Essential rosemary oil enrichment of minimally processed potatoes by vacuum-impregnation

Abstract

Vacuum impregnation (VI) has been recognized as a promising tool for the introduction of solutes into the internal structure of some porous food products. The enrichment of minimally processed potatoes with aromatic compounds could represent an interesting method for product innovation. This study was aimed at applying VI with rosemary essential oil on a minimally processed potato product in order to obtain an innovative fresh-cut potato product, and to evaluate its influence on the physico-chemical, sensorial and microbiological properties of potato sticks during refrigerated storage and after frying. A pressure of 60 mbar was applied for 30 min followed by a relaxation time at atmospheric pressure of 30 min to potato sticks immersed in rosemary oil solutions in concentration between 0 and 12%. Prepared samples were packed and stored at 4 °C for 14 days. Analytical determinations were carried out on the fresh and fried product. The weight gain of potatoes promoted by VI was in the range of 6–14%, depending on the concentration of rosemary essential oil. The rosemary essential oil concentration gradients of impregnated potato sticks were detected by GC analysis and sensorial test, evidencing their persistency during storage and after frying. The treatment seemed to improve microbiological stability, not affecting the texture, moisture, but slightly deteriorating the product color. Results obtained in the present study confirm the potentiality of VI for fresh products innovation.

Introduction

Fresh cut fruit and vegetables are defined as any fresh fruit or vegetable or combination thereof physically altered from its original form, but remaining in a fresh state, which offer consumers high nutrition, convenience and value. Since their origin in the early 1980’s, the consumption of fresh-cut fruits and vegetables has been characterized by a tremendous growth due to their health and convenience benefits (Rico et al. 2007). However, comparing to total volume of fruits and vegetables sold in Europe, the market share of fresh cut products represents only few percent points. In 2010, the fresh cut fruits and vegetables shared only 1% and 5% of total consumption of fruits and vegetables in Europe (Baselice et al. 2014). Therefore, the development of innovative products is still needed in order to improve the consumption of fresh cut fruit and vegetables.

The odor and flavor attributes are important factors influencing the purchasing motivation of consumers, in addition to the appearance characters of fresh cut fruits and vegetables, which has already been extensively studied (Forney 2008; Toivonen and Brummell 2008). Essential oil is a concentrated liquid containing volatile chemical compounds extracted from plants characterized by a high odorous impact. It is acceptable by consumers at appropriate concentrations when applied to foods (Dias et al. 2015) and can improve the flavor of potato chips when added in the oil used for frying (Chammem et al. 2015). Potato (Solanum tuberosum) is the third largest food crop in the world (Wang et al. 2015), and minimally processed potatoes are popular commodities due to the high consumer demand of potato chips, French fries and baked potatoes. For these reasons, aromatic minimally processed potatoes could be very attractive for consumers as well as industries.

However, potato tissues are relatively tight, impermeable to water and gases. Previous study confirmed that inserting an external solution into potato tubers by simple immersion proved to be very difficult (Hironaka et al. 2011). An auxiliary technology may be therefore needed in order to impregnate potatoes with an aromatic solution.

Vacuum impregnation (VI) satisfies that requirement as, thanks to the action of hydrodynamic mechanisms promoted by pressure changes, is able to efficiently impregnate a porous structure. Fruits and vegetables have a great amount of intercellular space which are occupied by gas and offer the possibility to be impregnated by external solutions (Fito et al. 2001). VI has been widely used to incorporate various solutes into the internal structure of fruit and vegetable porous matrices. Previous studies have successfully investigated the impregnation of fruit and vegetable tissues with solutions containing anti-browning agents, microbial preservatives, cryoprotectants (Panarese et al. 2014) or components for quality and nutritional improvements (Alzamora et al. 2005; Betoret et al. 2007, 2015), including potatoes (Sapers et al. 1990; Hironaka et al. 2011, 2014, 2015). To the best of our knowledge the odor and/or flavor enrichment of minimally processed potatoes by VI has not been studied yet.

Rosemary essential oil is characterized by a widely accepted flavour and suitability to potato in terms of sensorial properties. Moreover, its antimicrobial and antioxidant activities are well known and are due to the synergy between its components (Jiang et al. 2011). For these reasons, it could represent an interesting product to be used for the impregnation of potatoes, exploiting both its flavor and its stabilizing properties.

The aim of the present research was to study the enrichment of minimally processed potato sticks with rosemary essential oil through VI in order to obtain an innovative aromatic minimally processed potato product. The physico-chemical, sensorial and microbiological properties of impregnated potato sticks during refrigerated storage and after frying were evaluated.

Materials and methods

Raw materials

Potato tubers (Solanum tuberosum), cultivar Daisy, normally used for industrial processing of french-fries, were grown in Emilia Romagna region and collected in January 2017 by the company Pizzoli S.R.L. (Italy). Tubers of uniform size (40–50 g) without superficial defects and with an average dry matter content of 19.6 ± 0.3% were selected and stored in the dark at 10 °C and 90% relative humidity (RH) for a maximum of 2 weeks, before treatment.

Aromatic solutions

The rosemary oil suspension used in this work was provided by DKS AROMATIC S.R.L. (Italy). The composition of the suspension was: water, propylene glycol (E1520), hydroxy propyl distarch phosphate (E1442), octenil succinate (E1450), rosemary essential oil, xanthan gum (E415) and potassium sorbate (E202). The different additives have different uses: E1520 as humectant, E1442, E1450 and E415 as emulsifiers and thickeners, while E202 is added as preservative on account of its antimicrobial activity.

Vacuum impregnation process and sample preparation

The vacuum impregnation (VI) treatment was performed using a system that allowed the control of both the pressure acting on the impregnating solution during the process and the velocity of vacuum level and atmospheric pressure restoration. The potato samples, obtained as described below, and the impregnating solution were placed in a cylindrical glass chamber (10 L volume), which was connected with a rubber tube to a vacuum pump (SC 920, KNF ITALY, Milan, Italy). The system was controlled by an electro-mechanical control unit (AVCS, S.I.A., Bologna, Italy).

Potato tubers were washed with tap water and then dipped in a 200-ppm sodium hypochlorite solution for 2 min for sanitizing the surface. The skins were peeled using a sharp knife and then cut by a manual cutter into pieces of rectangular shape of 1 × 1 × 7 cm. The VI solutions were prepared at 4%, 8% and 12% (w:v) of essential oil suspensions in distilled water.

Potatoes sticks were immersed in the rosemary solutions (product weight/solution weight 1:1.5) and a sub-atmospheric pressure of 60 ± 10 mbar was applied for 30 min followed by a relaxation time at atmospheric pressure of 30 min. VI parameters (vacuum time and relaxation time) were chosen on the basis of preliminary tests (data not shown). Afterwards the excess liquid on the samples surface was removed lightly with absorbent paper. Control samples were obtained by immersing potato sticks in distilled water at atmospheric pressure for the same time of the whole VI treatment (about 70 min). The prepared samples were the following: control (dipped in distilled water); 0% (subjected to VI in distilled water); 4% (subjected to VI in 4% rosemary oil suspension); 8% (subjected to VI in 8% rosemary oil suspension) and 12% (subjected to VI in 12% rosemary oil suspension). Each VI treatment has been replicated at least three times.

After the treatment, potato sticks were packed in polypropylene trays, sealed with a medium permeability polyethylene film of 200 μm thickness. 40 packages with about 100 g of potatoes sticks for each sample were prepared. The samples were stored at 4 °C for 14 days and sampled regularly for analytical determinations. Analyses of minimally processed potato samples were carried out after 0, 3, 7, 10 and 14 days on sticks from at least three packages for each sampling time.

Moreover, after 0, 7 and 14 days of storage, analytical determinations were carried out also on the product after frying. Frying was carried out with a home fryer mod. F989 (De Longhi, Italy), using peanut oil (w:w oil:sample ratio 20:1) at 180 °C for 5 min. After frying, the potatoes were drained of excess oil and gently dried on paper towels for 5 min before the analytical determinations.

Analytical determinations

Physico-chemical properties of potatoes and rosemary solutions

Water activity (a_w) of fresh potatoes and rosemary oil solutions were measured by Aqua LAB (3TE, Decagon Devices, Inc.).

The pH value and viscosity of the solutions were measured using a pH-meter (Cdberscan pH 510, Eutch Instruments, Singapore) and a vibrational viscometer mod. Viscolite 700 (Hydramotion Ltd., York, England).

The bulk density (ρ_b) of fresh potatoes was determined by measuring the volume of the sample (about 3 g) by displacement using a pycnometer with glycerin as the reference liquid. The solid–liquid density (ρ_s) was measured on the sample previously de-aired in order to eliminate pores and air. The porosity (ε) of fresh potato tissue was calculated from the values of ρ_b and ρ_s by the following equation (Nieto et al. 2004):

$$\epsilon =\frac{{\rho }_{\text{s}}-{\rho }_b}{{\rho }_{\text{s}}}$$

All physical chemical properties were analyzed in triplicate measurements.

Mass transfer parameters

To determine the total mass change due to VI, the weight of the potato sticks was measured before (M₀) and after (M_t) the VI treatment. The mass changes (ΔM) were calculated by the following equation (Neri et al. 2016):

$$\mathrm{\Delta }M=\frac{M_{\text{t}}-M_0}{M_0}\times 100$$

Mass change was measured in triplicate on three independent samples.

Moisture content

The moisture content of the samples was determined gravimetrically in triplicate by difference in weight before and after drying at 70 °C in a vacuum oven, until a constant weight was achieved (AOAC 2000).

Respiratory gases in the package headspace

The composition of O₂ and CO₂ (%) in the package headspace during storage was determined by a gas analyser “Check Point O₂/CO₂” model MFA III S/L (Witt-Gasetechnik, Witten, Germany). At each sampling time, measurements were obtained for at least three packages for each sample.

Color measurement

A spectrophotocolorimeter model Colorflex (HUNTERLAB ColorFlexTM, Reston, Virginia) was used to measure surface color of minimally processed and fried samples (D65 illuminant and 10° standard observer). For each piece, measurements were performed on each side. The L* and a* parameters of the CIELAB scale were considered. Results were expressed as average of 10 measurements for sample.

Texture measurement

The texture of the potato samples was evaluated by subjecting each sample to a penetration test using a dynamometer model Texture Analyzer TA. HDi500 (Stable Micro Systems, Surrey, U.K) equipped with a load cell of 5 kg, using a cylindrical shape stainless steel probe of 2 mm diameter. The penetration rate was 1 mm/min and the depth was 0.5 cm. From the graphs obtained, the parameter of hardness (N) that represents the maximum force required to make the penetration of the sample, was extrapolated. Texture measurements were carried out on 10 different sticks.

Microbiological analysis

The total aerobic counts, psychrophilic bacteria, molds and yeasts counts were carried out for microbiological analysis of potato samples during 14 days of storage. 50 g of each sample were added to 50 mL of saline (0.85% sodium chloride solution) in a sterile polyethylene bag and mixed by a stomacher (Seward Stomacher 400, UK) for 2 min at high speed. Further decimal dilutions were made with sterile saline. Spread plate method was applied to enumerate the total aerobic counts, psychrophilic bacteria, molds and yeasts counts using Plate Count Agar (PCA) and Yeast Extract-Peptone-Dextrose agar (YPD) with 200 ppm antibacterial as culture medium, respectively. The storage of the plates took place at 30 °C for 48 h for total aerobic and molds and yeasts counts and at 10 °C for 10 days for psychrophilic bacteria.

The microbiological analysis was conducted on the raw product after 0, 7, 14 days of storage. Each micro-organism was determined in two sample trays and two replicates for each treatment at each sampling time, and the results were transferred to log₁₀ (CFU/g).

Analysis by gas chromatography with mass spectrometry (GC–MS)

Potato sticks impregnated with rosemary essential oil, followed or not by frying, were analyzed for volatiles using a VG Platform II GC–MS system equipped with a DB-5MS capillary column (30 mm × 0.25 mm i.d.; film thickness 0.25 m), both for raw potato sticks and fried ones.

The solid phase microextraction (SPME) was used to extract the volatile components from the essential oil of rosemary. After being taken out from refrigerated storage room (4 °C), the potato trays were maintained at 30 °C for 20 min. The volatile components were isolated using a SPME fiber for 5 min at room temperature (22 °C).

For GC–MS detection, an electron ionization system of electrons with ionization 70 eV energy has been used. Helium was employed as the carrier gas, at a flow rate of 0.8 mL/min. The temperature of the injector and transfer line of the MS detector were set at 160 °C and 265 °C. The column temperature was initially set at 50 °C and maintained for 10 min, and then increased gradually to 160 °C at a rate of + 5 °C/min, kept at this temperature for 2 min and finally brought to 280 °C at the speed of +5 °C/min. The components were identified by comparison of their relative retention times and mass spectra with those of the standard (for major components)—NIST library data of the GC–MS system and the literature data (Jiang et al. 2011). The GC–MS analysis was conducted at 0, 7, 14 days of storage, both to the minimally processed potatoes and fried ones.

Sensory descriptive analysis (DA)

Sensorial assessment of fried potato samples has been performed by a panel of 10 fully trained assessors (age between 25 and 50 years, five females and five males) recruited because of their previous experience in descriptive sensory analysis (staff and Ph.D. students at the Campus of Food Science, University of Bologna, Cesena, Italy) and their familiarity with the product.

In order to prevent panelist fatigue, the attribute list has been minimized. After the descriptors selection, training sessions have been carried out, following the guidelines of the ISO 13299:2010. The test sessions were performed in a closed room in separate tasting booths. A final list of seven descriptors have been selected and a hedonistic scale from 0 to 8 has been used. In Table 1 sensory terms, definitions and reference of each descriptor are reported.

Table 1.

Sensory terms, definitions and reference of each descriptor

Sensory modalities	Descriptors	Definitions	Referencesa and their values on scale
Appearance	Colour	Typicality and homogeneity of French fries colour	Weak (0); strong (8)
Aroma and flavour	Potato odour	Intensity of fried potato odour	Weak (0); strong (8)
	Rosemary odour	Intensity of rosemary odour	Weak (0); strong (8)
	Potato flavour	Intensity of fried potato flavour	Weak (0); strong (8)
	Rosemary flavour	Intensity of rosemary odour	Weak (0); strong (8)
Texture	Crispness crust	Intensity of crust crispness	Weak (0); strong (8)

^aReferences established by the panel during the training section

Randomized blocks of fried potato samples, labelled with random three-digit codes, have been analysed. During sensorial analysis, water has been used to cleanse the palate and between each sample analysis 2-min break has been allowed.

Sensory analysis was performed after frying and room temperature reconditioning for 3 min, until the samples reached an acceptable temperature for consumption (50 °C).

Statistical analysis

The statistically significant differences among the treatments were analyzed by SPSS 22.0 (SPSS Inc., Chicago, IL, USA) by analysis of variance (ANOVA) using the LSD test for comparison of the data (p < 0.05).

Results

Physico-chemical properties of potatoes and rosemary solutions

The pH, water activity (a_w), viscosity and porosity values of raw potatoes and rosemary oil solutions are reported in Table 2.

Table 2.

Physico-chemical properties of potatoes and rosemary solutions

Parameters	Potatoes	Rosemary oil solutions
		0%a	4%	8%	12%
pH	6.30 ± 0.01a	–	2.99 ± 0.01b	2.98 ± 0.01b	2.96 ± 0.01b
aw	0.997 ± 0.003b	1.000 ± 0.000a	0.997 ± 0.001b	0.998 ± 0.002b	0.997 ± 0.001b
Viscosity (cP)	–	1.0 ± 0.0a	0.97 ± 0.6a	0.97 ± 0.06a	0.97 ± 0.06a
Porosity (%)	1.87 ± 0.45	–	–	–	–

Values followed by different letters are significantly different (p < 0.05)

^a0% solution was considered as distilled water

The solutions used for the impregnation, with concentration of 4, 8 and 12% of essential oil were not significantly different for either pH (between 2.99 and 2.96), water activity (between 1.000 and 0.997), nor viscosity (between 1 and 0.97).

The pH of the solutions was much lower than that of the potatoes, while the water activity showed no significant differences. The porosity of the potatoes was 1.87 ± 0.45%. This finding is in agreement with previous studies (Alzamora et al. 2005; Hironaka et al. 2015).

Weight gain after treatment

In the control sample, immersed in distilled water for 70 min, a weight gain of about 8% was observed. By applying vacuum to the sample immersed in water (0% solution), the weight increase was approximately 12%. The results also showed that increasing the concentration of rosemary oil proportionally decreased weight gain, until reaching values similar to the ones achieved by immersion in distilled water at atmospheric pressure. However, all weigh gain was in the range of 8–12% (data not shown), significantly higher for the 0 and 4% sample and similar to the control for the 8 and 12% sample.

Physico-chemical parameters and microbial loads of minimally processed potatoes during storage

The moisture content of fresh potatoes was 80.37 ± 2.76%. This parameter did not show significant differences during storage, ranging from 78.83 ± 2.19 to 82.96 ± 1.57% (data not reported).

The headspace gas evolution of respiratory gases is reported in Table 3. All the samples showed a progressive decrease of O₂ and an increase of CO₂ during storage, promoted by potato tissue respiration and packaging permeation. The sample subjected to vacuum impregnation with only distilled water (0%) showed lower O₂ and higher CO₂ values compared to the sample immersed in water at atmospheric pressure (control), indicating a higher respiration rate. The samples impregnated with solutions containing rosemary extract showed results similar to sample 0%.

Table 3.

Physico-chemical and microbial loads of packed minimally processed potato sticks during storage at 4 °C

Storage time (days)	0	3	7	10	14
Sample
Headspace gases
O2 (%)
Control	20.90A, a	15.77B, abc	9.47C, ab	7.03D, a	1.13E, a
0%	20.90A, a	15.55B, b	8.00C, b	1.35D, b	0.55D, a
4%	20.90A, a	14.67B, c	8.55C, ab	3.80D, c	0.45E, a
8%	20.90A, a	15.70B, abc	8.25C, b	2.37D, bc	1.57D, a
12%	20.90A, a	14.97B, abc	9.87C, a	3.85D, c	1.37E, a
CO2 (%)
Control	0A, a	4.10B, a	7.30C, a	8.43C, b	10.43D, a
0%	0A, a	4.00B, a	7.77C, a	10.85D, a	10.85D, a
4%	0A, a	4.47B, a	7.90C, a	10.55D, a	11.50D, b
8%	0A, a	4.05B, a	7.60C, a	10.93D, a	11.47D, ab
12%	0A, a	4.50B, a	7.40C, a	10.00D, a	10.53D, a
Colour
L*
Control	70.15A, a	66.20AB, a	66.93AB, a	64.67B, a	63.44B, a
0%	62.90A, b	67.77A, a	66.87A, a	65.81A, a	64.90A, a
4%	63.69A, b	66.50A, a	64.92A, a	63.20A, ab	62.21A, ab
8%	65.03A, b	66.81A, a	65.21AB, a	62.71AB, ab	61.45B, ab
12%	64.51A, b	66.65A, a	65.48AB, a	61.71AB, b	59.81B, b
a*
Control	− 0.46A, a	0.21AB, a	0.27AB, a	0.59B, c	2.09C, ab
0%	− 1.08A, b	0.16B, a	0.14B, a	1.09BC, bc	1.57C, b
4%	− 1.04A, b	0.45B, a	0.40B, a	2.18C, ab	2.66C, a
8%	− 0.99A, b	0.27B, a	0.63B, a	2.58C, ab	2.78C, a
12%	− 0.75A, ab	0.18B, a	0.37B, a	3.12C, a	2.94C, a
Microbial load (log CFU g−1)
Total aerobic count
Control	3.13A, a	–	3.46A, a	–	3.36A, a
0%	3.35A, a	–	3.33A, a	–	3.53A, a
4%	3.54A, a	–	3.63A, a	–	3.67A, a
8%	1.75A, b	–	3.18B, b	–	3.83C, a
12%	2.12A, b	–	3.52B, a	–	3.76B, a
Yeasts and moulds
Control	0A, a	–	0A, a	–	2.48B, a
0%	0A, a	–	0A, a	–	2.84B, a
4%	0A, a	–	0A, a	–	2.90B, a
8%	0A, a	–	0A, a	–	0.56B, b
12%	0A, a	–	0A, a	–	0.91B, b
Psychrophilic bacteria
Control	1.60A, a	–	3.02B, a	–	3.80B, a
0%	1.45A, a	–	3.23B, a	–	3.78C, a
4%	1.60A, a	–	2.98B, a	–	3.57C, a
8%	0A, b	–	1.52B, b	–	3.58C, a
12%	0A, b	–	1.95B, b	–	3.15C, b

Capital letters indicate significant differences (p < 0.05) among the same sample at different sampling time, lowercase letters indicate significant differences (p < 0.05) among samples at the same sampling time

In Table 3, the brightness (L*) and red index (a*) values measured in minimally processed potato samples during storage are reported. Immediately after the treatment, the vacuum-impregnated samples showed significantly lower L* and a* values compared to the sample immersed in water at atmospheric pressure. During storage, a progressive decrease in the value of L* and an increase of a* were observed for the control sample, probably due to enzymatic browning. In the 0% and 4% impregnated samples, no significant differences were found with regard to the brightness (L*) between the beginning and the end of storage, but only in relation to a*. However, samples impregnated with solutions containing 8% and 12% of essential oil, were characterized by a higher browning level until the 10th day of storage, but only in terms of a*.

Hardness of samples was in the range 8–10 N. Immediately after treatment, the samples subjected to VI showed lower average hardness values (data not reported) compared to the sample impregnated at atmospheric pressure (control), although the differences were not statistically significant because of the high natural variability of the data (typical of texture measurements of cell turgid vegetables). In any case, a reduction in hardness may be due to an alteration of the potato structure due to the application of the vacuum. On the contrary, during storage, the hardness was almost unchanged and there were no differences among the various samples (data not reported).

The microbial loads related to, respectively, total aerobic count yeasts and molds and psychrophilic bacteria have been detected in minimally processed potato samples during storage (Table 3). Samples impregnated with the higher essential oil concentrations (8 and 12%) showed a reduced load (1.75–2.12 log CFU/g) compared to the others (3.1–3.5 log CFU/g), probably due to the antimicrobial activity of rosemary. During storage the values increased for 8 and 12% samples, but only slightly, up to values between 3 and 4 log CFU/g after 14 days, without significant differences between the samples, while remaining unchanged for the other samples.

Yeast and molds count was below the limit of detection (1 log CFU/g) until the 7th day of storage, rising above it in only at the end of the storage, remaining significantly lower in samples impregnated with the 8 and 12% concentrated solutions. Similarly, psychrophilic bacteria were reduced in the 8 and 12% samples to values below the detection limit, while in the other samples were in the range 1.45–160 log CFU/g. In all samples loads increased during storage. However, at the end values were still significantly lower in the 12% sample compared to all the others.

Microbiological results indicated that the impregnation with rosemary essential oil had an antimicrobial effect on the natural-occurring classes of microorganisms considered, proportional to its concentration.

In Table 4 the evolution of the amount of volatile compounds in fresh potato sticks during storage is reported. The main detected compound was eucalyptol, followed by camphor, 3-methyl-apopinene, α-pinene, 1,3,8-p-menthatriene, α-camphene and linalool. The quantity of the main volatile compounds like eucalyptol, camphor, generally decreased with storage time and in a similar way for the different tested concentration. At the end of storage, camphor, 1,3,8-p-menthatriene and α-linalool were no longer detectable in all samples. On the other hand, the quantity of some other compounds like 1,3,8-p-menthatriene, α-pinene increased during the first week and then decreased. Behavior of many compounds followed a similar trend independently of the concentration, this may be due to some specific formation/degradation pattern that has yet to be clarified. At the last day of storage, the presence of ethanol was detected, probably due to the endogenous cell metabolism and/or growth of microorganisms.

Table 4.

Volatile compounds (expressed as area/weight 10⁻⁶) detected in minimally processed potato samples at 0, 7 and 14 days of storage at 4 °C

Storage time (days)	Control			0%			4%			8%			12%
	0	7	14	0	7	14	0	7	14	0	7	14	0	7	14
Volatile compound
Ethanol	ND	ND	0.48A, a	ND	ND	0.53A, a	ND	ND	0.32A, a	ND	ND	0.53A, a	ND	ND	0.33A, a
3-methyl-apopinene	ND	ND	ND	ND	ND	ND	4.52A, b	8.99B, a	4.99A, b	8.85A, a	11.23A, a	4.34B, b	9.56A, a	10.44A, a	15.84B, a
Camphene	ND	ND	ND	ND	ND	ND	1.97A, a	2.40A, a	0.47B, a	3.84A, b	3.89A, b	0.42B, a	4.30A, b	3.76A, b	3.52A, b
α-pinene	ND	ND	ND	ND	ND	ND	2.45A, a	5.26B, a	3.34A, a	6.07A, b	7.11A, b	2.69B, a	6.76A, b	6.59A, b	8.82B, b
Eucalyptol	ND	ND	ND	ND	ND	ND	54.83A, a	19.71B, a	11.1C, a	94.71A, b	36.01B, b	12.75C, a	112.91A, b	32.24B, b	14.66C, a
1,3,8-p-menthatriene	ND	ND	ND	ND	ND	ND	5.32A, a	6.01A, a	ND	9.46A, b	8.18A, b	ND	6.72A, a	8.41A, b	ND
3-octanol	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	0.55A, a	ND	ND
Camphor	ND	ND	ND	ND	ND	ND	15.26A, a	3.11B, a	ND	19.09A, b	8.82B, b	ND	20.64A, b	7.13B, b	ND
α-linalool	ND	ND	ND	ND	ND	ND	3.32A, a	ND	ND	4.61A, b	ND	ND	4.45A, b	1.47B, a	ND
p-Menth-1-en-4-ol	ND	ND	ND	ND	ND	ND	ND	ND	ND	0.20A, a	ND	ND	0.77A, b	ND	ND
Borneol, (1S,2R,4S)-(-)-	ND	ND	ND	ND	ND	ND	2.44A, a	2.31A, a	ND	3.95A, b	6.36B, c	ND	3.96A, b	4.99B, b	7.08C, a
p-Menth-1-en-8-ol	ND	ND	ND	ND	ND	ND	1.85A, a	ND	ND	2.71A, b	ND	ND	2.50A, b	ND	ND

ND not detected

Capital letters indicate significant differences (p < 0.05) among the same sample at different sampling time, lowercase letters indicate significant differences (p < 0.05) among samples at the same sampling time

Physico-chemical and sensorial parameters of fried french-fries

The brightness (L*) and red index (a*) values detected on fried potatoes were consistent with those of minimally processed samples in terms of changes during storage (Table 5).

Table 5.

Volatile compounds (expressed as area/weight 10⁻⁶) and color parameters detected in fried potato samples at 0, 7 and 14 days of storage at 4 °C

Storage time (days)	Control			0%			4%			8%			12%
	0	7	14	0	7	14	0	7	14	0	7	14	0	7	14
Volatile compound
2-methyl-propanal	0.84A, a	0.77A, a	0.56A, a	0.78A, a	0.66A, a	0.60A, a	0.59A, a	0.71A, a	0.87A, a	0.67A, a	0.82A, a	0.57A, a	0.85A, a	0.69A, a	0.77A, a
Eucalyptol	ND	ND	ND	ND	ND	ND	93.22A, a	65.40B, a	37.21C, a	113.41A, b	92.35B, b	50.75C, ab	163.21A, c	122.40B, c	53.45C, b
Pyrazine	4.38A, a	3.59A, a	5.28A, a	4.43A, a	5.25A, a	5.07A, a	5.13A, a	3.87A, a	4.46A, a	4.59A, a	5.58A, a	5.29A, a	5.48 A, a	3.52A, a	3.99A, a
Camphor	ND	ND	ND	ND	ND	ND	28.59A, a	16.38AB, a	11.20B, a	44.26A, ab	19.29AB, b	11.08B, b	63.65A, b	36.02B, c	15.71C, c
Colour
L*	62.49B, b	66.77A, a	64.28AB, a	63.97A, a	64.83A, ab	62.52A, a	67.42A, a	67.56A, a	63.13B, a	65.24A, a	61.43B, b	62.41AB, a	66.86A, a	62.20B, b	58.93B, b
a*	− 0.97A, a	− 0.04A, c	0.86B, d	− 1.69A, b	− 0.01B, c	1.68C, c	− 1.71A, b	− 0.28B, bc	1.90C, c	− 2.09A, b	0.86B, b	2.38C, b	− 1.90A, b	0.33B, a	4.16C, a

ND not detected

Capital letters indicate significant differences (p < 0.05) among the same sample at different sampling time, lowercase letters indicate significant differences (p < 0.05) among samples at the same sampling time

Immediately after frying, the L* and a* values were higher in samples impregnated with the 4, 8 and 12% rosemary oil solutions compared to the control and 0% samples. However, while values did not change during storage for 0% impregnated samples the 8 and 12% samples have undergone a decrease of L* and increase of a*. Actually, the a* value after frying showed a progressive increase with the increase of the storage time in all the groups. Nevertheless, while at the beginning the differences among the samples were minimal, after 14 days of storage, VI samples showed significantly higher values than the control sample, impregnated at atmospheric pressure. Moreover, the 12% sample was characterized by the highest value compared to the others. In terms of volatile compounds, a lower number of components was detected. In the control and 0% sample, only 2 components were present (2-methyl-propanal and pyrazine). Their concentration was constant and unchanged in all samples during storage. In samples impregnated with 4, 8 and 12% of rosemary essential oil, proportional concentration of camphor and eucalyptol were detected. During storage they decreased in all samples (Table 5).

In terms of sensorial properties (Fig. 1), at day 0 (Fig. 1a) the parameter linked to the appearance (color uniformity) did not show significant difference among the samples. The control group has maintained similar values up to the 14th day of storage (Fig. 1c), while although some differences were observed among samples at day 7 (Fig. 1b), at the end of the storage values were similar in all samples. Furthermore, the samples impregnated with higher concentrations of essential oil showed significantly lower values at the end of storage, which was consistent with the findings of the instrumental assessment of the color. For parameters related to smell, typical smell of potatoes and rosemary were evaluated. In general, the typical potato odor was perceived in a similar manner in all samples at day 0, but, at the end of the storage, sample 8 and 12% showed lower values, probably the high concentration of essential oil has limited its perception. Rosemary oil was perceived proportionally to the amount, although significant differences (p < 0.05) were not always found among samples. In addition, only in the 4% sample a decrease in the value was observed during storage, while the 8 and 12% samples have shown similar values up to the 14th day. Parameters related to flavor, typical potato and rosemary flavor, showed similar results to those obtained by the odor evaluation. The perception of the typical potato flavor tended to diminish with the increase of the concentration of rosemary of the impregnating solution, although not always significantly. Furthermore, the perception of flavor of rosemary increased with the concentration of the solution, but in the 4% sample decreased during the storage, until the 14th day of storage, in which it was not different from control and 0% samples.

Fig. 1.

Results of sensory descriptive analysis (DA) performed on fried samples on day 0 (a), day 7 (b) and day 14 (c)

Discussion

Vacuum impregnation has been previously used for nutritional enrichment of vegetable products (Alzamora et al. 2005) and only once for the aromatic enrichment of apple slices (Comandini et al. 2010), proving to be an effective method for introducing compounds of interest into a vegetable porous tissue and hence for product innovation. However, it is important to evaluate the effect of the treatment on the qualitative properties of the obtained products and its stability during storage.

In the present study, VI has been used to introduce aromatic components in potato sticks intended for frying and some qualitative and stability aspects of the obtained products have been evaluated.

The porosity of the potato has shown to be relatively low compared to other vegetables, as already observed by other authors (Hironaka et al. 2011, 2014). This parameter is a key factor to consider for vegetable tissue to be vacuum impregnated, because it gives an indication of the total amount of the intercellular spaces that are normally filled with air. The impregnation levels obtained are significantly higher compared to the porosity of the potato (Table 2), indicating that apart from the intercellular spaces, some diffusion phenomena occurred and cell membrane selectivity has caused the entry of water into the cells (Yadav and Singh 2014). The application of vacuum increased further the weight gain promoting the inflow of the solution inside the pores in the tissue, hence fostering the mass exchanges due to osmotic phenomena, which has been shown in previous studies (Fito et al. 2001; Shi et al. 1995). Tylewicz et al. (2013) demonstrated that vacuum impregnation of apple tissue resulted in the formation of membrane vesicles inside the cells; this phenomenon may also occur in potato tissues, but should be further clarified.

In minimally processed products, the evaluation of the package headspace is important because it’s related to the product shelf-life as the development of anaerobic metabolism can lead to the formation of off-flavors and odors. In the present study, the application of vacuum may have lead, as evidenced by differences found in samples of day 10 for both CO₂ and O₂ in the internal packages’ atmosphere (Table 3).

As suggested by Sanzana et al. (2011) that studied the effect of VI on various vegetables, this CO₂ increase could be due to the mechanical tissue stress as a result of vacuum application. Some authors found that after VI the onset of an anaerobic metabolism was observed; however results are generally inconsistent (Castelló et al. 2006; Igual et al. 2008; Sanzana et al. 2011). Considering that the packaging film used in this experiment was characterized by a medium barrier to gas permeability, it is not possible to obtain precise information about the type of respiratory metabolism occurring.

In previous studies, the effect of the impregnating solution was observed to decrease the respiration rate of samples (Sanzana et al. 2011; Tappi et al. 2017), indicating a possible effect of the bioactive compounds to compensate the stress caused by the application of vacuum to the tissue. In the present research, the essential oil effect on metabolic changes in the tissue was not so evident, in disagreement with previous studies that have pointed out that rosemary essential oil could cause biophysical perturbation of membranes (Pérez-Fons et al. 2009). This difference may be explained by the concentration of the essential oil.

An important aspect in minimally processed potatoes is the enzymatic browning caused by the action of polyphenoloxidases (PPO). The initial difference observed in the present study (Table 3) in the color of vacuum impregnated samples compared to the fresh one was probably due to the variation of the refractive index of the tissue as a result of the filling of the intercellular spaces with the impregnating solution, as already observed in previous works (Neri et al. 2016). Since decrease in L* and increase in a* values are considered indexes of enzymatic browning, the color data suggested a reduction of this phenomenon following vacuum impregnation. A similar result has already been observed and was attributed to the reduction of the presence of oxygen in the tissue (Tylewicz et al. 2013). However, the higher essential oil concentration lead to an increase of these parameters suggesting an increased browning during storage, possibly because of a low stability of the essential oil components, that may have undergone oxidation phenomena. This change in the appearance of the product was detected also after frying, both by instrumental color analysis (Table 5) and sensorial test (Fig. 1).

Tappi et al. (2017) found that impregnating apples with a green tea extract lead to a significant change in samples color during storage. This variation was reduced by the presence of ascorbic acid in the impregnating solution that probably acted as antioxidant preserving the green tea components. The use of an antioxidant in the impregnating solution may, also in this case, reduce the color variation during storage.

Microbial development (Table 3) was not affected by the application of vacuum itself. Tappi et al. (2016) observed a faster microbial spoilage in minimally processed melon subjected to VI during storage. This effect was attributed to the irreversible alteration to the visco-elastic properties of the fruit tissues caused by the application of a vacuum pressure that may enhance nutrients availability for microbial growth. However, as shown by other qualitative parameters such as water content and texture, in the present study the structure of the potato tissue was not negatively affected by the vacuum treatment. On the other side, the presence of essential oil at its highest concentrations (8 and 12%) allowed to reduce the concentration of spoilage bacteria (total aerobic count and psychrophilics) at the beginning of the storage and the development yeast and moulds at the end. This result may be due to the known antimicrobial effect of rosemary essential oil (Jiang et al. 2011), although the presence of potassium sorbate (E202) in the industrial formulation of the oil could have possibly played a role.

Concerning the aromatic enrichment, that was the main aim of the treatment, the number of volatile compounds detected in this study (Table 4) was lower compared to previous literature data on rosemary essential oil (Tawfeeq et al. 2016; Tomi et al. 2016), probably due to the dilution of the oil (4–12%) and to its low penetration into the potato tissues. In addition, other chemical and/or bio-chemical compounds in the solution and in the potato tissue may have blocked the release of volatile compounds. However, the aromatic components were well perceived by the panel (Fig. 1), proportionally to their concentration and, at the highest concentration consistently during storage for both raw and fried product.

Conclusion

The vacuum impregnation process resulted an effective method for the aromatic enrichment of potatoes intended for frying. The aroma was successfully incorporated in the tissue although the aromatic profile of the potato samples indicated a loss of the aromatic compounds during storage. Sensory analysis has virtually confirmed the data obtained instrumentally indicating a reduction in rosemary aroma with the storage time. Frying procedures significantly reduced the number of the detected volatile components. The microbial load of the samples was reduced by the higher concentration of the essential oil, confirming its potentiality as antimicrobial. However, the presence of potassium sorbate (E202) in the oil formulation, could have also played a role. However, all samples maintained values below the threshold for spoilage (6 log CFU/g) until the end of storage. However, there are few issues that need further investigation in order to obtain a product with high quality characteristics and stability. The color of the potatoes during storage appeared adversely affected proportionally to the content of essential oil, probably due to oxidation phenomena. The addition of an antioxidant compound in the impregnating solution could be tested. Furthermore, the vacuum treatment seems to promote an alteration of metabolism measured by the respiration rate that should be better clarified. Anyhow, vacuum impregnation presents high potentiality to modulate the sensorial profile of porous vegetable tissue, being a cold formulation process that does not cause the thermal degradation of specific aromatic compounds of the impregnating solution.