Food Processing Potential of the Oyster Mushroom (Pleurotus florida)

Can./llst. FoodSci. Techllol. J. Vo!. 14, No. I, pp. 36-41, January 1981 Pergamon Press Ltd. Printed in Canada.

Food Processing Potential of the Oyster Mushroom (Pleurotus jlorida) L. Oddson and P. Jelen Department of Food Science University of Alberta Edmonton, Alberta T6G 2N2

all of the mushrooms consumed in Canada are the "Button Mushrooms" (Agaricus bisparus), The "Oyster Mushroom" (Pleuratus spp,), which is now being commercially produced in some Asian and East European countries (Weijer, 1974), may be an attractive alternative variety for Canadian market use, The Oyster Mushroom can be cultivated on unsupplemented wheat straw at much lower cost and with more ease than the Button Mushroom (Stanek and Rysava, 1971), In addition, it may be possible to use the fruited substrate as an animal feed (Zadrazil, 1976) because of the apparent capacity of the fungus for nitrogen fixation (Ginterova, 1973), The best known species of the Oyster Mushroom is Pleuratus astreatus, However, the strain usually referred to as Pleuratus jlarida has several unique features from a cultivation standpoint, such as advantages in the mycelial growth phase, a fructification temp, requirement of 19-25°C, higher yields per unit area, and a lower proportion of stems to caps on a weight basis (Zadrazil and Schneidereit, 1972). Growth requirements and cultivation methods for Pleuratus spp, have been described (Block et ai" 1959; Volz, 1972; Macaya-Lizano, 1975; Cailleux et aI., 1976; Kalberer, 1976), However, very little has been published on the use of this mushroom for industrial processing, and some of the results are conflicting (Andreotti et ai" 1975; Gormley and O'Riordain, 1976), The objectives of the present work were to assess the potential of P. flarida for freezing, canning and drying preservation and to elucidate certain effects of these techniques on sensory properties of the processed mushrooms. A. bisparus was included in the experiments where appropriate for comparative purposes,

Abstract The effects offreezing, canning and drying on the Oyster Mushroom (P.f1orida) were evaluated in various products. Sensory tests demonstrated that an off-flavordeveloped in the unblanched mushrooms during frozen storage. This effect was correlated to a rise in peroxide number, and decreases in the levels of unsaturated fatty acids as found by gas liquid chromatography analyses. Free fatty acid production, determined by thin layer chromatography, was related to a decrease in phospholipid content. Canned caps in cream or butter sauce had greater sensory acceptability than brine canned caps. Texture of the stems was improved by canning in brine. Sun or hot air drying proved to be satisfactory for the P.f1orida mushroom without blanching. Polyphenoloxidase activity and the concentration of its substrates, as demonstrated by Hunter colorimeter measurements, were lower in samples of P.florida than in A. bisporus.

Resume Nous avons evalue l'effet de la congelation, de la mise en boite et du sechage sur le champignon Oyster (P. f1orida) dans un certain nombre de produits. Les tests d'evaluation sensorielle montrent que les champignons non-blanch is developpent une certaine flaveur pendant l'entreposage it I'etat congele. Cet effet est lie it une augmentation de I'indice de peroxyde et it une diminution des taux d'acides gras insatures reveJee par l'analyse chromatographique en phase liquide et gazeuse. La production d'acides gras libres, mesuree par chromatographie sur couche mince, est liee it la diminution des phospholipides. Les tetes de champignons en sauce creme ou sauce au beurre et mises en boite ont ete jugees plus favorablement que celles mises en boite avec saumure. La texture des pieds de champignons s'ameliore en presence de saumure. Finalement, le sechage au soleil ou it l'air chaud donne des resultats satisfaisants pour le champignon P. f10rida sans blanchiment. L'activite des polyphenol oxydases et la concentration de leurs substrats, mesurees it I'aide du colorimetre Hunter, sont plus faibles dans les echantillons de P. florida que dans ceux de A. bisporus.

Introduction Although mushrooms are Canada's third largest horticultural crop, demand exceeds supply, About 15% of the fresh mushrooms and over 60% of the canned products are imported (Agriculture Canada, 1975), During the period 1964 to 1974 Canadian consumption rose by approximately 150% (Burns and Curry, 1971; Agriculture Canada, 1976), Practically Copyright

©

Materials and Methods Raw Materials p,jlarida mushrooms were grown in the facilities of the Department of Genetics, University of Alberta,

0315-5463/81/010036.()6$2.00/0 1981 Canadian Institute of Food Science and Technology

36

using a method similar to that described by Weijer (1974). Wheat straw was pasteurized at 60°C for 72 h, cooled, inoculated with spawn and incubated at 25°C in a dark chamber for 14 days. Fructification of the mycelium permeating the straw was accomplished at 22°C and 80% R.H. in a growth chamber continuously illuminated under banks of fluorescent lights. A. bisporus mushrooms (Prairie Mushrooms Ltd., Edmonton, Alta.) were purchased at local supermarkets as required.

Enzymatic Browning in P. florida and A. bisporus Visual observations indicated that P. j/orida is much less susceptible to browning than A. bisporus. In an attempt to explain this observation, the two mushrooms were compared as to the relative activities of their polyphenoloxidase (PPO), an enzyme which has been implicated in the browning of mushrooms. Fresh samples of sliced A. bisporus and P.florida were wetted with 15 mL of 0.0 I %, 0.10%, 0.50%, 1.0%, 2.0%, 3.0% or 10% 4-methyl catechol solutions. The lightness (L values) of the treated samples was determined after 5 min using a Hunter Colorimeter (HunterLab Color/ Difference Meter 025-2, Fairfax, Va.). Relative concentrations of the indigenous browning reaction substrates were also compared in P.j/orida and A. bisporus slices by the application of 15 mL of a 0.1 % solution of mushroom polyphenoloxidase (Sigma Chemical Co., Product # T-7755, lot # 17C-9520). Lightness was measured as above at I, 2, 3, 4 and 5 mm.

Processing and Chemical Analyses Freezing Un blanched or steam- blanched P. florida caps were frozen in bulk in a walk-in freezer at -18° C for 8 h. All blanching treatments were in an enclosed stainless steel steam table for 3 min. The effects of freezing and frozen storage were tested by the measurement of drip loss, color and flavor immediately after freezing and after 4 mo frozen storage at -30°e. Drip loss was determined by thawing the frozen mushrooms at 23° C for 2 h in a funnel placed over a calibrated cylinder and recording the amount of separated liquid; color and flavor were determined by Hunter lightness measurements and sensory tests. These comparisons showed that although the freezing per se of un blanched mushrooms resulted in a better quality product, a strong off-flavor described as "strawy" developed during frozen storage. To examine the cause ofthis problem, eight samples (250 g) of unblanched P. florida from a single harvest were slow-frozen and stored at -18°C for 1,2,4, and 8 weeks. Four samples were used in comparative taste panel evaluations with fresh P.florida. The remaining four samples were lyophilized at the specified storage time, extracted with peroxide-free anhydrous ethyl ether and tested for peroxide number by the micromodified method of Lee and Wagenknecht ( 1951 ).

Can. Inst. Food Sci. Technol. J. Vol. 14. No. I. January 1981

Lipid extracts of P. florida samples were obtained by the chloroform/ methanol extraction method of Bowyer and King (1977). The extracts prepared from lyophilized fresh and unblanched frozen tissue (stored for 2 mo at -30°C) were used to identify the major lipid fractions and their oxidation products by the thin layer chromatography (TLC) procedure of Freeman and West (1966). Components were identified by their Rf values after development with Rodamine 6G (0.05%). Phospholipid fractions were carefully scraped off undeveloped plates, eluted and esterified with boron trifluoride (14% w/w). The fatty acid methyl esters (FAMEs) thus obtained were dissolved in carbon disulphide and two microliters injected into a Bendix 2500 Gas Chromatograph. A 2.4 m glass column (o.d. = 6.4 mm) packed with Silar 5CP (10%) on Chromosorb W was used for the separations. Additional extracts were prepared from fresh and unblanched frozen tissue (stored for 12 mo at -30°C) and used to observe changes in the phospholipid fraction with prolonged storage. Two TLC plates were quantitatively spotted with 200 jJ.g of these extracts and chromatographed using the same solvent system as above. To one plate, lipid standards (monoglyceride, diglyceride and a free fatty acid mixture) were applied. Phospholipid fractions were scraped off the second plate, eluted and analyzed for phosphorus according to the procedure of Bowyer and King (1977). Total phospholipid content of the lipid extracts was determined as percent phosphorus x 25.

Canning Stems and caps of P..f/orida were blanched separately in steam for 3 min and hot filled with a 2% (w / v) NaCI brine into 306 x 411 cans. Blanched caps of P. Jlorida were also canned in butter sauce (I part flour, 3 parts butter, 9 parts water) and cream sauce (I part flour, 2 parts butter, 15 parts whole milk). Cream of mushroom soups were prepared from fresh caps and stems of P. j70rida and fresh A. bisporus by a procedure similar to the basic method described by Lopez (1975). All cans were processed for 12 min at 121 ° C in a pilot plant retort (Reid Boiler Works, Bellingham, Wash.). Drying Several batches of unblanched P. florida caps were dried in direct sunlight on perforated screens (average drying time 6 to 8 h). Air temp. was normally 20-25°C and R. H. 20% to 50%. One batch of steam-blanched P. florida caps were sun-dried in a similar manner. Hot air cabinet drying (65°C for 3 h) and freeze drying (24 h, Virtis Co., Gardiner, N.Y.) were also attempted using unblanched caps only.

Sensory Evaluation A panel of eight individuals was chosen from sixteen members of the Department of Food Science, University of Alberta, who had participated in seven preliminary screening sessions. The final selection was based on the panelists' ability to differentiate mushroom flavors and intensity as well as textural differen-

Oddson and Jelen/37

ces (Oddson, 1979). With one exception, the sensory procedures utilized hedonic scale ratings of the appropriate quality parameters (flavor, texture, etc.) on a 9-point scale (Larmond, 1970). Where appearance characteristics were not to be measured, samples were presented under red light. All cut mushrooms were cooked in butter for 3 min before serving; dried products were first rehydrated for 5 min. Canned cream soups and sauce products were rewarmed to approximately 60° C. A standard triangle difference test (Larmond, 1970) was used to compare unblanched sun-dried mushrooms, stored for 10 mo at 25°C, with two freshly prepared sun-dried controls. All samples were rehydrated in water for 5 min before cooking in butter and serving. Computer assisted analyses of the sensory parameters involved the use of the Analysis of Variance (ANOV A) and the Duncan's test from the University of Alberta computer program library.

Results and Discussion Enzymatic Browning Figure I shows that, while the initial lightness readings of the untreated controls of P. f10rida and A. bisporus were very similar, a marked difference in the PPO activity existed between the two mushrooms. As the concentration of 4-methyl catechol increased, the degree of tissue darkening was much lower for P. f10rida than for A. bisporus in the PPO activity tests. Lightness values of 19.4 and 46.9 were obtained for A. bisporus and P. florida, respectively, with the most concentrated test solution. These results suggested

that the activity ofPPO in P. f10rida is substantially lower than that found in A. bisporus. A distinct difference in the levels of phenolic substrates also appeared to exist between the two mushrooms (Figure 2). After application of the 0.1 % PPO solution, the total decrease in lightness for A. bisporus was 24.2 lightness units while that observed for P. f10rida was only 8.8 units. It therefore seems that P. f10rida has both a lower enzyme activity and lower concentration of phenolic su bstrates than A. bisporus. Enzymatic browning in A. bisporus is a well-documented problem. The observation that P. florida is subject to much lower levels of browning could be an interesting and indeed important result for the commercial processing of this mushroom.

Freezing Experiments Samples of P. f10rida blanched prior to freezing were inferior to freshly frozen unblanched mushrooms (Table I). Unfortunately, after 4 mo of frozen storage, a pronounced deleterious flavor change had developed in the unblanched materials.

75 lJ A.

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35

Relative activity of phenolic substrates in P.florida and A. bisporus as measured by the rate of browning.

30 Table I. Effect of blanching on quality parameters of P. florida freshly frozen and stored 4 mo at -30°C.

25 20 15

0.01

0.1

1.0

10

4-Methyl Catechol Concentration (%)

Fig. I.

Relative activity of the enzyme polyphenoloxidase in P. florida and A. bisporus as measured by the extent of browning.

38/0ddson and Jelen

Flavor' Color 2 Drip loss3

Freshly frozen

Stored frozen

Blanched U nblanched

Blanched U nblanched

5.7 38.5 N.D.4

7.2 54.4 N.D.

7.0 34.9 35.0

1.5 44.2 27.0

'Score code: I = very poor, 9 = very good. 2Lightness (L value) after th·awing. 3Drip loss % by weight. 4Not determined.

J. Inst. Can. Sci. Technol. Aliment. Vo!. 14. No. J. Janvier 1981

Figure 3 illustrates the correlation between mean taste panel scores and peroxide numbers for the un blanched mushrooms during frozen storage. The trend was clear; as peroxide number increased, taste panel scores decreased. The off-flavor development therefore appeared to be related to lipid rancidity. TLC separation of the lipid extracts revealed that phospholipids were the major mushroom lipids. The remainder consisted of neutral Iipids and sterols. A lower proportion of unsaturated fatty acids (i.e., 18: I and 18:2) in the phospholipid was demonstrated by G LC (Figure 4). It was evident from these results that oxidation had occurred after only 2 mo frozen storage. A notable observation was the appearance of free fatty acids (Figure 5), most likely due to the breakdown of the phospholipid. The total phospholipid content decreased from 70% in the fresh mushroom extract to about 20% after 12 mo frozen storage. While other causes of off-flavor development cannot be discounted, these results have demonstrated a change in the composition of the lipid which appears to lead to off-flavors. Lee and Mattick (1961) reported similar results for peas and presented evidence which tends to support the above conclusion.

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A summary of results from the four sensory evaluation trials with fresh P..florida and brine canned products made from them is shown in Table 2. A significant difference in texture (p::::':O.O I) was found between fresh stems and fresh caps, the stems being described as "woody," "rubbery" and "chewy" by several panelists. Differences between the stems and caps in both texture and overall scores were also significant after brine canning ( p ::::':0.0 I). This time, however, stems were rated better than caps. A comparison of brine canned and freshly cooked caps showed that the fresh caps were significantly better in terms of all sensory attributes evaluated. Comments such as "mushy,"

o Mean Taste Panel Score

Fresh

600

Canning Experiments

7

D

Fig. 4.

Comparison of saturated and unsaturated fatty acid methyl esters (FAM Es) from the phospholipid fraction of fresh P. florida and frozen unblanched P. f10rida stored for 2 mo at -30 0 C.

70

[] Peroxide Number 1 = phospholipid 2 = monoglyceride 3 = sterols 4 = free fatty acids 5 = diglyceride 6 = triglyceride

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Fig. 3.

Relationship of peroxide numbers and mean taste panel navor scores for unblanched frozen P. f1orida.

Can. Inst. Food Sci. Technol. J. Vol. 14, No. I. January 1981

Samples

Lipid extract from fresh tissue (Sample F). Lipid extract from unblanched tissue stored 12 mo at-30°C (Sample S).

Fig. 5.

Effects of frozen storage on P. f10rida lipids as demonstrated by the thin layer chromatography.

Oddson and Jelenj 39

Table 2. Mean scores' from the sensory analyses of fresh and brine canned P. florida prod ucts. Trial

2 3 4

Product

Flavor Texture

Fresh caps Fresh stems Canned caps Canned stems Fresh caps Canned caps Fresh stems Canned stems

4.6 4.8 3.6 4.6 5.1 3.4"" 3.8 4.6

Appearance

Overall

4.4 4.6 4.5 4.8 4.5 4.0" 4.2 4.8

4.8 4.5 3.8 5.0"" 4.8 4.0"" 4.0 5.0""

5.4 3.8"" 4.4 5.4"" 4.8 4.1"" 4.6 5.6"

'Score code: I = very poor, 6 = excellent. "Significantly different at 5% level. ""Significantly different at I% level.

"sloppy" and "sour" indicated that poor flavor and appearance of the brine canned caps were major factors in the low ratings. The comparison of fresh and canned stems showed the reverse trend, with canned stems rated significantly better than fresh stems in terms of texture (p:S:O.05) and overall scores (p:S:O.O I). A 6-point hedonic scale was used in these four trials; thus the absolute values are not directly comparable to the subsequent results. Because the brine canned caps were rated so poorly, it was decided that other canning packs should also be examined. Table 3 shows that the butter or cream sauces improved the acceptability of the canned caps. Flavor and overall quality of the brine canned products were rated significantly lower (p:::::O.O I), while appearance of the cream sauce product was rated significantly higher (p:::::O.05). As a final test on suitability of the Oyster Mushroom for canning, cream of mushroom soups made from P.j7orida and A. bisporus were compared in one experiment. A summary of the results from this test is shown in Table 4. Despite the flavor differences between A. bisporus and P. j7orida, statistical significance was not established for the different mean values obtained. Based on this trial and other cursory obserTable 3. Mean scores' from the sensory evaluation of P. florida caps canned in brine, butter sauce and cream sauce. Attributes Brine Butter sauce Cream sauce

Flavor

Texture

Appearance

Overall

4.0"" 7.4 7.0

6.6 6.6 6.8

5.0 5.8 7.2"

4.6"" 7.0 7.2

'Score code: 1 = very poor, 9 = very good. "Significantly different at 5% level. ""Significantly different at I% level. Table 4. Mean scores' from the sensory evaluation of soups made from P. florida caps, P. florida stems and A. bisporus. Attributes

Pleuro/us florida caps Pleuro/us /7orida stems Agaricus bisporus 'Score code: 1 =very poor, 9

40/0ddson and Jelen

Flavor

Consistency

5.8 7.2 7.4

6.6 7.1 6.4

=very good.

vations, it appears that either of the mushrooms is equally acceptable in soup form. The above results suggest that development of certain canned products from P.florida may be successful. In particular, brine canning could be an excellent means for utilization of the stems. It also appears that this mushroom could be used in some processed products normally prepared with A. bisporus.

Drying Experiments Sun or cabinet drying methods for unblanched mushrooms were equally successful. The dried products contained from 5 to 7% moisture on the average and were a dark to light tan in color. Mushrooms blanched before drying, on the other hand, were very dark in color and had a semi-translucent appearance. The removal of moisture appeared to be much more difficult due to the compaction effect of blanching, and rehydration characteristics were poor. The freeze-dried mushrooms (moisture content 23%) were white in color but their fragile nature made handling difficult; upon rehydration they became excessively limp. Data from the sensory evaluation of unblanched, sun-dried and freeze-dried P. florida are given in Table 5. The sun-dried mushrooms were significantly less tender (p:S:O.O I) but no trend as to texture preference was evident. Flavor was not affected by either drying process. After storage for 10 mo, the unblanched sun-dried mushrooms were not substantially different from freshly dried material. In a standard triangle test, only five out of eight panelists correctly identified the old sample(a minimum of six would have been required to establish statistical significance at the 5% level). It appears that sun drying may be a suitable and effective procedure for drying un blanched P. florida caps.

Conclusion Of the three processing techniques examined freezing, canning and drying - both drying and canning showed a good potential for further industrial product development with P. j70rida mushrooms. Freezing preservation of P. j70rida appears difficult; much further work is still needed especially in the area of mechanisms for the retardation of off-flavor production in the unblanched product, including the use of permitted food additives in place of blanching.

Table 5. Mean scores from the sensory evaluation of sun-dried and freeze-dried P. florida as compared to a fresh reference sample. Sample Sun-dried Freeze-dried

Flavor

Tenderness

4.5 4.8

2.3"" 4.5

""Significantly different from reference sample at 1% level. The numerical-verbal scales for flavor or tenderness comparisons to the reference were from I - extremely inferior flavor or less tender, through 5 - equal, to 9 - extremely superior flavor or more tender. The reference was arbitrarily assigned a score of 5. J. Inst. Can. 5ci. Technol. Aliment. Vol. 14. No. J. Janvier 1981

However, the apparent industrial processing potential demonstrated in this work, together with the ease of cultivation, economy of production and low levels of enzymatic browning, could make the Oyster Mushroom an attractive alternative for A. bisporus in some processed products.

Acknowledgements The co-operation of Or. Jan Weijer, Department of Genetics, University of Alberta, in providing the cultures of P. florida and arranging for the use of the growth chamber facilities was very much appreciated. We are grateful to Mr. Ron Currie, Department of Food Science, University of Alberta, for his expert assistance with the lipid extraction and identification procedures.

References Agriculture Canada. 1975. Detailed Food Expenditures. Publication No. 74/5. Economics Branch, Agriculture Canada, Ottawa. ON. Agriculture Canada. 1976. Handbook of Food Expenditures. Prices and Consumption. Publication No. 75/6. Economics Branch, Agriculture Canada, Ottawa, ON. Andreotti, R., Tomasicchio, M. and Castelvetri, F. 1975.11 PleuroIUS os/rea/us, nuovo fungo di coltivazione: Caratteristiche e idoneita all preparazione di conserve. Industria Conserve 50( 1):29. Block, S.S., Tsao, G. and Han, L. 1959. Experiments in the cultivation of Pleuro/us os/rea/us. Mush. Sci. 4:309. Bowyer, D.E. and King, J.P. 1977. Methods for the rapid separation and estimation of the major lipids of arteries and other tissues by thin-layer chromatography on small plates followed by chemical assays. J. Chromatography 143:472. Burns, J.R. and Curry, F.G. 1971. The mushroom industry of Canada. Can. Farm Economics 5(6):28. Cailleux, R., Diop, A. and Macaya-Lizano, A. W. 1976. PleurolUs os/rea/us et formes affines: Comportement cultural, influence des sources carbonees et azotees sur le developement mycelien et la fructification. Mush. Sci. 9(1 ):596.

Can. Inst. Food Sci. Technol. J. YoL 14. No. I. January 1981

Freeman, c.P. and West, D. 1966. Complete separation of lipid classes on a single thin-layer plate. J. Lipid Res. 7:324. Ginterova, A. 1973. Nitrogen fixation by higher fungi. Biologia (Bratislava) 28:199. Gormley, T. R. and O'R iordain, F. 1976. Quality evaluation offresh and processed Oyster Mushrooms (Pleuro/us os/realUs). Lebensm.-Wiss. u. Technol. 9(2):75. Kalberer, P. 1976. The cultivation of Pleuro/us os/rea/us: Experiments to elucidate the influence of different culture conditions on crop yield. Mush. Sci. 9(1 ):653. Larmond, E. 1970. Methods for Sensory Evaluation of Food. Publication No. 1284. Agriculture Canada, Ottawa, ON. Lee, F.A. and Wagenknecht, A.C. 1951. On the development of off-flavor during the storage of frozen peas. Food Res. 16:239. Lee, F.A. and Mattick, L.R. 1961. Fatty acids of the lipids of vegetables. I. Peas. J. Food. Sci. 26:273. Lopez, A. 1975. A Complete Course in Canning, 10th Edition. The Canning Trade, Baltimore, MD. Macaya-Lizano, A. W. 1975. Pleuro/us os/rea/us (Jacq. Ex Fr.) Quel., formes et especes affines: Comportement cultural et systematique. Revue de Mycologie, Tome XXXIX:3. Oddson, L. 1979. An evaluation of the Oyster Mushroom (PleuroIUS florida) for food processing potential. M.Sc. Thesis. Department of Food Science, University of Alberta, Edmonton, AB. Stanek, M. and Rysava, J. 1971. Application of thermophillic microorganisms in the fermentation of the nutrient substrate for the cultivation of Pleuro/us os/rea/us. Mykologicky Sbornik (CSSR) 8:59. Volz, P.A. 1972. Nutritional studies on species and mutants of Lepis/a, Cantharellus, Pleuro/us and Volvariella. Mycopathologia et Mycologia applicata 48(2-3): 175. Weijer, J. 1974. A reconnaissance of the technology of protein enrichment of straw by PleurolUs os/rea/us. Report compiled by GlM Foundations Ltd. (Edmonton, AB) for Alberta Agriculture. Zadrazil, F. and Schneidereit, M. 1972. Die grundlagen fur die inkulturnahme einer bisher nicht kultivierten PleurolUs-Art. Der Champignon 135:25. Zadrazil, F. 1976. The ecology and industrial production of PleuroIUS os/rea/us, Pleuro/us florida. Pleura/us cornucopiae and Pleuro/us eryngii. Mush. Sci. 9( 1):621. Accepted May 30, 1980

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