Acta Alimentaria, Vol. 34 (4), pp. 427–440 (2005)

RESPONSE TO HOT AIR DRYING OF SOME OLIVE CULTIVARS OF THE SOUTH OF ITALY A. PIGAa, B. MINCIONEb, A. RUNCIOb, I. PINNAa, M. AGABBIOa and M. POIANAb,* aDipartimento

di Scienze Ambientali Agrarie e Biotecnologie Agro-Alimentari, Università degli Studi, Viale Italia, 39/A, 07100 Sassari. Italy bDipartimento di Biotecnologie per il Monitoraggio Agroalimentare ed Ambientale, Università degli Studi Mediterranea, Località Vito, 89060 Reggio Calabria. Italy (Received: 20 September 2004; accepted: 16 August 2005) Olive fruit dehydration is always done locally in non-industrial ovens. This technology poses concern about the quality and safety of the end product. Most of the problems involved in this empirical technology may be solved by a proper control of process parameters. Olive fruits of fourteen Italian cultivars underwent hot air dehydration in mild conditions in a tangential airflow cabinet dryer. At the start, at regular intervals and at the end of the process, sampling was performed to calculate dehydration curves and quality loss. Pre-treatments such as as blanching in hot brine, piercing of the skin and salting after blanching were applied. The drying kinetic is strongly affected by pre-treatments and olive characteristics: fruit size, flesh to pit ratio, dry matter. Results showed that mild drying temperature led to slow drying kinetics, even if pre-treatments reduced drying time to a certain extent. Blanched olives showed, in general, the highest polyphenols content. The fastest drying was measured in the pierced olives, but the best taste was achieved for the salted product. Keywords: blanching, drying kinetic, olive, piercing, process parameters, salting

According to the definition reported in “the Unified Qualitative Standard Applying to Table Olives in International Trade” (IOOC, 1980), table olives are “the sound fruit of specific varieties of the cultivated olive tree harvested at the proper stage of ripeness and whose quality is such that, when they are suitably processed produce an edible product and ensure its good preservation as marketable goods”. Processing is aimed mainly at removing the natural bitterness of the olive fruits, and different technologies have been developed for this purpose. The main preparations are (GARRIDO FERNANDEZ et al., 1997): – Spanish style pickled green olives in brine; fruits are treated with alkali solution, washed, then fermented in brine; – Californian style pickled black olives in brine; fruits are treated with alkali and oxidized to obtain a uniform colour; – Greek style natural olives in brine; fruits are placed in a suitable brine and a natural fermentation takes place. * To whom correspondence should be addressed. Tel.: +39 0965 682688; fax + 039 0965 682710; e-mail: [email protected]

0139-3006/$ 20.00

2005 Akadémiai Kiadó, Budapest

428

PIGA et al.: AIR DRYING OF SOME OLIVE CULTIVARS

The international trade rule (IOOC, 1980) has classified different preparations. The list comprises some typical products of the Mediterranean area: shrivelled black olives, black olives in dry salt and dehydrated olives. “Maiatica di Ferrandina” cultivar has been used for a long time in Italy to obtain dried product. Olives are blanched, stored in salt for 2 or 3 days, and dehydrated in an oven. The technology gives a particular taste to the product. Dehydration not only prevents natural losses from foodstuffs but also gives them particular sensorial attributes. In Greece, the Thrubolea cultivar is used to prepare the “Thruba-style olives”. By this method, olives, which lose their bitterness on the tree, probably by means of a fungal enzyme, are sun-dried to remove most of their moisture, then a small quantity of salt is added to obtain the final product. In some cases, dried olives are submitted to lye treatment (GARRIDO FERNANDEZ et al., 1997). The air-drying process is driven by the properties of the air used and of the food being dehydrated. The production of dried olives at non-industrial level is carried out using empirical oven devices, by which the drying parameters cannot be accurately controlled, thus the end product lacks uniformity and has a lower quality level, mainly from the microbiological standpoint. The literature lacks reports on the effects of forced air-drying applied to olives, in relation to starting material (cultivar), pre-treatments and process parameters. In particular, the cultivar has an important role due to the great variability of the fruit characteristics such as carpological parameters, texture and composition (in particular, bitter taste). Few data are available about the so-called local cultivars, mainly used for oil production. Southern Italy, in this context, offers a certain number of olive cultivars. In a previous work, GAMBELLA and co-workers (2000) reported data about the effect of different pre-treatments on drying green table olives of the “Ascolana” cultivar. Bitter free olives of good taste were obtained by dipping in 10% brine at 50 °C before the dehydration process. MARSILIO and co-workers (2000) studied the effects of treatments on the microscopic structure, pectic fraction and firmness of Cassanese olives processed by the “Ferrandina” method. The softening of the olive tissues increased after heat treatment and correlation between protopectin and firmness was observed. Based on this experience, some research was carried out on the air-drying performance of fourteen local olive cultivars (three from Apulia, eight from Calabria and three from Sardinia), as well as on quality changes after processing. 1. Materials and methods 1.1. Plant material The following olive cultivars (and corresponding growth area) were used for the experiments: – “Borgese”, and “Tonda di Strongoli” from the Calabria region and particularly from the Kroton district, North Ionian Sea coastline; – “Cassanese”, “Dolce di Rossano” and “Roggianella” from northern Calabria; – “Carolea”, typical of the Calabria region; – “Geracese”, from the Calabria region, South Ionian Sea coastline;

Acta Alimentaria 34, 2005

PIGA et al.: AIR DRYING OF SOME OLIVE CULTIVARS

429

– – – – –

“Leccino”, harvested in the Apulia region; “Manna”, from the island of Sardinia; “Mele”, from the Apulia region; “Nera di Gonnosfanadiga”, from Sardinia island, particularly the Cagliari district; “Nocellara Messinese”, from the east of Sicily and widespread also in Calabria where the sample was collected; – “Pizz’e carroga”, from the island of Sardinia, particularly from the Cagliari district; – “Termite”, from the Apulia region. Fruits were harvested between the end of September and the first decade of November at a maturity stage suitable for drying and immediately transported to the laboratory, where only fruits without or with minimal peel defects were selected. Calibration by weight was performed in order to have uniform fruit callipers, and not to affect dehydration times. 1.2. Pre-treatments

Before drying, the whole fruits were pre-treated, in order to try to speed up the drying process. Pre-treatments were: – Blanching in 2% NaCl water solution at 90 °C for 2 min, cooling to ambient temperature with tap water (solution/fruit was 10:1 v/w) – “blanched olives”; – Blanching with water at 90 °C for 2 min followed by 3 days of storage in barrels where the olives were disposed in alternate layers with salt (salt/fruit ratio was 1:10) to reproduce the “Maiatica di Ferrandina” process – “salted olives”; – Mechanical skin piercing with a steel brush – “pierced olives”. Fruits were examined again to discard damaged olives then air-dried. 1.3. Drying equipment and process parameters Fruits were dried in a laboratory pilot dryer. The air dryer was a tangential air flow cabinet (C.R.S.I., Lugano Switzerland) equipped with automatic temperature and air moisture control devices. Air flows tangentially to fruits, while a particular air recycling system allows mixing exhaust air with fresh air and then reheating and redirecting to the product, in order to regulate the desired moisture of air. The particular construction of the dryer permits continuous air flows on the fruits, avoiding turbulences, and consequently it is particularly suitable for calculating drying kinetics. Fruits were placed on steel shelves (product load from 10 to 13.7 kg m–2); three shelves were loaded for each treatment (the drier can hold ten shelves). The process was stopped when the fruits reached an estimated 20–25% water content (based on weight loss calculations). Process parameters were as follows: air temperature at ambient conditions (25 °C), drying air temperature 50 °C, relative air humidity at entrance 40%, volumetric flow rate 1840 m3 h–1and air recycling to keep relative humidity below 40%. Dried fruits were packaged inside water barrier plastic bags (coextruded polyethylene/polypropylene, thickness of 95 m ) and stored at –18 °C until analysis.

Acta Alimentaria 34, 2005

430

PIGA et al.: AIR DRYING OF SOME OLIVE CULTIVARS

1.4. Assessments and determinations A sample of 100 fresh fruits of each cultivar was used to determine mean weight, pulp and stone mean weight and pulp to stone ratio. Fresh and dried fruits were inspected for water and dry matter content (%), water activity, and polyphenols content. Drying kinetics were calculated by plotting the water content revealed at regular intervals during processing versus process times, while drying rates were computed from water loss and process times. Water content was determined in an oven for 4 h at 105 °C until constant weight was reached. Water activity of fruit pulp was assessed by an aw meter (model Aqualab, Decagon, Pullman, WA). Phenols were extracted according to AMIOT and co-workers (1986) and determined spectrophotometrically at 760 nm, after reaction with the Folin-Ciocalteu reagent (expressed as mg of gallic acid/kg of flesh). An informal panel of ten untrained assessors evaluated rehydrated samples and gave a preference judgement. 1.5. Statistical analysis Data of polyphenols content of dry matter were subjected to one-way ANOVA using MSTAT-C software. Means were separated, when necessary, by Duncan’s multiple range test at P 0.01. 2. Results and discussion 2.1. Fruit characteristics The carpological parameters of the olives are reported in Table 1. Carpological data reveal that some cultivars are unsuitable for processing as table olives. The fourteen cultivars were divided by fruit weight, according to the IOOC (2000) standard classification: – medium weight fruits (from 2 to 4 g): “Borgese”, “Cassanese”, “Dolce di Rossano”, “Geracese”, “Roggianella” and “Leccino”; – high weight fruits (from 4 to 6 g): “Carolea”, “Nocellara Messinese”, “Tonda di Strongoli”, “Manna”, “Nera di Gonnosfanadiga”, “Pizz’e carroga”, “Mele” and “Termite”. It must be pointed out that some cultivars of the first group seem too small to be dried. However, other technological parameters, such as the flesh to pit ratio, made us consider them valuable for processing. We decided, therefore, to use the classification proposed by BRIGHIGNA (1998). – flesh/pit