Cristina Fernández & Wenceslao Canet, M. Dolores Alvarez*

Cristina Fernández & Wenceslao Canet, M. Dolores Alvarez*

Original article

The effect of long-term frozen storage on the quality of frozen and thawed mashed potatoes with added cryoprotectant mixtures

Cristina Fernández & Wenceslao Canet, M. Dolores Alvarez*

Department of Plant Foods Science and Technology, Instituto del Frío-CSIC, José Antonio de Novais nº 10, E-28040 Madrid, Spain

Running title
Frozen storage effect on quality of mashed potatoesC.Fernández et al.

*Correspondent: Fax: +34 91 549 36 27;

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Summary Cryoprotectant mixtures were added to frozen/thawed (F/T) mashed potatoes in the form of amidated low-methoxyl (ALM) pectin and xanthan gum (XG), kappa-carrageenan (κ-C) and XG, and sodium caseinate (SC) and XG, and the effect of frozen storage was examined.F/T mashed potatoes without added biopolymers had higher storage modulus G' after freezing and frozen storage, associated with sponge formation due to amylose retrogradation. Oscillatory measurements indicated weakening of the structure of mashed potatoes without biopolymers and with added κ-C/XG and SC/XG mixtures at the end of storage due to ice recrystallization, whereas the structure of samples with added ALM/XG mixtures was reinforced by increasing time in storage.Mashed potatoes with added mixtures exhibited water-holding capacity for one year.Samples with added κ-C/XG mixtures were more structured, although when both κ-C/XG and SC/XG mixtures were included in mashed potato, very acceptable sensory quality was maintained in usual frozen storage conditions.

Keywords Potato puree, freeze-thaw stability, frozen food storage, overall acceptability, cryoprotectant mixtures, quality, water-holding capacity.

Introduction

Starch is one of the most important functional food biopolymersand is added as a functional ingredient to manyproducts such as sauces, puddings, confectionery and avariety of low-fat products; also mashed potatoes as preparedin this study contain nativepotato starch. In food processing, gelatinization and pasting ofstarch granules occur during heating process along withshear leading to changes in starch granules and viscosity.On cooling, retrogradation is induced by reassociationof starch molecules, causing gel formation or increasedviscosity (Hermansson & Svegmark, 1996).

Previous studies showed that mashed potato madefrom Kennebec potatoes should be quick-frozenand microwave-thawed to obtain a product quite similarto freshly made mashed potato (Alvarez et al., 2005). In corn starch pastes, higher freezing rates also preserved textural characteristics and produced less exudate (Ferrero et al., 1993, 1994). The rapid transition from the melt to the glassy state prevents nucleation and propagation of amylose and amylopectin crystals. Amylose retrogradation is commonlyassociated with rheological changes in the system;amylopectin retrogradation can usually be measuredby differential scanning calorimetry (Ferrero & Zaritzky, 2000). However, increasing time in frozen storage produces a firmer texture in mashed potato (Alvarez et al., 2005). During distribution and storage,starch pastes may undergo transformation of starchbiopolymer molecules: namely, chain aggregation andrecrystallization.Moreover, it is difficult to keep frozenfood products constantly in an optimum frozen statewhen they undergo repeated freeze–thaw cycles during the supplychain, which leads to changes in syneresis and relatedrheological properties (Lee et al., 2002).

Incorporation of an appropriate amount of hydrocolloids mayimprove or maintain desirable textural properties and stabilityof most starch-based products during prolonged storage (Downey 2002, 2003). Five different hydrocolloids and two dairy proteins were added at five concentrations to fresh and frozen/thawed mashedpotatoes to investigate ways of improving the effects offreezing and thawing (Alvarez et al., 2008a, 2008b; Fernández et al., 2008). Dairy proteins affected the taste and odour and were judged unacceptable in the sensory analysis, while samples containing 0.5 and 1.5 g kg–1 added xanthan gum (XG) were preferred organoleptically due to the creamy mouthfeel they produced.In addition, the product yielded by adding XG was softer than the control without added cryoprotectants in F/T samples, and it was therefore suggested that XG mightbe suitable for use as anadditive to mitigate the thickening effect caused by long-term frozen storage (Alvarez et al., 2008b).

Knowledge of the mechanics of protein-polysaccharide systems is also important in order to develop desirable properties in food products (Hemar et al., 2001). The purposes of the present study weretherefore (a) to investigate the effect of adding mixtures of: (1) Amidated low-methoxyl (ALM)pectin and xanthan gum (XG); (2) Kappa-carrageenan (κ-C) and XG; and (3) Sodium caseinate (SC) and XG on the quality of F/T mashed potatoes in order to determine how the presence of gelling (pectin and carrageenan) and non-gelling (xanthan) hydrocolloids and proteins (sodium caseinate) and a non-gelling hydrocolloid (xanthan) modifies instrumental parameters and sensory attributes of processed mashed potatoes; and (b) to determine the freeze-thaw stability of these systems in order to be able to control and maintain the quality of mashed potatoes during long-term frozen storage.

Material and methods

Test material

Data presented in this report were obtained using potato tubers (Solanum tuberosum, L., cv. Kennebec) (cv Kennebec) from Aguilar de Campoo (Burgos, Spain). The material was stored in a chamber at 4±1°C and 85% relative humidity during two days to the maximum before processing (Nourian et al., 2003).

Cryoprotectant mixtures

Amidated low-methoxyl (ALM) pectin (GENU pectin type LM-104 A; pectin was methylated to a degree of 27%, and in addition a further 20% of the residues was amidated (DA = 20%)), kappa-carrageenan (κ-C) (GENULACTA carrageenan type LP-60), and xanthan gum (XG) (Keltrol F [E]) were donated by Premium Ingredients, S.L. (Girona, Spain). Sodium caseinate (SC) EM-6 was supplied by Manuel Riesgo, S.A. (Madrid, Spain). Following range finding experiments (Alvarez et al., 2008a,2008b; Fernández et al., 2008), the level of each biopolymer to be used in mixtures was fixed at 1.5 gkg-1.Notations used to refer to each of the samples were: C, F/T mashed potatoes without added biopolymers; ALM1.5/XG1.5, κ-C1.5/XG1.5, and SC1.5/XG1.5, F/T mashed potatoes with 1.5 g kg-1 added ALM pectin, κ-C orSC, respectively and 1.5 g kg-1 added XG.

Preparation of mashed potatoes

Tubers were manually washed, peeled and diced. Mashed potatoes were prepared in 650-g batches from 607.7 gkg-1 of potatoes(total starch content, 736 ± 26 g kg-1 dry basis, amylose content, 256 ± 21 g kg-1 of starch), 230.8 g kg-1 of semi-skimmed in-bottle sterilized milk (fat content, 15.5 gkg-1), 153.8 gkg-1 of water, and 7.7 gkg-1 of salt (NaCl) using a TM 21 thermal mixer (Vorwerk España, M.S.L., S.C., Madrid, Spain). Mixtures of cryoprotectants were added at this point; the appropriate amount (1.5 g kg-1) of ALM pectin and XG, κ-Cand XG and SC and XG was added to the rest of ingredients in form of a dry powder. The ingredients were cooked for 25 min at 100 °C (blade speed: 100 rpm), as described elsewhere (Alvarez et al., 2005; Fernández et al., 2006). The mash was immediately ground for 40 s (blade speed: 2000 rpm). The product was immediately homogenized through a stainless steel sieve (diameter 1.5 mm). Following preparation, mashed potato sample was immediately packed in 300×200 mm2 rectangular polyethylene plastic, sealed under light vacuum (−0.05 MPa) on a Multivac packing machine (Sepp Haggenmüller KG, Wolfertschwenden, Germany), and frozen and thawed according to procedures indicated below.

Freezing and thawing procedures

Mashed potato was frozen by forced convection with liquid nitrogen vapour in an Instron programmable chamber (model 3119-05, −70°C/+250°C) at −60°C until their thermal centres reached −24°C. Air and product temperatures were monitored by T-type thermocouples (NiCr/NiAl; −200 to +1000°C) using the MMS3000™ Multi Measurement System™ (Mod. T4, Commtest Instruments, Christchurch, New Zealand). After freezing, samples were placed in a domestic freezer for storage at −24°C. For microwave thawing process, frozen mashed samples were unpacked and then thawed in a Samsung M1712N microwave oven(Samsung Electronics S.A., Madrid, Spain). Samples were irradiated for 20 min with an output power rating of 600 W, as described previously (Fernández et al., 2006; Alvarez et al., 2008a, 2008b).

Long-term frozen storage

Mashed potato was prepared and frozen, stored at −24°C for 0 (1 day), 3, 6, 9 and 12months and thawed by microwave as described above.Each testing date was replicatedtwice for each type of mashed potato.

Heating of samples

After thawing, all samples were brought up to55°C by placing them in a Hetofrig CB60VS (Heto Lab Equipment A/S,Birkerød, Denmark) water-bath, where water and product temperatures were monitored by T-type thermocouples as described elsewhere(Fernández et al., 2006, 2008; Alvarez et al., 2008a, 2008b). The selected sample testingtemperature was 55°C, as results from different analyses showed that this is the preferred temperature for consumption of mashed potato (Canet et al., 2005).

Oscillatory and steady rheological measurements

A Bohlin CVR 50 controlled stress rheometer (Bohlin Instruments Ltd, Gloucestershire, UK) was used to conduct small amplitude oscillatory shear experiments and steady shear using a plate-plate sensor system with a 2 mm gap (PP40, 40 mm) and a solventtrap to minimize moisture loss during tests. Samples were allowed torelax for 5 min before rheological measurements were made (Fernández et al., 2006). Temperature control at 55°Cwas achieved with aPeltier Plate system (-40 to +180°C; Bohlin Instruments Ltd,Gloucestershire, UK). Linear viscoelastic domain for each sample was determined from stress sweepsat 1 rad s-1. Next, three frequency sweeps were carried out over the range 0.1-100 rad s-1 at very small strains, mostly below 10-3. The dynamic rheological parameters used to evaluate viscoelastic properties of mashed potatoes were the phase angle (), the storage modulus (G’) and the loss modulus (G”) at 1 rad s-1. A power-law type relationship was verified for dynamic rheological data; linear regressions of ln (G’) and ln (G”) versus ln () were carried out and the magnitudes of slope and intercepts were computed as described in previous works(Alvarez et al., 2007).

In order to describe the variation in rheological properties of mashed potato under steady shear, data obtained from increasing shear rate measurements were fitted to the well-known power law model (Rao, 1999). All the rheological measurements in each experimental combination were carried out in duplicate.

Instrumental objective texture measurements

Texture profile analysis (TPA) tests were carried out with a TA.HDi Texture Analyser (Stable Micro Systems Ltd, Godalming, UK) using a 250 N load cell. During tests, mashed potato samples were maintained at 55°C by means of a Temperature Controlled Peltier Cabinet (XT/PC) coupled to a separate heat exchanger and proportional-integral-derivative (PID) control unit. For TPA tests, a flat 35-mm diameter aluminium plunger (SMS P/35) was used to move within a 60-mm diameter stainless steel cylinder containing 50 1 g of sample. The experimental conditions were: deformation rate (180 mmmin-1), compression level (33.3%), with a rest period of 5 s between cycles (Alvarez et al., 2005). There were four replicates for eachexperimental unit. Program software (TextureExpert for Windows, version 1.0; Stable MicroSystems, Surrey, England, UK) automaticallycalculates the textural parameters from the curvegenerated by such a test, as follows: consistency, CON (N), adhesiveness, ADH (N s), springiness, SPR (dimensionless), cohesiveness, COH (dimensionless) and gumminess, GUM (N).

Other quality parameters

Colour measurements

The colour of the mashed potatoes in thepots was measured using a HunterLab model D25(Reston, VA, USA) colour difference meter fittedwith a 5 cm diameter aperture, and results were expressed in accordance with the CIELAB system with reference to illuminant D65 and a visual angle of 10°. Parameters determined were L*, a* and b*, recommended by the International Commission on Illumination (CIE 1978), although colour was also expressed as L*/b*, i.e., the white/yellow ratio (O'Leary et al., 2000). The total colour difference (∆E*) between F/T control (C) and F/T mashed potatoes with added cryoprotectant mixtures was calculated as described elsewhere (Baixauli et al., 2002).

Drip Loss

Drip loss (DL) was measured by centrifugal force. The centrifuge tubes containing the sample (approximately 10 g of mashed potato) were centrifuged at 6000 rpm (15000×g) for 30 min in a Sorvall®, RC-5B apparatus (Global Medical Instrumentation, Inc,Minnesota, USA). DL was expressed as the percentage of liquid separated per total weight of sample in the centrifuge tube (Eliasson & Kim, 1992).

Total Soluble Solids Content

Total soluble solids (TSS) content g/100 g (w/w) as measured by the refractive index was determined with an Atago (Itabashi-ku, Tokyo, Japan) dbx-30 refractometer.

Determination of pH

The pH of the F/T mashed potatoes was measured with a Schott CG pH meter (Model 842; Schott-Geräte GmbH, Mainz, Germany). Measurements of all the quality parameters were performed in quadruplicate and the results averaged.

Sensory analysis

Sensory TPA was done by a four-member panel trained specifically in sensory analysis of mashed potato (Alvarez et al., 2005).Each sample was tested twice and average scores calculated, so that each sample was tested eight times in all. The Texture Profile system (UNE 87025, 1996) was modified to evaluate frozen mashed vegetables (Canet et al., 2005). Scores for sensory attributes were based on a 10-point descriptive intensity scale converted to a 1–10 numerical scale for statistical analysis, with 1 = not detectable and 10 = extremely intense. Profile attributes are classified in four groups as described in a previous work (Fernández et al., 2006). Description of the sensory attributes evaluated by the trained panel during the texture profile analysiscan be found elsewhere (Alvarez et al., 2008a).Mashed potato samples were also subjected to an overall acceptability (OA) test based on all sensory attributes (texture, colour, taste), on a 10-point hedonic scale (10 = like extremely, 1 = dislike extremely).

Statistical analysis

For analysis of the effect of cryoprotectant mixtures and long-termfrozen storage on the quality parameters studied, results were subjected to multifactor analysis of variance (two way-ANOVA) using STATGRAPHICS (version 5.0, STSC Inc., Rockville, MD, USA) forone control and three cryoprotectant mixtures (C, ALM1.5/XG1.5, κ-C1.5/XG1.5, SC1.5/XG1.5) and five test dates (0, 3, 6, 9, 12 months). Cryoprotectant mixtures and times were compared using the least significant difference test (LSD, 99% for instrumental parameters and 95% sensory attributes). Where interaction was significant, long-term results were analysed for each mixture type using one-way analysis of variance onfive test dates, andtimes were compared using LSD test as indicated above.

Results and discussion

Oscillatory and steady rheological measurements

Addition of cryoprotectant mixtures to mashed potatoes significantly affected all the rheological parameters (P < 0.01, Table 1). Ingeneral terms, addition of binary biopolymer mixtures increased phase angle (δ), magnitudes of the slopes n’ and n”, and steady-shear rheological properties (n and K) with respect to the control (C). Nevertheless, G’ values were significantly lower in all the samples with added mixtures than in C mashed potatoes. Higher magnitudes of storage modulus (G’) in starch pastes submitted to freezing and abusive frozen storage conditions were associated with the formation of an elastic, opaque structure due to amylose retrogradation (Ferrero & Zaritzky, 2000). Mashed potatoes with added κ-C1.5/XG1.5 mixture presented greater viscosity (G”)than C control, indicating a moreviscous nature. XG, which was present in all the added mixtures, does not form gels and therefore its presence has a greater impact on the viscous response than on the elastic response (Rodríguez-Hernández Tecante 1999). Thefact that the lowest δ values were found in F/T samples without added biopolymers could be ascribed to the presence of XG in all the other systems, where it would affect G” more because of its thickening properties. In all the systems, the slopes of viscous modulus G”exhibited more solid characteristics (lower values of the slopes) than those of the elastic componentG’.

In foodsystems likemashed potatoes containing disintegratedstarch granules, rheological propertiesare governed by the continuous phase, especiallyby the formation of networks of solubilized and highly entangled macromolecules released from the granules(Hermansson & Svegmark, 1996).Oscillatory measurementsshowed that the addition of cryoprotectant mixturesto mashed potatoes weakened the gel structure of the products as compared to F/T control.Probably, freezing of C mashed potatoes produced high local starch concentrations and allowed chain crystallization of both amylose and amylopectin to occur (Ferrero et al., 1993). Certainly, a spongy structure was observed in thawed samples which had been frozen and stored without cryoprotectants. In starch pastessubmitted to slow freezing and frozen storage, hydrocolloids prevent the formation of a sponge-like structure and the production of exudates due to amylose retrogradation (Ferrero & Zaritzky, 2000; Lee et al., 2002; Mali et al., 2003). Specifically, it has been stated that in starch pastes with added XG, amylose-XG interaction competes with amylose-amylopectin aggregation, reducing the probability of amylose retrogradation or retarding it.It is possible that the amylose chains leached during cooking and cooling were readily exposed to the XG present in the added biopolymers mixtures, and the amylose would compete in the chain association between XG molecules and other amylose chains. Slade and Levine (1987) suggested that the stabilizing properties of XG might be products of its ability to undergo molecular entanglement within the frozen concentrated matrix.

Moreover, significant differences were found between mashed potatoes with added binary mixtures, regardless ofallcontaining XG (Table 1).Samples withadded κ-C1.5/XG1.5 mixture exhibited stronger elastic and viscous characteristics than mashed potatoes with added ALM1.5/XG1.5 and SC1.5/XG1.5 mixtures (in that order). Therefore, samples with added κ-C and XG were more structured, confirming previous findings for mashed potatoes with addedκ-Cand other biopolymers alone (Fernández et al., 2008). Mashed potatoes as prepared here are themselvescombined systems of native potato starch/denatured milkprotein/water/salt plus the added biopolymer mixture. Also,mashed potatoes contain either sugars, supplied mainly by the potato and the milk, or Ca ions, supplied principally by the added milk, and complex interactions influence properties of these products.

In a few cases, synergistic gelation occurs when two hydrocolloids are combined. κ- and i-carrageenans are considered the most suitable hydrocolloids for commercial dairy products because of their ability to combine into double helices and to interact with casein to form network structures (Tárrega et al., 2006).A carrageenan–casein network cannot be expected to form in mashed potatoes containing denatured milk protein. Therefore, the fact that the increase in the structure’s rigidity produced by addition of κ-C1.5/XG1.5 mixturewas higher could be ascribed to their ability to combine into double helices, and to interactions between the anionic sulphated polysaccharide and denatured milk proteins present in mashed potatoes. A stronger synergistic effect was observed in κ-C/denatured soy protein systems associated with greaterincompatibility because of thermal denaturation of theprotein (Baeza et al., 2002).Then again, it has been found that addition ofstarches accelerates gelation of κ-C, possibly due tocoupling effects between κ-C and soluble starch molecules (Faria-Tischer et al., 2006).The positive effect on rheological properties associatedwith κ-C1.5/XG1.5 mixtures could also be caused by the presence of potatostarch, resulting in a decisive synergistic effect andhelping to enhance intermolecular binding.Nevertheless, it has been reported that the addition of salt exerts a considerable influence onthe gelation of carrageenans (DeFreitas et al.,1997). In the case of κ-C, alkaline ions bind to thehelix of the hydrocolloid, thus partially neutralizing thesulphate groups. This causes aggregation of the doublehelixes, increasing gel rigidity.