Comparative Study and Quality Evaluation of Italian Focaccias Seasoned with Extra Virgin Olive Oil

Abstract

An extra virgin olive oil was used as seasoning for three different types of focaccia produced on an industrial scale. This oil and samples of the oil extracted from the focaccias after baking were submitted to routine analyses and to silica gel column chromatography to separate polar compounds. These were then subjected to high performance size-exclusion chromatographic (HPSEC) analysis and it allowed to determine oxidative and hydrolytic degradation products. The results were compared with those obtained from artisan focaccias, seasoned with the same oil and toppings, by statistical analyses. The different technologies and the toppings employed directed in different ways the oxidative and hydrolytic processes in the oil.

Keywords:

• Focaccia
• Extra virgin olive oil
• Oxidation
• HPSEC
• Baking technologies

INTRODUCTION

Focaccia is a bakery product that is typical of the cooking of some Italian regions but widespread also in many other geographic areas. It is much appreciated for its organoleptic characteristics and versatility: it is usually consumed as a snack, appetizer, or even as a tasty alternative to bread. The traditional focaccia is prepared with few and simple ingredients: flour, yeast, water, salt, and oil. Different variants, traditional or new, even sweet are characterized by the topping added before baking on the surface of the dough. Nowadays, the market offers an industrial product as an alternative to the traditional artisan focaccia. It is a pre-cooked and frozen food, to be consumed after the completion of the baking. In the literature, little research has been carried out about the impact of baking technologies on this product. Some authors[1] compared different heating methods to evaluate the effects on colour and structure of breads; others[2] assessed the mass and heat transfer phenomena in pizza. Some investigation[3] concerned trans fatty acids content in bakery products, such as pizzas, biscuits, pastry, etc. The European multicenter TRANSFAIR Study, performed in 14 countries, revealed that trans fatty acids content varied greatly among the examined products, depending on the fat used and was lower in Southern European countries.[4] In light of the most recent researches, which have pointed out that the consumption of trans fatty acids would increase the risk of cardiovascular disease, Quìlez et al.[5] suggested to add phytosterols, α-tocopherol, and β-carotene to the bakery products with a total content in saturated fatty acids (SFA) + trans fatty acids (TFA) ≤20% of the fat fraction for their ability to reduce LDL-cholesterol. Priego-Capote et al.[6] proposed an alternative method (FMASE) to Folch reference method for the extraction of trans fatty acids from bakery products because it enables to reduce times and costs. In order to acquire some information about the influence of baking on the fat fraction, which is closely related to quality characteristics of baking products, an investigation was carried out regarding the oxidative and hydrolytic degradation of an extra virgin olive oil employed as a seasoning of focaccias produced on an industrial scale, as influenced by technology and the type of topping. The analytical data acquired were then compared with those obtained from a previous investigation[7] on focaccias produced by artisan technology, with the aim to evaluate the influence of production technology on the quality of the finished product.

MATERIALS AND METHODS

Three different types of focaccia were examined. They were prepared with the same dough obtained by durum wheat re-milled semolina, water, natural yeast, and salt. The dough, added with extra virgin olive oil on the surface, was manually arranged in loaves, which were put in apposite containers to be transferred in the leavening cell. The leavened loaves were manually rolled and rounded and then seasoned with the fixed amount of 15 g of extra virgin olive oil. Before cooking, toppings (onions, potatoes, and rosemary) were spread on the surface of the loaves.

The three types of focaccias resulted as follows:

• Onion-focaccia (O) consisted of a loaf of 400 g topped with about 200 g of sliced and frozen raw onions;

• Potato-focaccia (P) consisted of a loaf of 400 g topped with about 200 g of diced and frozen raw potatoes;

• Rosemary-focaccia (R) consisted of a loaf of 400 g topped with about 2 g of dry rosemary.

Cooking took place in a deck oven for 15–18 min at 220–230°C; the temperature at the core of the product was about 85°C at the end of cooking. After cooling at room temperature for 20–30 min, the focaccias were sprayed with extra virgin olive oil in the amount of 20 g, and transferred to the deep-freezing tunnel at a temperature of −40 to −38°C for 25–30 min, time occurring to reach a temperature at the core of −20 to −18°C. Each focaccia was then wrapped in a shrink film. Then, after automatic weighing and labelling, the focaccias were stored in a cold room at a temperature of −20 to −25°C until distribution. Once in the laboratory, the focaccias were kept in a freezer at −18°C and analysed within the storage time suggested by the manufacturer. The focaccias were thawed out for 30 min, then subjected to completion of cooking in a domestic oven at a temperature of 200°C for 15 min, following the instructions reported on the label.

After cooling at room temperature, diameter and thickness of each focaccia were measured and moisture[8] and fat content[9] were determined. Routine analyses were performed on the uncooked oil as well as on oil extracted from the baked samples by the Folch method, to determine percent free fatty acids (FFA), peroxide value (PV), UV absorption[10] and the p-anisidine value (p-AV).[11] Unconventional analytical parameters were also examined to better evaluate the degree of oxidative and hydrolytic degradation of the oils. Silica gel chromatography was performed according to the IUPAC method[12] to separate polar compounds (PC) from the oils and high performance size-exclusion chromatography (HPSEC) of the PC to determine the triglyceride oligopolymers (TGP), oxidized triglycerides (ox-TG), and diglycerides (DG). The HPSEC system consisted of a series 200 pump (Perkin-Elmer) with a 7125 S sample injector, 50 μL injector loop, a PL-gel guard column (5 cm × 0.75 cm i.d., Perkin-Elmer) and a series of two PL-gel columns (30 cm × 0.75 cm i.d., Perkin-Elmer) packed with highly cross-linked styrene divinylbenzene copolymer with a particle diameter of 5 μm and pore diameters of 500 Å. The elution solvent used was CH₂Cl₂ for HPLC at a flow rate of 1 mL/min. A differential refractometer (series 200, Perkin-Elmer Ltd., Beaconsfield, UK) was utilized as the detector. Peak identification and quantification was carried out as described elsewhere.[13,14] The fatty acid trans isomers were determined as described in EC Regulation no. 1429/92[15] utilizing a HRGC Mega 2 series gas chromatograph (Fisons, Milan, Italy) gas-chromatograph with a flame ionization detector equipped with a SPTM 2340 fused silica capillary column (60 m in length × 0.25 mm i.d. and film thickness 0.20 μm, Supelco, Bellefonte, PA, USA). The temperature was set to a range from 160 to 200°C with an increasing rate at 1.3°C/min, temperature of the split injector was 210°C with a split ratio of 1:100, and detector temperature was 220°C. The carrier gas was hydrogen. The data obtained were compared by ANOVA, Tukey Test for multiple comparisons and principal components analysis (PCA) using the XLStat software (Addinsoft, New York, USA).

RESULTS AND DISCUSSION

The focaccias showed moisture ranging from 30 to 35 g/100 g, a diameter of 26–28 cm, and a thickness of 2–3 cm. The routine analyses performed both on the uncooked oil and on the baked oil samples provided the results shown in Table 1; they provided initial information about the hydrolytic and oxidative degradation of the oils as affected by the production technology. The percent free fatty acids increased in all baked oils, in particular in the oil sample extracted from the onion-topped focaccia. The results obtained indicate that all values exceed the limit of 0.8% fixed for the extra virgin olive oil.[16] Also, the indices of oxidative degradation proved a worsening of the quality. In fact, K₂₇₀ increased after baking from 0.14 in the raw oil to values ranging from 0.84 to 1.08 (legal limit for the extra virgin olive oil: 0.22) in the oils extracted; K₂₃₂ reached values that were about 1.5–2 times greater than that found in the raw oil; whereas ΔK showed only slight increases. Total oxidation of the baked oils, expressed as TOTOX (2PV + p-AV), was higher than that of extra virgin olive oil; in particular, the increase was above 60% in rosemary-topped focaccia. More detailed information on the oxidative and hydrolytic degradation of the baked oil samples were obtained by HPSEC analysis of polar compounds. Figure 1 reports the HPSEC chromatogram of the PC of the oil extracted from the potato-topped focaccia and shows the good separation of the substance classes constituting the PC. Table 2 reports the results of the HPSEC analysis of the PC and the results of statistical elaboration of the data. The oils extracted from the three types of focaccia showed a two-fold greater increase in TGP (mean value 0.14%, p < 0.05) than the value observed in the raw oil. No statistical difference was found comparing the TGP of the oils extracted from the three types of focaccia. The percentage of ox-TG measured in the uncooked oil was 0.76%, statistically different from those found in all the baked oils; the latter values ranged from 1.0 to 1.33% and resulted in a significant difference from each other. The production technology, as well as the toppings, did not lead to significant increases of DG in the oils extracted from focaccias (mean value 1.77%) compared with raw oil (1.63%). The overall level of oxidation, expressed as (2TGP% + ox-TG),[17] confirmed the greater extent of oxidative degradation of the oil employed as seasoning of the rosemary-topped focaccia. In fact, it showed a value of overall oxidation equal to 1.61, against the 0.91 of the extra virgin olive oil. In Table 3, the percent amounts of trans isomers of unsaturated fatty acids are reported together with the results of the statistical analysis. The oils extracted from the focaccias after baking had total amounts of trans isomers approximately three times greater—ranging from 0.07 to 0.08%—than the level in the uncooked oil (0.02%), with significant differences in comparison with the uncooked oil, but not from each other. The two different stages of baking and the time of storage caused the different levels of oxidation found, which were higher in the oil sampled from the rosemary-topped focaccia than in the other oils. These findings seem to be ascribable to the different amounts, percent humidity, and properties of the toppings used. The industrial technology provides, with the exception of the rosemary-topped focaccia, for the addition of frozen toppings, which have the effect—due to the thawing out and subsequent evaporation of the water—of blandishing the thermal stress on the seasoning oil during baking. The topping with diced potatoes (moisture 78.5%) seems to have better protected the oil from the oxidative degradation. In the case of the focaccia flavoured with dry rosemary, no protective effect took place during heating and the oil extracted after baking presented a greater extent of degradation, in spite of the well-known anti-oxidant activity of rosemary.[18,19] The level of oxidation found after baking of the focaccias proved to be rather little and lower than that generally found in refined oils.[17,20] These results testify to the good quality of focaccia seasoned with extra virgin olive oil. The data obtained were statistically compared by analysis of variance (ANOVA) and principal component analysis (PCA) with the results of our previous paper on the focaccias produced with artisan technology.[7] The studies differed only for the production technology because all the examined focaccias were seasoned with the same extra virgin olive oil and prepared with the same ingredients. The statistical analysis was carried out comparing the unconventional (TGP, ox-TG, and DG) and conventional analytical parameters (FFA, PV, K₂₃₂, K₂₇₀, ΔK, trans isomers).

Table 1 Mean results of the routine analyses for each oil sample examined. ^a

Samples	FFA (%)	PV (meq O2/kg)	K232	K270	ΔK	TOTOX
Extra virgin olive oil	0.60	16.4	2.11	0.14	−0.02	39.02
Oil extracted from:
Onion-topped focaccia	1.40	19.9	3.92	1.08	0.03	46.11
Potato-topped focaccia	1.19	20.8	3.16	0.84	0.02	50.61
Rosemary-topped focaccia	1.12	29.7	4.14	1.02	0.02	63.80
^aMean values of two independent repetitions. FFA: free fatty acids; PV: peroxide value; K232: specific absorption at 232 nm; K270: specific absorption at 270 nm; ΔK = K270 − (K266 + K274)/2; TOTOX = 2PV + p-AV (p-AV, p-anisidine value).

Table 2 Percentage values (w/w) in oil of the main substance classes constituting the polar compounds for each oil sample examined. ^a

Samples	TGP	ox-TG	DG	2TGP + ox-TG
Extra virgin olive oil	0.08 ± 0.01a	0.75 ± 0.04a	1.63 ± 0.16a	0.91
Oil extracted from:
Onion-topped focaccia	0.15 ± 0.01b	1.16 ± 0.08b	1.81 ± 0.11a	1.46
potato-topped focaccia	0.14 ± 0.00b	1.00 ± 0.01c	1.74 ± 0.02a	1.28
rosemary-topped focaccia	0.14 ± 0.00b	1.33 ± 0.01d	1.76 ± 0.01a	1.61
^aResults of statistical analysis at p < 0.05. Mean values of three independent repetitions ± SD; one common letter following an entry indicates no significance. TGP: triglyceride polymers; ox-TG: oxidized triglyderides; DG: diglycerides.

Figure 1 HPSEC chromatogram of the polar compounds of the oil extracted after baking from the potato-topped focaccia. TGP: triglyceride oligopolymers; ox-TG: oxidized triglycerides; DG: diglycerides; ST: free sterols and triterpene diols; FFA: free fatty acids.

Table 3 Percentage values (w/w) in oil of the trans-isomers of fatty acids for each oil sample examined. ^a

Samples	C18:1t	C18:2t+C18:3t	Total trans-isomers
Extra virgin olive oil	0.022 ± 0.002a	tr.a	0.022 ± 0.002a
Oil extracted from:
Onion-topped focaccia	0.061 ± 0.008b	0.008 ± 0.001a	0.069 ± 0.009b
Potato-topped focaccia	0.054 ± 0.007b	0.024 ± 0.013a	0.079 ± 0.019b
Rosemary-topped focaccia	0.074 ± 0.003b	0.008 ± 0.011a	0.082 ± 0.013b
^aResults of statistical analysis at p < 0.05. Mean values of three independent repetitions ± SD; one common letter following an entry indicates no significance. tr., traces (not integrated).

Two-way ANOVA was applied to unconventional parameters. The amounts of TGP and ox-TG in the oils extracted from focaccias after baking registered significant differences (p < 0.05) in relation both to the type of topping used and to the technology employed (Table 4). Specifically, the potato-topped focaccia showed lower levels of oxidative degradation, regardless of the production technology employed. The industrial technology showed to determine lower levels of oxidation compounds. The content in DG did not seem to be influenced by the production technology; as regards the toppings, only the potato influenced significantly the hydrolytic degradation of the finished product. The effect of the interaction of the two variables considered resulted very significant, as attested by the low p-values. The principal components analysis performed considering both the conventional and unconventional parameters, allowed to describe approximately the 94% of the total variability with the first three principal components (PC1, PC2, PC3). Scores and loadings are reported in the biplots in Figure 2. The biplots put in evidence the relationships between the principal components and the original variables. The PC1 expresses a great amount of the variability of the peroxide value (PV) and of the classes of substances constituting the polar compounds (TGP, TG-ox, DG). The PC2 is mainly the expression of variability of spectrophotometric constants (K₂₃₂, K₂₇₀, ΔK) and free fatty acids (FFA). The PC3, finally, is correlated with the amount of trans isomers and also with the indexes of secondary oxidation (TGP, K₂₇₀, ΔK) and peroxide value (PV). The uncooked oil showed negative values on all components. The oils extracted from the artisan focaccias had negative scores on PC2, very close to each other. As regards the PC1, the onion-topped and the rosemary-topped focaccias had positive scores, whereas the potato-topped focaccia had a negative one. Regarding the PC3, all artisan baked oils showed increases in scores compared with the uncooked oil. The oils extracted from the focaccias produced by industrial technology, compared to the raw oil, showed significant increases with positive scores on PC2, and values near to the uncooked oil on PC1 and PC3. Overall, these oils were much less different from each other than those extracted from the artisan focaccias.

Table 4 Results of two-way ANOVA, with interactions, performed on the data. ^a

	Topping			Technology
	Onion	Potato	Rosemary	Artisan	Industrial	Topping × technology
TGP	A	C	B	a	b	p < 0.0001
ox-TG	B	C	A	a	b	p < 0.0001
DG	A	B	A	a	a	p = 0.001
^aA common letter indicates no significant difference (p < 0.05). A > B > C, a > b.

Figure 2 Biplot of PCA, principal component analysis. EVO: extra virgin olive oil; O: onion-topped focaccia; P: potato-topped focaccia; R: rosemary-topped focaccia; A: artisan focaccia; I: industrial focaccia.

Comparing the technologies, it appears that the industrial technology leads to a remarkable increase of the spectrophotometric constants and free fatty acids in respect to the uncooked oil. This increase is higher than that observed in the artisan focaccias. The industrial technology seems to have partially flattened the effects of the toppings used. On the contrary, the characteristics of the oils extracted from artisan focaccias appeared much more heterogeneous. In fact, the indexes of oxidation (PTG, ox-TG, PV) hydrolytic degradation (DG), and isomerization of the fatty acids seem strongly influenced by the topping. The spectrophotometric constants and the free fatty acids showed a slight increase in the baked oils compared to the raw oil and appeared to be little influenced by the toppings used to flavour the focaccias.

CONCLUSIONS

The present comparative investigation showed that the production technology combined with specific toppings influenced significantly on the characteristics of the oil employed as seasoning and, therefore, on the quality of the finished product. The technologies considered and the different toppings employed directed in a different way the oxidative and hydrolytic processes in the oil. Specifically, the focaccias produced by artisan technology were very different, putting in evidence a greater influence of the topping on the characteristics of the finished product. On the contrary, the industrial production technology partly flattened this influence. Among the toppings used, the diced potatoes seemed to better protect the quality of the seasoning oil.