Extending the shelf-life of cottage cheese using monolaurin

Food Research Inter~rutionul26 (1993) 203-208

RESEARCH

NOTE

Extending the shelf-life of cottage cheese using monolaurin D. A. Bautista,” M. D. Durisin,” S. M. Razavi-Rohani,’ A. R. Hill” & M. W. Griffiths” “Food ScYence Department, University of Guelph, Gurlph, Ontario, Canada NIG 2 I+‘/ “Department of Food Technolug~~.b’niwrslt~~qf’Urmiu, c’rmiu, Irrm

Shelf-life problems due to premature spoilage of cottage cheese result in approximately 5% return of product to the manufacturer each year. While preservatives are not permitted in Canada, it may be possible to use naturally occurring compounds to extend the storage life of cottage cheese. The monoglyceride monolaurin has been shown to possess anti-microbial properties as well as being an emulsifying agent. Incorporation of monolaurin into naturally contaminated cottage cheese at levels of 250 and 500 ppm resulted in >90% inhibition of both Pseudomoms spp. and coliforms during 7 days of storage at 6, 15, and 2 1°C. There was also >90% inhibition of growth of yeasts and moulds under the same storage conditions in the presence of monolaurin. These results suggest that the use of monolaurin as an emulsifying agent in cottage cheese will have beneficial effects by extending microbial shelf-life by approximately 35’%1at 6°C. Monolaurin also increased the shelf-life of cottage cheese (approximately 5-10 days) when assessed by organoleptic analysis. Keywords: monolaurin.

shelf-life.

micro-organisms,

emulsifier.

cottage

cheese.

ties. A report by Kabara (1984) has indicated that lauricidin can extend shelf-life for many food products at room temperature for several days. The report also showed that the shelf life of food products could be extended to 4 months when monolaurin and lactic acid were used in combination. The antimicrobial activity of monolaurin is primarily directed against Gram-positive organisms (Robach rt al., 198 1; Kabara, 1982; Lindsay. 1985) but Gram-negative organisms can be affected by combining monolaurin with heat treatments, chelators or sorbic acid (Robach et al., 1981; Kabara, 1984). The possibility of monoglycerides as a means to control undesired levels of microbial activity in foods has been investigated. The objectives of this study were to determine the level of inhibition of monolaurin in raw milk and to ascertain the functional application of monolaurin in cottage cheese.

INTRODUCTION Shelf-life problems associated with premature spoilage of cottage cheese results in approximately 5% returns of product to the Canadian manufacturer each year. Preservatives are an alternative to correct this problem but unfortunately, Canadian food regulations prohibits use of these types of additives. It may, however, be possible to use naturally occurring compounds to extend the product life of cottage cheese. Furthermore, the use of any natural preservative is extremely appealing to the consumer (Shibasaki, 1982) and, therefore, merits investigation. Monolaurin (lauricidin) is a monoglyceride that has been shown to possess anti-microbial properFood Research International 0963-9969/931$06.00 0 1993 Canadian Institute of Food Science and Technology 203

MATERIALS

AND METHODS

Raw milk study Raw milk was obtained from the Ontario Central Milk Testing Laboratory, Guelph, ON. Canada. All samples were pooled and mixed thoroughly to distribute the microbial population. The milk was divided into three separate batches and the pH of each milk lot was adjusted to 5.2. 6.0 and 6.73 with lactic acid. From each milk lot. three sub-samples of 75 g were retrieved and placed into resealable containers. A stock solution of monolaurin (Lauricidin, Inc.. Galena. IL) was prepared in 95% ethanol to give a final concentration of 5% (w/w). Monolaurin was added at concentrations of 0. 250 and 500 ppm to each pH-adjusted milk sub-sample. All sub-samples were mixed thoroughly and incubated at 6°C. A similar procedure was performed for incubation of pH-adjusted raw milk samples at 15°C. Duplicate tests were performed at both 6 and 15°C. The natural microflora was monitored every 24 11and 12 h for incubation temperatures at 6 and 15°C. respectively, until microbial counts exceeded -10 colony forming units (cfu)/ml. Total microbial count was enumerated on milk agar (Oxoid CM21 . Basingstoke. UK) and P.~&r~~rzas spp. were enumerated on Pseudommr.c agar with supplement (Oxoid: CM559+SR103E, Basingstoke. UK). All plates were inoculated using the Spiral plater (Spiral Systems, Inc., Spiral Plater Model I>, Cincinnati, OH) and counted after I 2 days of incubation at 2 1“C.

using standard method?; lor the microhiill ixa~n~nation o! dairy product7 (Marshall (sf *ii i9X5) pertaining 10 cottage cheese. The sample \t’;ih anaiysed also for microbial levels of yeast ;tnc.l rnouici. P.~r~lf/orllol2tr.s spp. and coliforms using poliito dc,trose agar. P.~f~lllfonzontr.sagar and Violet Red Bile (VRB) agar, respectively. All plates w’erc inoculated using the Spiral plater and counted :llter 1 .? days of incubation at 21 Y‘. Determination of minimum inhibitory concentration The minimum inhibitory concentrations (M Ic’) 01 monolaurin required to inhibit growth for a variety of micro-organisms Luctocwcu~ lrrcti.\. Etitcrw spp. were hater ~~i~rogcw.v and P.vr~domo~zu.s determined using the Spiral Plater according to the manufacturer’s instructions (Anon., 1985 ). Taste panel analysis A trained taste panel was used and samples analysed by -Triangle Test’ (Poste et al., 1991). Analyses were performed on cottage cheese treated with concentrations of monolaurin at 0, 250, 500 ppm at 6 and 15°C over a period of several days. Panelists were asked to score a variety of organoleptic traits (Table I). An indication of overall acceptability was obtained from an average of all scores on a particular sample.

RESULTS

AND DISCUSSION

Raw milk study Cottage cheese study Cottage cheese curd was obtained on two separate occasions from a commercial plant on the day of processing. Curd was kept in a refrigerated cooler (Koolatron Corporation, model P34A, Brantford, ON, Canada) during transport. Under complete sanitary conditions, curd was divided into two batches and cream [18’% (w/w) milk fat] was added to achieve an overall milk fat content of 4% (w/w). Monolaurin was administered in powder form to each batch at 0 and 500 ppm. Both cottage cheese batches were mixed thoroughly to evenly distribute the cream and monolaurin. Samples of the cheese were stored at 6, 15 and 21°C. The natural microflora of the cottage cheese was monitored daily until microbial counts exceeded IO5 cfu/ml. Enumeration was performed by

Monolaurin exhibited little effect on total bacteria or Pseudomonas spp. in In practically all cases, no inhibition when compared to controls (i.e. 0

the levels of milk (Fig. 1). was observed ppm) at the

Table 1. Judging criteria for assessing quality of cottage cheese

Appearance Whey Gloss Bubbliness Yellowiness Greyness Wetness

Taste

Texture

Sweetness Salty Sour Sharp Cardboard Rancid Buttery Bitterness Sour aftertaste Cheese aftertaste Chemical aftertaste

Saltiness Chewiness Stickiness Chalky mouthfeel

205

E.xtending the .she@jk qf’cottage cheese using monolaurin

1L

C

1

2

3

4

5

Time (Days)

Fig. 1. Microbial

inhibition

by monolaurin

at 0 (0). agar (

250 (0) and 500 (A) ppm in raw milk incubated ) and P.reudnmonu.~ agar (- ~~~ ).

various incubation temperatures/pH values combinations (results not shown). This lack of inhibitory activity of the monolaurin could be attributed to the partitioning of monolaurin in the cream layer in raw milk (Lindsay, 1985). Monolaurin is a highly lipophilic molecule (Wang & Johnson, 1992; Lindsay, 1985) and, therefore, would have become more associated with the milk fat lipids found in cream. This, would have effectively reduced the concentration of monolaurin in the aqueous phase. To achieve microbial inhibition in cheese, it is inadvisable to add monolaurin directly to raw milk as it inhibits lactic acid bacteria including Luctococcus lactis and Lactococcus cremoris (Table 2). This, however, would have a deleterious effect on acid development. Cottage cheese study

Growth

of yeast, moulds, coliforms and Pseuspp. was substantially inhibited in cottage cheese stored at 6°C in the presence of monolaurin at a concentration of 500 ppm (Fig. 2). in general, growth was suppressed by l-2 log units at 500 ppm monolaurin when compared to controls (i.e. 0 ppm). Similar results were found at storage temperatures of 15 and 21 “C. However, the inhibition of organisms capable of growth on VRB agar was more pronounced at the higher temperatures (15 and 21 “C). This may be due to differences in sensitivity of psychrotrophic and

at 6°C at pH 6.73 on milk

non-psychrotrophic strains since there was an effect on lag phase and generation times. The lag phase observed in cottage cheese incubated at 21°C‘was 24 h for control samples but was >48 h in the presence of monolaurin. .4 similar observation was found with cottage cheese incubated at 6°C; shelf-life (defined as the time for the plate count to reach 1 X IO6 cfu/ml) was extended by at least 5 days. At 15’C, inhibition was observed with coliforms and Pseudomonw spp. only after 4 and 5 days, respectively. A possible reason for the general success of monolaurin can be attributed to the distribution of milk fat in cottage cheese. Therefore, the microbial activity of monolaurin is improved in this type of medium. This is in contrast to the results of Robach ef ar’. (1981) who were unable to demonstrate an anti-microbial effect of monolaurin in cottage cheese. The physical appearance of the cottage cheese treated with monolaurin was different from control samples. For example, the cream dressing was

domonas

Table 2. A list of minimum inhibitory concentration of monolaurin of various micro-organisms Micro-organism

Minimum concentration Monolaurin

Luctococcus luctis Pseudomonas aeroginosa Pseudomonas putidu P.ceutlomonas,puorescens Ef7terohacter aerogenes

x >7942 >7942 >7942 >7942

inhibitory (MIC) pg/ml

Monolaurin 4 437 150 252 530

+EDTA

1

._____.

0

1

2

3

4 5 Time (Days)

6

7

8

Fig. 2. Microbial inhibition by monolaurin at 0 ppm (open symbols: solid line) and 500 ppm (closed symbols: dashed line) in cottage cheese stored at 6°C with plate count performed on potato dextrose agar ( Ok--: 0~ -), P.cc~u~k,llloncr.~ selective agar (--0 ; -H- ) and violet red bile agar ( A- : A~ 1.

more evenly distributed throughout the product and an improvement in colour was observed upon the addition of monolaurin at different concentrations. Minimum inhibitory concentrations monolaurin

(MIC) on

Monolaurin was inhibitory against Gram-positive bacteria including List&l monocytogenrs. Other researchers have also shown monolaurin to be inhibitory to the latter organism (Oh & Marshall, 1992; Wang & Johnson, 1992). Monolaurin was less active against Gram-negative organisms (Table 2) but the addition of a chelating agent such as EDTA improved activity against these bacteria. This result may explain the inhibitory action of monolaurin against Gram-negative in cottage cheese where citrate may be present as a chelating agent. It has been shown that the inhibitory effect of monolaurin against Escherichiu coli was increased in the presence of citric acid (Kato & Shibasaki, 1976) and lower pH values (Oh & Marshall, 1992). The MIC values observed in this study were similar to those reported by Kabara (1982). Taste panel study Trained panelists were unable to distinguish differences @>0.05) in organoleptic traits of the cottage cheese stored at 6°C with different concentrations

of monolaurin (i.e. 0, 250, 500 ppm) over a storage period of 11 days (Fig. 3). The only quality discernible to the panelists was a chemical aftertaste at concentrations of monolaurin at 250 and 500 ppm. On a scale from 0 to 10 where 0 represented no aftertaste, the scores for cottage cheese containing 0, 250, 500 ppm monolaurin prior to storage were 1’1, 1.6 and 1.7, respectively. To gain an insight on the overall acceptability of the cheeses, organoleptic scores for all the traits studied were averaged. There was little difference in the average scores obtained for cheese samples containing 0, 250, and 500 ppm monolaurin and stored at 6°C for 11 days (Fig. 3). These had scores averaging 2.64, 2.60 and 2.61, respectively. At 15°C. a noticeable difference in organoleptic quality was observed between control samples and those to which monolaurin had been added (Fig. 4). After 9 days of storage at 15°C. the cottage cheese samples not containing monolaurin were completely unacceptable to the taste panel. whereas, those containing monolaurin were still edible and no discernible taint was evident . Since very few consumers are trained to detect specific food defects when they buy a food product, the observation of a chemical aftertaste under the trained scenarios may not be indicative of most consumers. Future studies could include consumer panel evaluations to ascertain the acceptance or rejection of cottage cheese treated with monolaurin.

Extending

the sheljJife

ofcottagr

207

chcesr using monolaurin

; I I

I I I I I

I I I I

I

I

L

1

-_ 0

2

4

I

/

I

7

9

11

0

2

.L____ 4

I

I

7

9

Days

Days

Fig. 3. Overall taste acceptability by trained panelist of cottage cheese incubated at 6°C and treated with monolaurin at 0 (O), 250 (m) and 500 (A) ppm.

Fig. 4. Overall taste acceptability by trained panelist of cottage cheese incubated at 15°C and treated with monolaurin at 0 (01, 250 (m) and 500 (A) ppm. The control cottage cheese was rejected by all panelists and was arbitrarily scored as 10 after 9 days of storage at 15°C t -@-).

CONCLUSIONS

ducers Co-operative Ltd, Gay Lea Foods, Grindsted Products Inc. and McCain Refrigerated Foods. The technical assistance of Gay Lea Foods and Dr E. A. K. Gullett is acknowledged.

The present study indicated that monolaurin exhibited anti-microbial activity in cottage cheese. The inhibitory action observed at 6°C extended the shelf-life of cottage cheese by approximately 35% (approximately 5-10 days). This could benefit manufacturers by reducing the amount of returns due to premature spoilage of processed cottage cheese product. However, trained panelists were able to distinguish monolaurin concentrations at 250 and 500 ppm by a chemical aftertaste. Other applications for monolaurin are in low fat foods such as yogurt and beverages. In addition, monolaurin has emulsifying properties that may be useful for many types of foods. ACKNOWLEDGEMENTS The authors would like to acknowledge the financial assistance of Beatrice Foods Inc., Dairy Pro-

REFERENCES Anon. (1985). Determination of antimicrobial susceptibility by the Spiral Gradient Endpoint test. Spiral System, Inc., Bethesda, MD. Kabara. J. J. (1982). A new preservative in food. J. Food Sqf&y, 4, 13-m25. Kabara, J. J. (1984). LAURICIDIN: The nonionic emulsifier with antimicrobial properties. In Cosmetic and Drug Przservution; Principles and Pructicr. Marcel Dekker, New York, pp. 30552 1. Kato. N. & Shibasaki, I. (1976). Combined effect of citric and polyphosphoric acid on the anti-bacterial activity of monoglycerides. J. Antihact. Ant@q. ,,lg. (Jupan), 4, 254-6 1. Lindsay, R. C. (1985). Food additives. In Food Chemistry, ed. Marcel Dekker, New York, pp. 629 -88. Marshall, R. T., Adams, D. M., Morgan, D. R.. Olsen, N. F. & White. C. H. (1985). Microbiological methods for dairy

products.

In Sltrtdwti M~~tJxtr/.~ /tw 1h E\titt2ittulioti (!! Doit:l~ Proo’ut~~.~. 15th edn. American Public Health As,ociation. Washington, f)C‘. pp. 21 I l-1. Oh, 11-H. & Marshall. I> I (1992). Etrect of‘pfi on the IWE irnunl inhibitory concentration of’ monolaurin aga~n\t I i\/wit1 t?totroc:l’logc’ttc.\. ./. Food PtYttcY~/iott. 55, 349 SO. Paste.I... Mackie, II. A., Butler. C;. & Larnlond. f:. ( 1001).Luh~rcrfot~vMrthotA for tlw Setzsor~~ Attu(~:si.t o/ Fborl Research

Branch of Agriculture (‘anada. Publication 1WitJ. pp. 70 -tZ. Robach. M. C.. Hickey. C‘. S. & To, E. C. ( I X31). Comparison of antimicrobial action of monolaurin and sorbic aad.

(Received 2 September ber 1992)

1992; accepted

I3 Novem-