Monitoring quality changes in stored frozen strawberries with timc temperature indicators*

R. P. Singh and J. H. Wells Department of Agricultural Engineering, University of California, Davis, CA 95616, USA

Rece ived 9 February 1987

Commercially available time-temperature indicators were examined as quality change monitors for frozen stawberries during storage. Strawberries were stored under constant ( - 12, - 18 and - 35°C) and variable temperature treatments, and quality changes were evaluated at three-week intervals by sensory and chemical analysis methods. Indicator response was strongly correlated with firmness and ascorbic acid change in strawberries stored at -12°C. (Keywords: strawberries; food quality; food storage)

Monitorage/t l'aide d'indicateurs temps-temprrature des modifications de la qualitb des fraises congel es entrepos es

On examine les indicateurs temps-temperature disponibles dans le commerce en tant que mode de surveillance des modifications de la qualitb des fraises congelbes au cours de l'entreposage. Les fraises ont btb entreposbes tempbrature constante ( - 12, - 18 et - 3 5 ° C ) et ~t tempbrature variable et les modifications de la qualit~ ont bt~ bvalubes toutes les trois semaines par des mbthodes d'analyse sensorielle et chimique. La r~ponse de l'indicateur a btb correlbe fortement avec l'bvolution de la fermet~ et de la teneur en acide ascorbique dans les fraises entreposbes ~ - 12°C. (Mots cl~s: fraises; qualite des aliments; entreposage des aliments)

A time-temperature indicator is a device which responds to the combined effect of time and temperature. This definition includes those devices which have been developed from a conjectural basis (an assumed reaction rate model), as well as those which have been developed from empirical considerations (an observed phase change). The primary application of time-temperature indicators has been in the area of pharmaceuticals, to monitor the shipment of temperature sensitive vaccines. Perhaps the largest potential application for these indicators lies in the area of monitoring the temperature exposure of foods during distribution.

Several specific quality changes in refrigerated and frozen foods have been shown to be related to t ime- temperature exposure. The identification of these quality attributes, their quantitative definition and correlation with time-temperature indicator response, could provide a means of using indicators as food quality monitors. The specific objective of this Paper is to examine the application of t ime-temperature indicators as monitors of selected quality attributes in frozen strawberries.

Time-temperature indicator review

It is believed that the first practical t ime-temperature indicator offered to the frozen food industry was the Honeywell Time-Temperature Indicator ~, as reported by Renier et al. 1. This device was developed because 'the lack

* Based on a paper presented at the 1985 Commission C2 meeting of the IIR, Orlando, USA, 17-21 November 1985 * Mention of manufacturer names and particular indicator models does not constitute endorsement by the authors

0140-7007/87/0502964)5503.00 © 1987 Butterworth & Co (Publishers) Ltd and IIR

296 Int. J. Refrig. 1987 Vol 10 September

of an adequate quality reserve monitoring control scheme for frozen foods has been a source of problems to many'. Renier et al. presented the technical requirements that t ime-temperature indicators must meet, and stated that the most critical requirement was the ability of an indicator to monitor specific time-temperature relationships in frozen food products.

Hu 2 reported the development of a time-temperature indicator which utilized oxygen permeation through a plastic film. The rate of oxygen permeation through the film used in this indicator was both time and temperature dependent. By enclosing an oxygen reactive solution within a sealed pouch, the solution would change colour (red to clear) and reveal a preprinted message. Hu demonstrated this indicator for a wide range of storage temperatures (5.5-55°C), and discussed its possible applications to storage of semi-perishable military rations.

Schoen and Byrne 3 examined patented and unpatented time-temperature indicators, which operated with a variety of physical, chemical and biological mechanisms. These researchers stated that the ideal indicator for frozen foods should reliably inform an individual when to discard a product. It was pointed out that no such indicator had yet been developed because of the subjectivity in defining frozen food quality, and the difficulty in deciding which relevant storage variables should be measured.

The design and development of the I -POINT Time/ Temperature Monitor was discussed by Blixt and Tiru 4. The enzyme-substrate response mechanism of this indicator was presented, and it was claimed to be superior

Monitoring quality changes in frozen strawberries: R. P. Singh and d. t4. Wells

to purely chemical or physical indicating methods and devices because it more closely responded to the biological changes which occur in food systems. Manske s outlined the motivation and criteria which was the basis for the 3M Monitor Mark time-temperature indicator. He believed that increased product quality consciousness and awareness of product liability had set the stage to develop a low cost device which could monitor the temperature history of perishable food products. Other commercially available time-temperature indicators, including the Ambitemp Temperature and Time Indicator and the Tempchron Temperature and Time Indicator, have been reviewed and described by Wells and Singh 6.

Materials and methods

Time-temperature indicators Several commercially available time-temperature indicators were evaluated at the beginning of this study, the results of which have been discussed previously by Wells and Singh 6. The I-POINT Time/Temperature Monitor was chosen for further evaluation in the context of a frozen food storage study using various commercially purchased frozen products ~. This Paper focusses on the use of two models of the I-POINT Time/Temperature Monitor (models 1020 and 3015) to monitor quality changes in whole frozen strawberries.

Frozen strawberry storage study Strawberries used in this study were harvested from the Watsonville (California, USA) growing area, and were individually quick-frozen seven to eight months prior to the beginning of this study. The product was USDA grade B or better, whole, unsweetened strawberries, packaged in 0.454 kg polythene bags, 12 bags to the case.

The storage study design consisted of placing packages of frozen strawberries, with time-temperature indicators attached, at constant and variable storage temperatures. Product temperatures of -12, - 1 8 and -35°C were maintained constant (+I°C maximum variation) by storing the product in insulated containers placed in cold storage rooms controlled at the appropriate tem- peratures. The variable temperature treatment consisted of - 18°C storage with three high temperature exposures. Each high temperature exposure lasted for 1 h at 20°C followed by 8 h at 5°C; this treatment was chosen to illu- strate the conditions encountered during frozen food transportation and handling. All samples stored with the variable temperature treatment were placed as individual packages on stainless steel wire cooling trays, spaced 3 cm apart on aluminium racks to ensure airflow over each tray. The product was sampled at a nominal three-week interval and evaluated with sensory and objective testing.

Sensory analysis procedures Sensory testing was conducted to determine whether trained panelists could detect and quantify changes in specific quality attributes. From a preliminary descriptive analysis, the attributes firmness, sourness and the darkness of the red colour were found to be important quality characteristics. During the storage study, each quality attribute was evaluated according to a complete randomized block design. A deviation from reference

scale was used to evaluate attribute changes as compared to a reference product stored at - 35°C (see Reference 8). Each experimental sample was rated in terms of how much it deviated from the reference on a 10cm anchored unstructured scale (marked at the mid-point to indicate reference).

Several bags of frozen strawberries from each treatment location were placed at - 18°C, 25 h before the beginning of a sensory analysis evaluation. Nine hours later, each bag was placed on a separate styrofoam tray, and was thawed for 12 h at 16°C. The thawed strawberries were drained for 15 min on a number 10 mesh screen prior to sensory presentation.

Firmness was evaluated orally, and was described as the force required to penetrate a whole berry with the molars. At least two whole berries were served in 89 ml paper cups, and berries were covered with the free liquid collected during product drainage to avoid fruit dehydration. To negate heterogeneity in the straw- berries within a treatment, a strawberry puree was prepared for the sourness and colour evaluations. The puree was prepared by combining drained strawberries in a blender until no fruit lumps remained. The firmness and sourness evaluations were presented under red light inside individual booths; the product serving temperature was ~ 10°C. The samples used to assess the darkness of the red colour were displayed in disposable petri dishes under white light.

A computer-aided method was developed to acquire and manage data from the sensory test. These procedures have been described elsewhere by Singh and Wells 9. The statistical software package SAS 1° was used for the analysis of variance and multiple means comparison computations.

Objective analysis procedure An objective evaluation of the product quality change was determined by analysing l-ascorbic and dehydro- ascorbic acid. A Turner fluorometer was used to measure percentage fluorescence on each blank, sample and standard according to a procedure adapted from Osborne and Voogt I

Results and discussion

Strawberry sensory evaluations The darkness of the red colour and the sourness attributes are believed to be dependent on the pigment and sugar concentration, respectively, and the firmness on fruit turgidity. Many packages were found to have small pinholes, or to be otherwise broken, when samples were removed from the treatment locations. This was presumed to have occurred during packaging and handling, and often was not evident until product thawing. The lack of packaging integrity caused excessive fluid drainage during thawing, and may have contributed to product variability as pigments and sugars were leached from the fruit.

Six quality evaluation sessions were conducted at 0, 23, 44, 65, 84 and 105 days after the beginning of storage. Three high temperature cycles were held at 24, 66 and 88 days after the beginning of storage. Analyses of variance were performed using the scores of 14 judges, and a complete summary of these analyses of the attributes firmness, sourness and colour are given in Table 1. No

Rev. Int. Froid 1987 Vol 10 Septembre 297

Monitoring quality changes in frozen strawberries: R. P. Singh and J. H. Wells

consistently significant treatment effects were observed for either firmness or sourness.

The mean values of the sensory evaluation scores and the multiple means comparison values for the red colouration are given in Table 2. The treatment effect for this attribute was significant for storage day 65 onward. Significant differences between - 12 and - 3 5 ° C samples were found for days 65 and 105. The lack of significant differences between these two samples at storage day 84 are believed to have been a result of experimental error.

In previous studies reported by Van Arsdel et al. ~2, sensory panels were used to determine the storage time

required to bring about noticeable, overall, unspecified changes at various storage temperatures. No specific product attributes were rated, and the noticeable differences were detected with the use of sensory comparison methods. Van Arsdel et al) 2 also reported that noticeable differences in strawberries stored at - 1 2 ° C were detected after 60 days. In this study, the colour for strawberries stored at - 1 2 ° C became significantly different from samples stored at - 3 5 ° C between 42 and 65 days. A more specific storage time for the generation of significant differences cannot be determined because sensory testing was conducted on a three-week interval.

Table l S u m m a r y o f ana lys i s o f v a r i a n c e resul ts for s t r a w b e r r y a t t r i bu t e s a, s h o w i n g t r e a t m e n t , T, r ep l i ca t ion , R, a n d j udge , J T a b l e a u 1 Rb.capitulation de l'analyse des rbsultats de variance de la qualitb des fraises a, indiquant le traitement, T, la rbp~tition, R, et le juge, J

S t o r a g e d a y

0 23 44 65 84 105

F i r m n e s s T ns ns ns ns ns ns R ns ns ns ns * ns J ns ns ns *** ns ** T x R ns ns ns ns ns ns T x J ns ns ns ns ns ns R x J ns ns ns ns ns ns

Sourness T ns ns ns * * ns R ns ns ns ns ns ns J ns ns * ns *** ** T x R ns ns ns ns ns ns T x J ns ns ns ns ns ns R x J ns * ** ns ns **

C o l o u r T ns ** ns *** * *** R • ns a s ns • ns J *** ns ** *** ** ns T x R ns ns ns ns ns ns T × J ** ns ns *** ** * R × J *** ns ns ns * ns

= Signif icance level g iven as : n s = n o t s ignif icant ; , = p < 0 . 0 5 ; * * = p < 0.01; *** = p < 0 . 0 0 1

Results of ascorbic acid measurements

Previous investigations on the keeping quality of strawberries indicated that total ascorbic acid decreases during frozen storage la. Furthermore, it was expected that higher temperatures would bring about a more noticeable decrease. These expectations were supported by the ascorbic acid test results which indicated a trend of decreasing amount for the -1 2 ° C and variable treatments, but no discernible decrease for the -18°C and reference products. The analyses of variance for each evaluation of total ascorbic acid showed no significant differences between the treatments.

Correlation of quality evaluations and time-temperature indicator response

Linear correlations between storage time, the mean response of each indicator model, the mean values of total ascorbic acid and the mean scores of each sensory attribute, were conducted for each constant temperature storage treatment. Significant negative correlations were found between the strawberry quality measurements of firmness and total ascorbic acid content, and both I -POINT indicator responses at the - 1 2 ° C storage condition (see Table 3). These parameters, in addition to being highly correlated with indicator response, were highly correlated with storage time. That is, these quality parameters appeared to be functionally related to storage time at -12°C. The quality attributes of firmness and ascorbic acid content are shown with the indicator

Table 2 M e a n scores , s t a n d a r d dev i a t i on a n d s ta t is t ica l c o m p a r i s o n of m e a n s for the senso ry e v a l u a t i o n o f the s t r a w b e r r y red c o l o u r T a b l e a u 2 Notes moyennes, bcart standard et comparaison statistique des moyennes pour I'bvaluation sensorielle de la couleur rouge foncb des fraises

N u m b e r o f obse rva t i ons ,

S to r age d a y n - 35°C - 18°C

M e a n s c o m p a r i s o n S t o r a g e t r e a t m e n t (p = 0.05)

Var i ab l e - 12°C L S D a M S D b

0 28 53.09 ~ 51.12 54.63 51.37 2.72 3.93 (7.09) d (9.79) (6.53) (9.57)

23 22 50.25 42.58 52.35 49.28 4.03 5.78 (8.22) (13.82) (8.17) (9.92)

44 26 52.67 55.48 54.93 51 .90 - - (8.50) (7.86) (7.88) (9.98)

65 28 52.86 45.79 47.63 45.03 3.68 5.29 (12.74) (8.36) (8.06) (11.82)

84 20 51.27 55.50 59.62 55.66 5.11 7.41 (4.69) (13.31) (15.96) (12.44)

105 24 49 .84 51.88 54.77 44 .30 2.67 3.85 (4.45) (5.03) (5.18) (7.29)

= Leas t s ignif icant difference (LSD) be tween t r e a t m e n t m e a n s , a c c o r d i n g to F i she r ' s mul t ip le m e a n s c o m p a r i s o n test (p = 0.05) b M i n i m u m s igni f icant dif ference ( M S D ) be tween t r e a t m e n t m e a n s , a c c o r d i n g to Scheffe 's mul t ip le m e a n s c o m p a r i s o n test (p = 0.05) c M e a n d a r k n e s s o f red c o l o u r score a S t a n d a r d d e v i a t i o n o f m e a n d a r k n e s s o f red c o l o u r scores

298 Int. J. Refrig. 1987 Vol 10 September

Monitoring quafity changes in frozen strawberries: R. P. Singh and J. H. Wells

Table 3 Pearson correlation coefficients, R, and probability significance levels, p, between the quality measurements of frozen strawberries and the response of I-POINT indicators stored at - 12°C Tableau 3 Coefficients de corrblation et niveaux d'importance entre les mesures de la qualitb des fraises congelbes et la rbponse des indicateurs du PO1N T-I pour les fraises entreposi?es it - 12°C

Ascorbic acid content Firmness

I-POINT model no. R p R p

Storage time - 0.943 0.005 - 0.793 0.060 1020 - 0.865 0.026 - 0.792 0.060 3015 -0.879 0.021 -0.724 0.104

65 .25

I ~==~Firmness Score Scale 40-60) ~_zlAscorbic Acid Scale 0-0.25

I-POINT t020 {Scale 0-3) 60 .2

H

55-

50-

45-

40

.15

.05

0 0 23 44 65

TIME (days)

- - - 3

-2

B4 105

Figure 1 Comparison of changes in frozen strawberry firmness score and total ascorbic acid content, and the response of the I-POINT Model 1020 indicator during -12°C constant temperature storage Figure 1 Comparaison de l'bvolution de la note attribuke it la fermetk des fraises congelbes et de la teneur totale en acide ascorbique et de la r~ponse de l'indicateur Model 1020 du POIN T-I au cours de l' entreposage it une temperature constante de - 12°C

g %

H

g

65 .25

60- .2

55- . I5

50- . ~.

'15 ~ .05

40 .?,

Firmness ScoPe Scale 40-60) Ascorbic Aci0 (Scale 0-0.25 I-POINT 3015 (Scale 0-3}

0 ?3 44

TIME

65 84

- r3 L

I '

-2

- 1

o 105

(days)

Figure 2 Comparison of changes in frozen strawberry firmness score and total ascorbic acid content, and the response of the I-POINT Model 3015 indicator during - 12°C constant temperature storage Figure 2 Comparaison de l'bvolution des notes attribubes it la fermetb des fraises congeldes et la teneur totale en acide ascorbique et de la rdponse de l'indicateur Model 3015 du POINT-I au cours de l'entreposage it une tempbrature constante de - 12°C

arising from the storage conditions. However, the significant correlations between the I-POINT indicator response, and the sensory attributes, firmness and total ascorbic acid change for products stored at - 12°C, are strong evidence that indicators of this type can be applied as quality change monitors for strawberries. In general, it is considered that the I-POINT time-temperature indicator may have extensive applications to a wide range of frozen food quality changes. Continued research is necessary to develop statistical sampling plans, adequate quality measurement and modelling techniques, and practical inventory management schemes to fully implement the use of time-temperature indicators as quality change monitors.

responses as bar graphs in Figures 1 and 2. The magnitudes of firmness and total ascorbic acid content are shown to be generally decreasing with time, while the indicator response is increasing.

Although indicator response was not significantly correlated with changes in the colouration, note that the I-POINT model 1020 indicator, stored at - 12°C, began to respond between 44 and 65 days of storage (see Figure 1). In this case, indicator response corresponds to the storage times between which significant changes in the colour began to be observed (see Table 2). It would appear that a time-temperature indicator which exhibits a delay prior to initiating a response, might be used to indicate storage conditions which give rise to significant differences in one quality attribute, as well as indicating the magnitude of change in other attributes.

Conclusions

For the duration of this study, frozen strawberries appeared to be a relatively stable product. A longer study would be necessary to determine when significant changes occur for sourness, firmness and total ascorbic acid content. It is believed that uncontrolled and non-uniform leaching of sugars and pigments during thawing may have negated the measurement of some product changes

Acknowledgements

This research was supported by US Department of Army Contract No. DAAK-60-83-C-0100. The authors wish to acknowledge the technical assistance provided by N. Raubach, E. Gonnet and K. Dolan.

References

1 Renier, J. J., Morin, W. T. et al. Time-temperature indicators I IR Bull Annexe (1962) 1 483

2 Hu, K. H. Time-temperature indicating system 'writes' status of product shelf-life Food Technol (1971) 26(8) 56

3 Sehoen, H. M., Byrne, C. H. Defrost indicators: many designs have been patented yet there is no ideal indicator Food Technol (1972) 26(10) 46

4 Blixt, K. G., Tiru, M. An enzymatic time/temperature device for monitoring the handling of perishable commodities Develop Biol Stand (1976) 36 237

5 Manske, W. J. Application of controlled fluid migration to temperature limit and time-temperature integrators I IR 16th Int Congr Refrig Commission C2 Prepr 632

6 Wells, J. H., SingK R. P. Performance evaluation of time- temperature indicators for frozen food transport J Food Sci (1985) 50(2) 369

7 Singh, R. P , Wells, J. H., Dolan, K. D., Gonnet, E. J., Munbz, A. M. Critical evaluation of time-temperature indicators for monitoring quality changes in stored subsistence. Report prepared for United States Army Natick Research and Development Center, Natick, Massachusetts, USA (October 1984) (Contract No. DAAK-60-83-C-0100)

Rev. Int. Froid 1987 Vol 10 Septembre 299

Monitoring quality changes in frozen strawberries." R. P. Singh and J. t4. Wells

8 Singh, R. P., Heldman, D. R. Quality changes in frozen foods. Paper presented at the 1983 Winter Meeting American Society of Agricultural Engineers, Chicago, USA ASAE Paper No 83-6510 (December 1983)

9 Singh, R. P., Wells, J. H. Use of time-temperature indicators to monitor quality of frozen hamburger Food Technol (1985) 39(12) 42-50

10 SAS Institute Inc. SAS User's Guide: Basics SAS Institute, Cary,

North Carolina, USA (1982) 11 Osborne, D. R., Voogt, P. The Analysis of Nutrients in Foods

Academic Press, New York, USA (1978) 12 Wm Arsdel, W. B., Coply, M. J., O l in , R. L. Quality and

Stability of Frozen Foods Wiley-Interscience, New York, USA (1969)

13 Labuza, T. P. Shelf-Life Datino of Foods Food and Nutrition Press, Westport, USA (1982)

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