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. 2019 Jul 5;56(10):4714–4721. doi: 10.1007/s13197-019-03918-z

Chemical and physical changes during storage of differently packed biscuits formulated with sunflower oil

Federica Balestra 1, Vito Verardo 2,3, Silvia Tappi 4,✉, Maria Fiorenza Caboni 1,4, Marco Dalla Rosa 1,4, Santina Romani 1,4
PMCID: PMC6801301  PMID: 31686703

Abstract

Various innovative materials are already present on the market, but there is scarce knowledge on their performances when used in real food system. In this study, two innovative packaging materials were investigated for the evaluation during storage of biscuits formulated with sunflower oil and compared to a traditional one. To this aim, three different flexible and composite film containing a metalized plastic layer and a paper layer were used. The control included orientated polypropylene (OPP), while the innovative materials contained poly-lactic acid or OPP with a pro-oxidant additive, ethylene vinyl acetate. The physical (moisture, water activity, hardness/crispness) and chemical (peroxide value, conjugated dienes and trienes, and hexanal and nonanal formation) changes of biscuits were monitored during accelerated storage (35 °C and 50% of relative humidity for 105 days). Packaging materials did not have significant impact on textural quality of biscuits. Instead remarkable differences were observed during storage in the evolution of different lipid oxidation compounds, moisture content and water activity among differently packed biscuits. A new ecofriendly packaging showed the best performances in terms of physico-chemical quality of biscuits. The obtained results provide useful information for industrial application.

Keywords: Biscuits, Packaging, Quality, Lipid oxidation, Structural properties

Introduction

Dry shelf-stable bakery products such as biscuits, together with crackers and bread-sticks, are largely popular and consumed around the world (Calligaris et al. 2008). In particular, biscuits are highly appreciated being relatively cheap and characterized by a wide range of tastes and long shelf-life (Romani et al. 2012; Vitali et al. 2009).

Dry biscuits are traditionally made with flour, sugar, eggs, fat and salt. Fats have different functions in biscuit formulation, mainly related to taste and structural properties (Pauly et al. 2013; Sudha et al. 2007).

Packaging of biscuits requires protection form oxygen and water vapour. In most cases, composite flexible materials with a high barrier to both are used for room temperature storage. Although these are shelf stable, some deterioration phenomena can occur during storage, mainly related to lipid oxidation and water uptake that results in textural changes (Galić et al. 2009; Robertson 2006; Smith et al. 2004).

Lipid oxidation causes the development of primary and secondary products. Primary ones, such as hydroperoxides, are characterized by high instability and are quickly decomposed in a wide range of secondary products (Mandić et al. 2013), that in turn lead to the development of off-odours and flavours that negatively influence products palatability and biscuits shelf-life. (Romani and Rodriguez-Estrada 2016; Smith et al. 2004).

In order to hinder or at least to mitigate the quality deterioration of biscuits during storage, a suitable packaging with materials characterized by high barrier properties to environmental factors such as light, oxygen and water vapour is necessary (Galić et al. 2009; Farris and Piergiovanni 2009; Robertson 2006).

Therefore, the quality of packaging materials is essential in order to contain undesirable processes that cause quality decay. Polypropylene (OPP) is generally used as polymer for biscuit packaging. It can be produced in different forms such as plain or pearlized, co-extruded or coated on both sides with acrylic. Acrylic-coating of OPP alone or in combination with PVC/PVDC copolymer is often used because it provides a higher barrier to oxygen but also offers a stronger barrier effect for flavour and aroma compared with plain OPP (Galić et al. 2009; Robertson 2006).

Sometimes, metalized films are used to limit the photoxidation (Piergiovanni and Limbo 2004), however, because these are made from petroleum-based polymers and are not biodegradable, can cause ecological problems. Thus, the increasing awareness related to sustainability of products and processes have increased the interest towards the research of alternative raw sources and innovative technologies to produce packaging materials with a lower environmental impact (Siracusa et al. 2008).

Nonetheless, the total replacement of synthetic packaging films is not feasible at date, but an increase of the utilization of bio-plastics in the sector of food packaging in the future is very likely (Siracusa et al. 2008).

Various innovative materials are already found on the market, however further studies on innovative bio-based and/or biodegradable films are needed. In particular, it is necessary to increase the knowledge on the behaviour of these materials in realistic environmental conditions, in order to evaluate interactions among food product, packaging films and environment. To our knowledge, only few works (Romani et al. 2015) have investigated the performances of sustainable packaging materials in terms of biscuits shelf-life. Based on the aforementioned, the aim of this research was to study the change of some quality characteristics of differently packed biscuit during accelerated storage. Briefly, biscuits formulated with sunflower oil were analysed. Herein, the determination of peroxide values, conjugated dienes and trienes and the determination of volatile compounds during shelf-life were used as lipid oxidation indexes. In addition, some physical parameters such as O2 concentration in the packaging headspace, moisture, water activity and textural parameters (hardness and fracturability) were also evaluated.

Materials and methods

Biscuit formulation

Biscuits were produced by Italian bakery company that declared the following formulation: wheat flour, brown sugar (19.2%), barley flakes (18.2%), sunflower oil, barley flour (7.1%), leavening agents, oatmeal flour (1.3%), iodized salt and flavourings. The total fat content, total carbohydrate, protein and fibre were 16.1%, 61.3%, 9.6% and 9.5%, respectively. Biscuits belonging to same batch were randomly divided into three sub-samples and packed by the food manufacturer using different materials and transferred to our laboratory in the same production day.

Packaging materials

As standard material, metalized orientated polypropylene (OPP)/paper (sample A-control) was used. The innovative materials instead were obtained by replacing the OPP layer with poly lactic acid (PLA-sample B) or by adding ethylene vinyl acetate (EVA) to OPP (sample C). EVA is an additive certified by FDA to came into food contact as reported on available literature data (www.italcombiodegradabile.com; www.ecmbiofilms.com) and it’s often used in extruded copolymers as sealing layer. That was applied for making the plastic layer biodegradable on account of its pro-oxidant (EVA-POA) activity.

Biscuit packages had an average filled weight of 300 g and a surface area of 1078 cm2.

Experimental conditions

A total of 21 bags for each sample (A, B and C) were used and placed in a climate chamber (Constant Climate Chambers with Peltier technology- mod. HPP 108/749, Memmert, Germany) with fixed temperature and relative humidity (RH). The temperature of 35 °C was chosen as a temperature representing a thermal abuse, while a RH value of 50% was selected as that corresponding to the average value of the data registered in a supermarket in a whole year period (Robertson 2006), and as the value with a minimum influence on the rate of fat oxidation (Lu and Xu 2009).

During a 105 days storage, at each selected sampling time (0, 15, 30, 45, 60, 85, 105 days), for each sample, three packages (replicates) were collected and analysed (21 packs × 3 packaging types: in total 63 packs).

Analytical determinations

O2 concentration in the packaging headspace

Before opening, each bag was analysed for the concentration (%) of O2 in the headspace, with a “check point O2/CO2” gas analyser (mod. MFA III S/L—Witt-Gasetechnik, Witten, Germany).

Moisture content

Samples moisture was determined according to AOAC official method by drying the grounded product in a vacuum oven at 105 °C till reaching constant weight (AOAC 1980).

Water activity

A dew point hygrometer Aqualab® series 3 TE (Decagon Devices, Inc., Washington, USA) at 25 °C was used for the determination of water activity of grounded biscuit samples.

Texture

A TA.HDi 500 Texture Analyser (Stable Micro Systems, Surrey, UK) was used for the determination of the textural parameters of hardness and fracturability. For the measurements, a 25 kg load cell was used. The chosen test was a three-point bending test as described in detail by Romani et al. (2012) carried out at room temperature. This type of test according to Tyagi et al. (2007) is able to simulate consumer handling for the determination of hardness.

From the curve of applied force versus time, the parameters of hardness and fracturability were calculated as respectively the absolute peak force (g) and the distance at the break (mm). The fracturability was expressed as 1/break point distance.

Fat extraction

According to Calligaris et al. (2007), a mixture (1:1, v/v) of diethyl ether/petroleum ether was used for fat extraction. Briefly, ground biscuits were added to the solvent mixture [1:2.5 (w/v) ratio] and agitated for 1 h at room temperature. The slurry was filtered with filter paper. Fat was separated by the solvent through evaporation and used for further analytical determinations of peroxide value, conjugated dienes and trienes content.

Peroxide value (PV)

Peroxide value determination was carried out on the extracted fat following the European Methods of Analysis (EC No 2568 1991).

Conjugated dienes and trienes

Extracted fat was opportunely diluted with iso-octane in triplicate. A spectrophotometer (Shimadzu 1601 UV spectrophotometer Duisburg, Germany) was used to determine specific extinction coefficients of conjugated dienes and conjugated trienes, respectively at 232 and 268 nm. Absorbance (1%/1 cm) was calculated according to IUPAC method II. D. 23 (IUPAC 1979).

Hexanal and nonanal content

The evolution of hexanal and nonanal levels during storage of biscuits samples was evaluated by headspace solid phase microextraction-gas chromatography/mass spectrometry (HS–SPME–GC/MS), according to Purcaro et al. (2008) with some modifications as described in detail by Marzocchi et al. (2017).

Volatile compounds were identified by comparing retention times and mass spectra with literature data (Mildner-Szkudlarz et al. 2009; Vichi et al. 2003) and the NIST Mass Spectral Database (NIST 08, National Institute of Standards and Technology, Gaithersburg, MD, USA). Values of hexanal and nonanal content were indicated as area counts.

Data analysis

Texture analysis was performed on 25 biscuits per sample at each sampling time. All other determinations were carried out in triplicates. Results were expressed as average and standard deviation. One-way ANOVA and Tukey post hoc test (p < 0.05) were used to investigate significance of differences.

Results and discussion

Determination of some physical parameters in packed samples

The content of O2 (%) in the package’s headspace did not change significantly during the storage period at 35 °C. In fact, oxygen values similar to the atmospheric air content (20.95%) were observed in all samples until the end of storage confirming the high gas barrier properties of the packaging materials.

The variations in the moisture content of biscuits packed with different materials during storage at 35 °C are shown in Fig. 1A.

Fig. 1.

Fig. 1

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Moisture change (%) (A) and water activity (B) of A, B, C biscuit samples during storage at 35 °C. A: Metalized OPP/Paper; B: metalized PLA/paper; C: metalized OPP–EVA–POA/paper. Different letters indicate significant differences among samples at the same sampling time (p < 0.05)

The initial value was 2.16%, that is in agreement with the range reported by Robertson (2006) for freshly baked biscuits that is typically between 1 and 5%. During storage, moisture content gradually increased similarly to a previous study of Romani et al. (2016). Water uptake may be explained by the hygroscopic nature of dried products, environmental conditions related to temperature and RH and characteristics of the packaging material. The evolution of biscuit hydration showed significant differences among the samples. The highest increase of moisture during the period of analysis was observed in the sample packed in the film containing PLA (B), that reached the highest moisture value (5.56%) at the end of the considered period. Conversely, sample C (OPP containing EVA-POA) was characterized by the lower increase of this parameter, remaining the significantly less hydrated sample (< 3%) after 105 days of storage.

Water activity followed a similar behaviour, increasing in all samples during storage (Fig. 1B), more pronouncedly in sample B that at the end of the storage exhibited values between 0.35 and 0.40. In A and C samples, aw remained respectively below 0.250 and 0.200 for the entire storage period.

Although the investigated materials are considered to offer high barrier to water vapour, a diffusion of water occurring during storage through all packaging materials could be the cause of the observed increase of moisture content and aw. However, this phenomenon was more pronounced in B samples (containing PLA) probably due to some structural modifications occurring during storage in the packaging material, which might have led to increasing its permeability characteristics.

Figures 2 and 3 show the structural modifications, in terms of respectively hardness and fracturability occurring in biscuit samples packed with different materials during storage at 35 °C.

Fig. 2.

Fig. 2

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Hardness (g) of A, B, C biscuit samples during storage time at 35 °C. A: Metalized OPP/Paper; B: metalized PLA/paper; C: metalized OPP–EVA–POA/paper. Different letters indicate significant differences among samples at the same sampling time (p < 0.05)

Fig. 3.

Fig. 3

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Fracturability (1/mm) of A, B, C of biscuit samples during storage time at 35 °C. A: Metalized OPP/Paper; B: metalized PLA/paper; C: metalized OPP–EVA–POA/paper. Different letters indicate significant differences among samples at the same sampling time (p < 0.05)

The hardness values tended to decrease in all samples showing a similar trend without significant differences among them. In the first 15 days of storage a decrease of hardness occurred simultaneously to an increase of water activity and moisture content.

At the same time, a slightly decrease of fracturability throughout the storage, which denotes a lower friability of biscuit samples, was observed. As expected, fracturability values decrease along the storage. This decrease occurred mainly up to about 45 days of storage very similarly in all samples.

During storage, textural parameters can be influenced by variation in moisture content and water activity that, in this study, can be mainly attributed to phenomena of migration of moisture and its redistribution within the sample, changes in the physical properties of biscuits components and in their interactions. On the other side, textural properties of dry foods are strongly affected also by other factors such as product formulation, amount of fat in particular, and its structural characteristics (Romani et al. 2016).

Determination of lipid oxidation parameters in packed samples

As index of primary oxidation of biscuit lipid fraction, extracted at each storage time, the evolution of peroxide value (PV) is shown in Fig. 4.

Fig. 4.

Fig. 4

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Peroxide values (meq O2 kg−1 oil) of biscuit A, B, C during the shelf life A: Metalized OPP/Paper; B: metalized PLA/paper; C: metalized OPP-EVA-POA/paper. Different letters indicate significant differences among samples at the same sampling time (p < 0.05)

At the beginning of the storage, a value of about 8 meq O2 kg−1 oil was observed. Considering this type of product, this can be considered quite a high level, caused probably by the quantity and characteristics of the fat included in the product formulation as well as by the baking process. In fact, the analysed biscuits included sunflower oil in formulation and, as reported in a previous work (Verardo et al. 2010), bakery products containing sunflower oil showed higher levels of lipid oxidation compared to those formulated with saturated fat (Verardo et al. 2011).

All samples showed the maximum PV between 45 and 60 days of storage, reaching values greater than 10 meq O2 kg−1 fat. In a previous study (Calligaris et al. 2008), it was demonstrated that the consumer rejection of a similar product (dry bakery product) increased over 50% when the PV increased over 15.6 meq O2 kg oil−1 fat.

After 60 days, PV decreased progressively for sample B until the end of the storage showing values lower than 4 meqO2 kg oil−1 fat, while in samples A and C PV decreased until the 85th day and then increased again. This trend may be explained by the equilibrium between formation and degradation of hydroperoxides. Indeed, during the steps of propagation and termination typical of the oxidative process, contemporary formation and decomposition of primary compounds occurs (Frankel 1991). However, the kinetic of oxidation reactions can also be affected by changes in moisture content and aw. The results obtained in this study pointed out that evaluating only the PV is not sufficient to determine the oxidative stability of this kind of product, but other indexes must be considered. For this reason, conjugated dienes and trienes and volatile compounds were considered.

Figure 5 shows that the formation of conjugated dienes tended to decrease in all the samples in the first 30 days of storage. Dienes showed a decreasing trend from 0 to 30 days in all samples. This decrease might be explained by a decomposition rate of these compounds higher than that of their production. After 30 days of storage the concentration of conjugated dienes increased; no significant differences were noticed in samples A, B and C at the end of shelf-life. It’s worth noting that the increase of conjugated dienes corresponds, for all samples, to a decrease of PV and to an increase of moisture and aw values.

Fig. 5.

Fig. 5

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Dienes (λ 232 nm) and trienes conjugated (λ 270 nm) of fat extracted from biscuit samples during storage time at 35 °C. Filled and Empty symbols represent dienes and trienes conjugated respectively. A: Metalized OPP/Paper; B: metalized PLA/paper; C: metalized OPP–EVA–POA/paper. Different letters indicate significant differences among samples at the same sampling time (p < 0.05)

At the beginning of the storage, conjugated trienes were not detected but their presence was found after 15 days (Fig. 5). The maximum concentration of trienes was observed between 60 and 85 days of shelf-life and then these were decreased again at the end of shelf-life in all samples.

As observed by other authors (Mildner-Szkudlarz et al. 2009; Purcaro et al. 2008), some volatile compounds that can be developed during lipid oxidation (such as hexanal and nonanal) could be considered as markers to determine the rancidity in biscuits. Hence, the development of hexanal and nonanal in the headspace of biscuit packages during storage was determined. Figure 6A shows the evolution of hexanal in the samples during shelf-life. Samples A and B had the same increasing trend until 60 days, showing significantly higher values compared to the values of sample C. Between 60 and 85 days, in sample B a sharp increase of hexanal content was observed reaching the highest value, while sample A, although still increasing reached a lower concentration compared to sample B. Finally, in the last part of the storage, samples A and B showed a declining trend, probably because of a result of degradation of hexanal. Conversely, in sample C hexanal content increased more slowly but progressively during all the storage period, maintaining always lower values compared to sample B and to sample A with the exception of the last sampling point.

Fig. 6.

Fig. 6

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Evolution of hexanal (A) and nonanal (B) of biscuit samples during storage time at 35 °C. A: Metalized OPP/Paper; B: metalized PLA/paper; C: metalized OPP–EVA–POA/paper. Different letters indicate significant differences among samples at the same sampling time (p < 0.05)

These results pointed out that the kinetic of hexanal formation was slower in the sample C, in which OPP was added with EVA-POA. On the contrary, where PLA was used to replace OPP (sample B), a faster formation of hexanal was observed, probably due to the higher water permeability, as demonstrated by the highest moisture content reached by the biscuits during storage which may favour lipid oxidation. This hypothesis agrees with the results reported by Park et al. (2014) that demonstrated the significant role played by moisture content in the rate of lipid oxidation and indicated that increasing moisture content could be related to a pro-oxidant effect. These results were also confirmed by Kim et al. (2014) that clearly showed that moisture may be a substrate for lipid oxidation, participating in the development of volatiles.

The development of nonanal in biscuit samples during storage is shown in Fig. 6B. This secondary oxidation product is a volatile compound originated from the degradation of 9-OOH and 10-OOH proceeding from oleic acid autoxidation (Choe and Min 2006) and is the second fatty acid in sunflower oil. Sample A showed a nonanal increasing trend from 0 to 45 days, after that a zig-zag trend was observed confirming an advanced state of oxidation. In sample B nonanal content increased from 0 to 60 days and then decreased until 105 days. Conversely, a slower but progressive increasing trend of nonanal content was noticed in sample C. As reported for the hexanal content evolution, a mechanism of formation and degradation (typical of advanced state of oxidation) of nonanal has been observed in samples A and B, while a lower degree of oxidation has been developed in sample C.

Conclusion

The effect of several packaging materials on some quality characteristics of biscuits was evaluated during accelerated storage for 105 days at 35 °C and 50% of relative humidity.

Remarkable differences in moisture, water activity and in the development of lipid oxidation products were found in biscuits packed with traditional and innovative packaging materials. In particular the product in flexible packaging with PLA (sample B) underwent the maximum hydration, which could be attributed to a probable increase in the water vapour permeation through the packaging during storage reaching the highest moisture and aw levels at the end of the storage time. On the other hand, packaging C showed the best performances in preserving the biscuits quality. In fact, sample C reported a lower hydration level and a lower degree of oxidation compared to sample A and B. Packaging material did not significantly impact on textural biscuit quality. The results of the present study show that the innovative packaging material tested (C) proved to be appropriate for storage of commercial biscuit, improving the protective performances on the products qualitative characteristics compared to the traditional one. Furthermore, these results contribute to increasing knowledge related to the innovative sustainable and environmental friendly packaging materials, encouraging their use at industrial level as a valid alternative to the standard petroleum-based ones.

Acknowledgements

Vito Verardo thanks Spanish Ministry of Economy and Competitiveness (MINECO) for his “Ramon y Cajal” contract.

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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