla, Hojiblanca and Cacereña), presented as plain and pitted (Table 1). The olives were packed in tin cans and glass bottles (jars) with contents ranging from 150 g in “12oz cans” of pitted Manzanillato 1.800 g in “A10 cans” of whole Gordal.
The experiment was initiated immediately afterprocessing the olives (supplied by Spanish processors) and the sample collection was coordinated by INTERACEITUNA (Spanish Inter-professional Association of Table Olives).
The samples were stored in the pilot plant of the Food Biotechnology Department of the Instituto de la Grasa (Sevilla, Spain). The storage temperature was periodically controlled, between 12:00 and 13:00 hours, at least every 10-14 days. The average temperature of storage was 22.7ºC, and ranged from 13 ºC in winter to 32 °C in summer (Sánchez-Gómez et al 2013), with a maximum daily fluctuation of about 3 °C. Only, ¼ gallon containers (pitted Hojiblanca, pitted and whole Cacereña) were exposed to entering light through the windows of pilot plant. Therefore, the applied storage conditions properly mimic those found in real life during the storage of such products.
Physicochemical analysis
Samplings were made upon receiving the product (0 months) and at 2, 6, 9, 12, 18, 24, 30 and 36 months of storage. At each sampling time, two samples (tin cans or glass bottles) for each of the commercial presentations were analyzed.
In the case of jars, the presence of sediment at the bottom of the containers was observed before opening. Then, regardless of the type of container, vacuum or overpressure (mm of mercury) the interior of the containers was monitored by introducing a gauge through the lid of the can or jar.
The determination of pH and NaCl concentration in the cover brines was carried out using the routine methods described by Garrido Fernández et al(1997).
The colour of the solutions was determined as the difference in absorbance at 440 and 700 nm (A440–A700), using a 1-cm cell path length and a Varian Cary 1E Spectrophotometer (Malgrave, Vi, Australia). Previously, the liquids were centrifuged at 12,000g for 10 min (Montaño et al1988). The colour was also expressed in terms of the CIE L* (whiteness or brightness/darkness), a* (redness/greenness) and b* (yellowness/blueness).
The surface colour of the fruits was measured using a BYK-Gadner Model 9000 Colour view spectrophotometer (Silver Spring, MD, USA). Any interference from stray light was minimized by covering the samples with a box, which had a matt black interior. Colour was expressed as reflectance at 700 nm (R700) (Garrido Fernández et al1997). Lower reflectance values indicate darker colours. In addition, colour was measured in terms of the CIE L* a* b* parameters and their derivatesChroma (C) and Hue angle (H). Results were the mean of 10 measurements.
Firmness was measured using a Kramer shear compression cell coupled to an Instron Universal Testing Machine (Canton, MA, USA). The cross head speed was 200 mm/min. The firmness of the olives, shear compression force in Newton (N), was expressed as N/100 g pitted olives and the value was the mean of 10 measurements, each of which was performed on one pitted olive for theGordal cultivar and on three pitted olivesfor the other cultivars.
Iron and calcium in the olive flesh was determined, in triplicate, by flame atomic absorption spectrometry in 3 samples of each commercial presentation (García et al2002).
Sensory evaluation
Canned fruits were evaluated by an 8-member trained panel, using the Department’s standardized testing room. Evaluations were made at product reception (0 months) and at 6, 12, 24 and 36 months of storage. This panel has a high level of training since it has been used for decades for different studies on various types of olives (Rejano et al1995; Medina et al2011; Sánchez-Gómez et al 2013) and, particularly, for all the works related to the development of the sensory method issued by the International OliveCouncil (2010).
The olives were tested according to the "Method for sensory analysis of table olives", COI/OT/MO 1/Rev.2 No 1 (IOC 2010), using the profile sheet also included in this methodology. This method employs the descriptors related to the perception of negative sensations (abnormal fermentation and other defects), gustatory attributes (salty, acid, bitter) and kinaesthetic sensations (hardness, fibrousnesses, crunchiness), in order to commercially classify the olives. When appropriate, specific descriptors were added to the sheet; these were related to external appearance: surface colour, brightness and skin defects; odour/flavour: typical flavour, soap taste (due to possible wrong neutralization) and metallic taste (due to iron addition); and texture:skin strength and pit release, using the same unstructured scale recommended in the IOC Standard (IOC 2010).
Three or four olives were presented to each taster in a normalized glass according to standard COI/T.20/Doc. No 5 (Glass for oil tasting). Panellists should indicate the intensity they perceived for each of attributes in the scales of the provided profile sheet. The left extreme indicates the absence of an attribute while the right end was maximum perception.
To determine the intensities of the attributes listed in the profile sheet the segment running from the origin of the scale to the mark made by the tester was measured using a ruler. The segment was expressed to one decimal place. The scale measured 10 cm long and the intensity ranged from 1 to 11. The statistic used to indicate the values of the attributes is the median of the individual data of the 8 testers (IOC 2010).
Degradation kinetic
For the degradation of surface colour and texture, kinetics of diverse orders was checked. Finally, a first-order kinetics was assumed. It was similar to that used by Sánchez-Gómezet al(2013) in Spanish green table olives and that applied to other parameters for ripe olives (García-García et al2008). If the quality factor is designated as F, its rate of destruction over time (month) is given by the equation:
dF/dt = - k * F (1)
wheredF/dt = degradation of parameter per unit of time, F = value of parameter at time t, k = rate constant (month-1) which leads to the integrated equation:
Ln (F/F0) = -k * t(2)
whereF0 represents the initial value of the studied parameter at time zero (month).
Statistical analysis
Statistica version 6.0 (StatSoft, Tulsa, USA) for windows was used for data analysis. Comparison of parameters among commercial presentations was carried out by superimposing their correspondingconfidence intervals (CI) at p<0.05. Differences were considered significant when the CI did not overlap. ANOVA post comparisons were also made considering the same level of probability (p<0.05).
Results and discussion
Visual observation of jars and vacuum
The jars of whole and pitted ripe olives did not present sediment atthe bottomof the containers orturbidityincover brines after three years of conservation.
Duringshelf-life, no significant variation in the determinations of vacuum in the olive containers was observed.As shown in Table 1, the highest vacuum values were observed in non-deformable containers such as jars (11.1-15.7 mm of mercury). In cans, the vacuum values were lower (3.2-8.3 mm of mercury), possibly because of the slight deformation produced when the can was pressed with the vacuum gauge as demonstrated by the fact that the lowest vacuum value (p<0.05) was shown in the largest can (A10 Gordal–Plain, Table 1), which deforms more easily. This also was observed in the shelf-life study of Spanish green table olives (Sánchez-Gómez et al 2013).
It is noteworthy that this vacuum retention means that the product was stable and no secondary fermentation was produced in the containers during shelf-life. Therefore, thermal sterilization treatments applied as wellthe closures of containerswere always appropriate.
pH changes in brines
The initial pH values (6.4-7.2) of cover brines were normal for this type of preparation. However, contrary to what happens with green olives (Sánchez-Gómez et al2013), pH did not remain stable over the shelf-life. As can be seen in Figure 1 there was always a rapid decrease in the initial levels followed by a subsequent increase after the 6th monthof storage. However, after the 9thmonth a new gradual decrease was noticed up to the end of storage.
Garcia-Garcíaet al (2008) observed that the pH of the ripe olive cover brine decreased with time according to a first-orderkinetic, the rate was higher when the temperature increased. The absence of the initial decrease and subsequent increase shown in Figure 1 in the ASLT tests could have been due to the constant (although different) temperatures used for the storage in the ASLT experiments.
On the contrary, in this study the storage temperature was not constant. During the first four months (March-June) the temperature increasedup to 32°C, a level which was maintained until the 6th month sampling at the end of August. These high temperatures can produce, according to the study of Garcia-García et al (2008), a more rapid decrease in pH, which may explain the initial drop in pH observed in Figure 1.
When trying to adjust the pH evolution to a 1st order kinetics as García-García et al (2008), the fit obtained was low (R2 <0.4) as expected from the evolution during the first few samplings (Figure 1). However, when removing the points corresponding to sampling at 2 and 6 months, the remaining points fit well (R2> 0.81) to a first-orderkinetics, as shown in Table 2.
Rate constant values (0.0015-0.0052 month-1) were of the same order as the average value (0.0033 month-1) for all commercial presentations at 20°C obtained by Garcia-García et al(2008).
The most influential factor on the pH change rate was the commercial preparation. Thus, for the same cultivar, the rate was statistically higher (p<0.05) in plain olives than in pitted olives (Table 2).This may be because during sterilization the pulp-liquid exchanges are favoured in the pitted olives.The cultivar had noeffect on the rate constant.This decrease in pH over time had nomeaning from the safety point of view because ripe olives are preserved by heat treatment and are then a sterile product.
Cover brine colour
As shown in Figure 2, the evolution of (A440-A700) parameter, brine colour,with time was similar in all commercial preparations. The profile showed a progressive increase in the coloration of the liquid during the first six months but, from this moment, (A440-A700) values were stable or slightly increased. The initial increase in the coloration of the liquid may be related, as in the pH evolution (Figure 2), with the marked increase in the environmental temperature during the first month’s storage as spring was advancing. The high summer temperatures may have favoured osmotic exchanges of the compounds responsible for the colour, and a rapid equilibrium between the pulp and the liquid in just 6 months.
The (A440-A700) rise in the cover brine during shelf-lifewas related to the increase in red tonality, because the CIE a* parameter values increased 5-15 units (Table 3);in addition, a very significant decrease in the luminance values (19-33 units) occurs simultaneously, which moved the colour toward darker tonalities.The CIE b* parameter has a variable evolution, suffering changes of ±15 units in some cases while it only changed slightly in others.
Accordingly, during the packed storage, the cover brine of ripe olives intensified the red tonality (a*) and simultaneously suffered a darkening as evidenced by the decrease in luminance values (L*).
Surface olive colour
In the plain olives of Gordal and Hojiblanca cultivars (Figure 3), a slightincrease in the reflectance at 700nm (R700) with storage time was observed. The highest R700increase (more than3 units after 3 years) was recorded for whole Cacereña olives.In pitted olives the colour practically remained stable throughout the three years of study.
The R700 increase in whole olives was related to a slight increase (1-2 units) in luminance (L*) and in red tonalities (a*) and a slightly larger increase (2-4 units) in yellowness (b*) (data not shown). In pitted olives, the CIE L*, a*, b* parameters did not statistically (p<0.05) change during shelf-lifein agreement with R700 stability. Therefore, only a small initial black discoloration was observed during plain olive shelf-life.
The colour stability of the pitted olives is due to the greater iron content (> 100 mg of Fe/ Kg of flesh) than in the whole olives (Table 1) and to the fact that the pH did not change during shelf-life (Breneset al 1985). It is known that higher amounts of iron in the olive flesh led to a more intense black surfacecolour (lower R700) (Garcíaet al2001). In fact, adding an amount of iron in packed cover brine, as is performed industrially,implies a greater amount of fixed iron in pitted olives than in whole olives (Garrido et al 1995).
García et al(2008) fit a first-order kinetic to the degradation of the surface colour of ripe black olives in aASLT test.However, in this study, the fit to the reflectance at 700nm(R700) curves from pitted samples of Manzanilla, Hojiblanca and Cacereñacultivars(Figure 3) of such kinetic was also rather poor (R2 <0.40, Table 2) due to the limited changes in R700 (Figure 3).