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Changes in chemical composition and sensory qualities of peanut milk fermented with lactic acid bacteria
International Journal of Food Microbiology, 13 (1991) 273-284
273
© 1991 Elsevier Science Publishers B.V. 0168-1605/91/$03.50 F O O D 00418
Changes in chemical composition and sensory qualities of peanut milk fermented with lactic acid bacteria Chan Lee and Larry R. Beuchat Department of Food Science and Technology, Unicersity of Georgia Agricultural Experiment Station, Griffin, GA, U.S.A. (Received 16 January 1991; accepted 10 April 1991)
The effects of fermentation of aqueous extracts of peanuts (peanut milk) with Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus, separately and in combination, on selected chemical and sensory qualities were investigated. Changes in pH, titratable acidity and viable cell populations indicated that there was a synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salicarius ssp. thermophilus during fermentation. Analysis of headspace volatiles revealed that hexanal, which is one of the c o m p o u n d s responsible for undesirable g r e e n / b e a n y flavor in peanut milk, completely disappeared as a result of fermentation. S. saliL'arius ssp. thermophilus was more effective than L. delbrueckii ssp. bulgaricus in reducing the hexanal content. The acetaldehyde content of peanut milk increased during fermentation. C h a n g e s in concentrations of these volatile c o m p o u n d s were correlated with sensory evaluation scores which showed that a significant (P_< 0.05) decrease in g r e e n / b e a n y flavor and a significant increase in creamy flavor occurred as a result of fermentation. Key words: Peanut; Lactic acid fermentation; Fermentation
Introduction There have been several studies to determine the suitability of soybean milk as a substrate for lactic acid bacterial fermentation. Some of these studies have been directed toward identifying bacterial strains capable of producing high amounts of acid (Angles and Marth, 1971a,b,c,d; Mital et al., 1974; Stern et al., 1977). The development of yogurt-like products with acceptable sensory qualities (Mital et al., 1973; Wang et al., 1974; Mital and Steinkraus, 1975, 1976, 1979; Kanda et al., 1976; Pinthong et al., 1980a,b,c) and shelf-life (Kanda et al., 1976) has been reported. Some strains of Lactobacillus acidophilus (Kanda et al., 1976) have been demonstrated to utilize stachyose and raffinose in soybean milk which are responsible, in part, for flatulence in humans. Correspondence address." L.R. Beuchat, D e p a r t m e n t of Food Science and Technology, University of Georgia Agricultural Experiment Station, Griffin, G A 30223-1797, U.S.A.
274 The g r e e n / b e a n y flavor of soybean milk is greatly reduced after fermentation with lactic acid bacteria (Wang et al., 1974). Kanda et al. (1976) reported that fermentation of soybean milk with L. acidophilus followed by the addition of lemon flavor yielded an acceptable product which could be stored at 5°C for up to 19 days without significant change. Pinthong et al. (1980b) reported that the g r e e n / b e a n y flavor of soybean milk and hexanal content in headspace above the milk were reduced after fermentation. This resulted in a general improvement of sensory qualities. Since bacteria behave differently on different substrates, growth patterns of lactic acid bacteria in peanut milk may not be the same as in soybean milk. Beuchat and Nail (1978) reported that a custard-like product resulted from fermentation of peanut milk with Lactobacillus delbrueckii ssp. bulgaricus and L. acidophilus. Schaffner and Beuchat (1986) concluded that L. delbrueckii ssp. bulgaricus and Streptococcus salicarius ssp. thermophilus may exhibit a synergistic relationship during fermentation of peanut milk. These observations give promise to the prospect of making a yogurt-type product for direct consumption. An objective of the investigation reported here was to determine the effects of fermentation of peanut milk with lactic bacterial starter cultures commercially used in the dairy industry on chemical, microbiological and sensory qualities. A second objective was to determine if a correlation existed between headspace gas volatile compound content and sensory qualities as determined by trained sensory evaluation panelists.
Materials and Methods
Seed and source Florunner cultivar peanuts (Arachis hypogaea L.) were purchased from the University of Georgia College of Agriculture, Southwest Branch Experiment Station, Plains, GA. Upon receipt, seeds were stored at 7°C and 60% relative humidity until used. Preparation of peanut milk A flow diagram for preparing peanut milk is shown in Fig. 1. Peanut kernels were dry blanched using a roller blancher (Ashton Food Machinery Co., Newark, N J) and then submerged in tap water (2: 1, w a t e r : p e a n u t ) containing 0.5% (w:v) N a H C O 3 and soaked for 18 h at 21-23°C. The soak water was then drained, and seeds were washed with fresh tap water, combined with water (2 : 1, water : peanut) and cooked at 100°C for 10 rain in a steam-jacketted kettle. After draining and washing with tap water, peanuts were mixed with distilled water ( 5 : 1 , water:peanut), coarsely ground using a Morehouse Mill (Electra Motors, Anaheim, CA, Model M-MS-3) and filtered through unbleached muslin cloth. The filtered peanut milk was passed through a laboratory homogenizer (APV Gaulin, Inc., Everett, MA, Model 15MR-8TBA) at a pressure of 4000 psi. Peanut milk was then sterilized at 121°C for 10 min and stored at 5°C until used for fermentation.
275 Peanuts
I I Soak
Dry blanch
(0.5% NaHC03, 18 hr) Discard
~
liquid
I
Drain I
I t
I
Wash in water, add water (2:1, water:peanut)
Cook
(lOOOC, 10 min) I q Grind Discard solids
i
Drain, wash, add water (5:1, water:peanut)
I
Filter I Homogenize
(4,000 psi)
I
Sterilize (121°C, tO min)
1
Peanut milk
Fig. 1. Flow diagram for preparation of peanut milk.
In tests designed to evaluate the effect of glucose on fermentation, dry-heatpasteurized (120°C, 16 h) glucose was added to sterilized peanut milk to give a concentration of 2% (w/v).
Bacterial strains and culture methods Mixed cultures of L. delbrueckii ssp. bulgaricus and S. saliuarius ssp. thermophilus were provided by Miles, Inc. (Culture 9085; Biotechnology Products Division, Madison, WI) and Chr. Hansen's Laboratory (Culture 14128; Milwaukee, WI). Cultures which had been activated in A P T (All Purpose plus Tween) broth (Difco, Detroit, MI) at 37°C were serially diluted and surface-plated on A P T agar and incubated at 37°C for 48 h to facilitate isolation of colonies of L. delbrueckii ssp. bulgaricus and S. salicarius ssp. thermophilus. For the purpose of discussion in this report, respective culture numbers have been assigned as strain numbers (9085 and 1 4 1 2 8 ) t o L. delbrueckii ssp. bulgaricus and S. saliuarius ssp. thermophilus isolates from each mixed culture. Colonies of each bacterium were picked from A P T agar and cultured in A P T broth. Pure cultures were then adapted to grow in a medium consisting of sterile A P T broth and peanut milk (3 : 2, v : v) by transferring three times at 48-h intervals. Stock cultures were held in the A P T : p e a n u t milk (1:1) at 5°C. Activation at 37015'. consisted of four successive transfers (1%
276
inoculum) in A P T : peanut milk (1:1, v :v) at 48-h intervals and one transfer (5% inoculum) after 24 h of incubation. These cultures served as inocula (2% for single strains, 1% each for two mixed strains) which were added to 200-ml quantities of sterile peanut milk containing 0 and 2% glucose and adjusted to 43°C.
Fermentation procedure Sterile peanut milk (200 ml) in 227-g commercial yogurt containers adjusted to 43°C was inoculated with 4 ml of a 24-h culture of L. delbrueckii ssp. bulgaricus or S. saliL~arius ssp. thermophilus (or 2 ml of each bacterium) grown at 37°C in the 1 : 1 mixture of A P T broth and peanut milk. Fermentation was carried out at 43°C. Analyses were performed after various periods of fermentation during a 24-h period.
Chemical analyses The p H was determined using an Accumet p H meter (Model 805MP; Fisher Scientific Co., Pittsburgh, PA). The titratable acidity was measured by titrating fermented milk with 0.1 N N a O H using 1% phenolphthalein as an indicator. Titratable acidity was calculated and expressed as percent lactic acid. Quantitation of hexanal and acetaldehyde in the headspace gas in sealed containers of unfermented and fermented peanut milk was done using a modification of a method described by Young and Hovis (1990). Peanut milk (1.5 ml) was deposited in a 5-ml vial and tightly sealed with a Teflon-lined silicone disc using a screw cap. After heating the vial at 120°C in a block heater for 15 min, headspace gas (1 ml) was withdrawn using a syringe and injected into a gas chromatograph (Hewlett-Packard, Avondale, PA, model 5890A) fitted with a flame ionization detector and a 1 m × 2 mm (i.d.) glass column packed with 80-100 mesh Propak P (Waters, Millipore Corp., Milford, MA). The carrier gas (nitrogen) flow rate was adjusted to approximately 40 m l / m i n so that acetaldehyde eluted at 0.95 _+ 0.03 min and hexanal eluted at 5.00 _+ 0.03 rain. The initial column temperature (120°C) was p r o g r a m m e d to increase at a rate of 20°C/min to 200°C where it was held for 3 min. The injector and detector t e m p e r a t u r e was 220°C. Peaks were integrated using a Hewlett-Packard HP3390A integrator. Hexanal and acetaldehyde standards were quantified using diffusion oil (Dow-Corning, Midland, MI) as a carrier.
Microbiological analyses Populations of L. delbrueckii ssp. bulgaricus and S. saliearius ssp. thermophilus were determined by serially diluting fermented peanut milk in 0.1 M potassium phosphate buffer (pH 7.0) and plating on L-S differential medium (Oxoid Inc., Columbia, MD). Colonies were counted after incubating plates for 48 h at 43°C.
Sensory et)aluation Color was measured using a G a r d n e r Colorimeter (Pacific Scientific Co., model X L 800, G a r d n e r Laboratory Division, Bethesda, MD) equipped with an X L 845 circumferential sensor. A white plate (XL-845 125D; L = 94.11, a = - 0 . 9 9 and b = 0.89) was used as a standard.
277 TABLE I Standards and intensity ratings used for descriptive analysis of flavor characteristics of unfermented and fermented peanut milk. Term
Reference standard
Soda Cooked apple Orange complex
Saltines (Nabisco) Apple sauce (Mott) Frozen orange concentrate (Minute Maid, reconstituted) Concord grape juice (Welches) Big Red gum (Wrigley)
Grape Cinnamon
Intensity rating ~ 2.5 5.0 7.5 10.0 12.5
~LRelative position on a 15-cm unstructured scale Descriptive sensory analysis of unfermented and fermented peanut milk was conducted using ten trained panelists. Sensory attributes were assigned scores for intensity using a 15-cm unstructured scale. Standards representing five flavor intensities (Table I) were provided to each panelist for judgment of flavor terms (Meilgaard et al., 1987) characteristic of raw peanuts. Flavor characteristics measured were creamy, g r e e n / b e a n y , cooked peanut, sulfur, sour, sweet, bitter and astringent. Scores for mouthfeei and color were also determined using the 15-cm scale. Anchors for mouthfeel characteristics were chalky (absent/present), viscosity (thin/thick), gummy ( a b s e n t / s t r o n g ) and smoothness ( s m o o t h / l u m p y ) . Anchor points for color (whiteness) were light/dark.
Statistical analysis Data represent means of three replicate trials and were analysed using a Statistical Analysis System (SAS, 1985) program package. Analysis of variance ( A N O V A ) was done initially to determine the main and interaction effects. Significant differences among treatment means were determined by Duncan's Multiple Range test.
Results and Discussion
Chemical analyses The pH of glucose-supplemented peanut milk fermented with S. salivarius ssp. thermophilus and a mixture of L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus decreased more rapidly compared to the pH of milk inoculated only with L. delbrueckii ssp. bulgaricus (Fig. 2A). A pH of 4.5 was obtained within 8 - 9 h in peanut milk fermented with mixed cultures and within 12-13 h in peanut milk fermented with S. salivarius ssp. thermophilus only, suggesting a synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus during fermentation. The titratable acidity of fermented glucose-supplemented peanut milk prepared using mixed cultures (Fig. 2B) did not reach that of yogurt made from cow's milk,
278
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oAZo_o_o
2
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12 16 20 24 0 4 --;me (hr)
8 12 16 20 24
Fig. 2. Changes in pH (A), titratable acidity (B), hexanal (C) and acetaldehyde (D) content in headspace gas and viable cell populations (E) of peanut milk fermented with single and mixed cultures of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus saliuarius ssp. thermophilus at 43°C. Key: ©, L. delbrueckii ssp. bulgaricus 14128; o , S. salicarius ssp. thermophilus 14128; A, mixed culture of L. delbrueckii ssp. 14128 and S. salit,arius ssp. thermophilus 14128; o, L. delbrueekii ssp. bulgaricus 9085; II, S. saliuarius ssp. thermophilus 9085; A, mixed culture of L. delbrueckii ssp. bulgaricus 9085 and S. saliuarius ssp. therrnophilus 9085.
which is about 1.0% at pH 4.4-4.5. Peanut milk fermented with the mixed cultures of L. delbrueckii ssp. bulgaricus and S. saliuarius ssp. thermophilus had higher titratable acidity values compared to peanut milk fermented with single cultures of L. delbrueckii ssp. bulgaricus or S. saliuarius ssp. thermophilus. The highest acidity was obtained (0.39%) in peanut milk fermented for 24 h with mixed cultures. Strains of combined cultures produced similar amounts of acid throughout fermentation. Changes in flavor components as determined by the analysis of headspace gas
279 volatile compounds indicate that the concentration of hexanal, which is one of the compounds responsible for an undesirable g r e e n / b e a n y flavor associated with legume seed products, steadily declined during fermentation of glucose-supplemented peanut milk (Fig. 2C). Hexanal essentially disappeared from headspace gas of peanut milk fermented with S. salivarius ssp. thermophilus or mixtures of L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus within 9 h. Essentially no difference was noted in the rate of decrease of hexanal content in peanut milk inoculated with S. salivarius ssp. thermophilus and mixed cultures. This suggests that S. salivarius ssp. thermophilus is mainly responsible for the disappearance of hexanal in glucose-supplemented peanut milk. The rate of decline in hexanal concentration was slowest in peanut milk inoculated with L. delbrueckii ssp. bulgaricus, strain 9085 being more effective than strain 14128. The effect of fermentation on hexanal was of great interest in this experiment, since Pinthong et al. (1980b) reported that the concentration of hexanal was substantially reduced during fermentation of soybean milk with lactic acid bacterial cultures. An overall reduction of hexanal in headspace gas can be associated with improvement of flavor and aroma of the fermented product and could be used as an indicator to evaluate changes in sensory qualities of peanut milk during fermentation. The concentration of acetaldehyde in glucose-supplemented peanut milk increased as a result of fermentation (Fig. 2D). The concentration of acetaldehyde in peanut milk at the end of the 24-h fermentation period ranged from 12 to 21 ppm. The acetaldehyde content of peanut milk inoculated with S. salivarius ssp. thermophilus and mixed cultures increased after 3 h of fermentation. The highest acetaldehyde concentration (21 ppm) was detected in peanut milk fermented for 24 h with L. delbrueckii ssp. bulgaricus 9085. The amount of acetaldehyde produced in peanut milk was lower than in good-quality yogurt produced from cow's milk, which ranges from 23 to 4l ppm (Rasic and Kurmann, 1978). In yogurt, glucose, which is a degradation product of lactose, is an important metabolic precursor for acetaldehyde synthesis through pyruvate and acetyl-CoA intermediates of glycolysis (Lees and Jago, 1976a). Amino acids such as threonine and methionine may also be direct precursors of acetaldehyde. Lactic acid bacteria convert threonine into acetaldehyde and glycine by threonine aldolase, whereas methionine is metabolically converted to threonine (Lees and Jago, 1976b, 1978a,b; Shankar, 1977). During fermentation of peanut milk, glucose produced by hydrolysis of sucrose by S. salivarius ssp. thermophilus as well as that added to peanut milk would be the primary precursor of acetaldehyde. Lactic acid contributes to the desirable refreshing taste of yogurt, while the by-products of lactic acid fermentation, e.g., carbonyi compounds, volatile fatty acids and alcohols, contribute to its pleasant, characteristic aroma. Yogurt aroma consists of a mixture of volatile compounds which originate from thermal degradation of milk constituents and the fermentative process. Some of these compounds play a principal role in the development of aroma and flavor of yogurt, while others only contribute to the balance of aroma compounds (Rasic and Kurmann, 1978). Carbonyl compounds consisted of acetaldehyde, diacetyl, acetoin, acetone and butanone-2. Of these, acetaldehyde is recognized as the principal flavor
280 component of yogurt (Pette and Lolkema, 1950). Other carbonyl compounds were not confirmed to be present in fermented peanut milk; however, gas chromatographic analysis revealed the presence of several volatiles in the headspace gas of peanut milk fermented with all lactic acid bacteria strains tested.
Microbiological analyses Growth of S. saliearius ssp. thermophilus in peanut milk was more rapid than growth of L. delbrueckii ssp. bulgaricus (Fig. 2E). The initial population (approx. 1.8 × 105) in peanut milk inoculated with mixed cultures of L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus was less than that in milk inoculated with S. salivarius ssp. thermophilus alone, which is better able to adapt to the peanut milk medium. The synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus, resulting in more rapid reduction of pH and increase in titratable acidity, is correlated with changes in viable populations. Similar observations from experiments using different strains of the same species were reported by Schaffner and Beuchat (1986). Changes in sensory characteristics Based on observations of the synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus during fermentation of peanut milk, yogurt-like products were made from peanut milk using both mixed and single cultures. Both 2% glucose-supplemented and unsupplemented peanut milk were used as substrates. Sensory evaluations were done to determine if changes in flavor, color and mouthfeel occurred as a result of fermentation. Analysis revealed that creamy flavor significantly increased (P_<0.05) and g r e e n / b e a n y flavor significantly decreased upon fermentation of peanut milk (Table II). Fermentation was also effective in significantly reducing sulfur, sweet and bitter flavors, and increasing the sour flavor of peanut milk supplemented with 2% glucose. The results from sensory analysis correlated with those obtained from analysis of headspace gas volatiles. The decrease in hexanal (Fig. 2C) and increase in acetaldehyde (Fig. 2D) concentrations in headspace gas after fermentation of glucose-supplemented peanut milk appear to be positively correlated with improved flavor. Hexanal has been associated with g r e e n / b e a n y flavor in soybean milk and acetaldehyde is a major compound in yogurt. Lactic acid production during fermentation (Fig. 2B) resulted in significantly increased intensity scores for sourness, which in turn may also affect scores for other sensory attributes, e.g., the g r e e n / b e a n y flavor thought to be caused, in part, by hexanal. On the other hand, the significant decrease in g r e e n / b e a n y flavor scores may actually be a result of decreased hexanal content as a result of catabolic activities of lactic acid bacteria. Whiteness of peanut milk tended to increase upon fermentation. Mouthfeel of fermented peanut milk differed greatly from that of unfermented milk in terms of viscosity, gumminess and smoothness. All three attributes were significantly enhanced, presumably due to curd formation caused by reduction in pH. The significant increase in gummy mouthfeel without the addition of gelling agents is undoubtedly a reflection of rheological changes resulting from fermentation. In
281 TABLE II Intensity scores for unfermented and fermented peanut milk supplemented with 0 and 2% glucose ~ Sensory attribute
Unfermented 0% 2%
Fermented Culture 14128
Culture 9085
0%
2%
0%
2%
Flavor
Creamy Beany/green Cooked peanut Sulfur Sour Sweet Bitter Astringent Color Whiteness
1.08 b 4.38 a 2.01 a 1.52 a 0.60 b 1.09 bc 2.48 a 2.67 a
1.57 b 3.83 a 2.12 a 1.38 ab 0.52 b 2.37 a 1.72 b 2.30 ab
2.41 a 3.03 b 1.25 b 1.19 abc 5.17 a 0.56 c 1.29 bc 2.16 ab
2.48 a 2.29 b 1.62 ab 0.83 c 4.71 a 1.19 b 0.99 c 1.62 c
2.23 a 2.88 b 1.16 b 1.00 bc 5.07 a 0.85 bc 1.36 bc 2.14 ab
2.39 a 2.71 b 1.64 ab 0.68 c 5.01 a 1.28 b 0.97 c 1.96 bc
1.14 b
1.23 ab
1.35 a
1.70 a
1.60 a
1.35 ab
2.09 a 1.39 b 0.18 b 0.55 b
2.07 a 1.30 b 0.18 b 0.60 b
2.43 4.01 0.92 2.36
2.15 3.58 0.79 2.06
2.43 3.57 0.79 2.28
1.95 a 3.58 a 0.73 a 1.94 a
Mouthfeel
Chalky Viscosity Gummy Smoothness
a a a a
a a a a
a a a a
Mean values in the same row which are not followed by the same letter are significantly different ( P _< 0.05). contrast, chalkiness was unaffected. Peanut milk can exhibit chalkiness, even after homogenization and the addition of stabilizers to formulas for the purpose of i m p r o v i n g e m u l s i o n s t a b i l i t y ( R u b i c o e t al., 1987). C i v i l l e a n d S z c e s n i a k ( 1 9 7 3 ) r e p o r t e d t h a t c h a l k i n e s s o c c u r s w h e n p a r t i c l e s a r e r e l a t i v e l y l a r g e r t h a n t h o s e in t h e s u r r o u n d i n g m e d i u m . N e v e r t h e l e s s , c h a l k i n e s s is n o t c o n s i d e r e d t o b e a s e r i o u s n e g a t i v e c h a r a c t e r i s t i c in f e r m e n t e d p e a n u t m i l k . In summary, chemical and sensory qualities of peanut milk change significantly u p o n f e r m e n t a t i o n w i t h L. delbrueckii ssp. bulgaricus a n d S. saliuarius ssp. thermophilus. M i x e d c u l t u r e s e x h i b i t e d a s y n e r g i s t i c i n t e r a c t i o n d u r i n g f e r m e n t a t i o n . T h e m a j o r c h a n g e s in h e a d s p a c e g a s v o l a t i l e c o m p o u n d s in f e r m e n t e d p e a n u t m i l k i n c l u d e a d e c r e a s e in h e x a n a l c o n t e n t a n d t h e p r o d u c t i o n o f a c o n s i d e r a b l e amount of acetaldehyde. These changes are correlated with results of sensory e v a l u a t i o n , w h i c h s h o w e d a s i g n i f i c a n t d e c r e a s e in g r e e n / b e a n y flavor and a s i g n i f i c a n t i n c r e a s e in c r e a m y f l a v o r u p o n f e r m e n t a t i o n o f p e a n u t m i l k . F e r m e n t e d p e a n u t m i l k r e p r e s e n t s a p r o d u c t w i t h p o t e n t i a l u s e as a n i n g r e d i e n t in a v a r i e t y o f f o o d s , a n d its u s e f o r t h i s p u r p o s e is u n d e r i n v e s t i g a t i o n in o u r l a b o r a t o r y .
Acknowledgements
T h i s s t u d y w a s s u p p o r t e d in p a r t f r o m g r a n t s f r o m t h e U . S . D . A . O f f i c e f o r International Development (cooperative agreement number 58-319R-7-013) and
282 the U.S. Agency for International Development,
Peanut Collaborative Research
Support Program (grant number DAN-4040-G-SS-2065-00). Recommendations do not r e p r e s e n t an official p o s i t i o n or policy of U S D A or U S A I D . W e are grateful to Carlos Bandaras for technical assistance.
References Angles, A.G. and Marth, E.H. (1971a) Growth and activity of lactic acid bacteria in soy milk. !. Growth and acid production. J. Milk Food Technol. 34, 30-36. Angles, A.G. and Marth, E.H. (1971b) Growth and activity of lactic acid bacteria in soy milk. II. Heat treatment of soy milk and culture activity. J. Milk Food Technol. 34, 63-68. Angles, A.G. and Marth, E.H. (1971c) Growth and activity of lactic acid bacteria in soy milk. II1. Lipolytic activity. J. Milk Food Technol. 34, 69-75. Angles, A.G. and Marth, E.H. (1971d) Growth and activity of lactic acid bacteria in soy milk. IV. Proteolytic activity. J. Milk Food Technol. 34, 123-128. Beuchat, L.R. and Nail, B.J. (1978) Fermentation of peanut milk with Lactobacillus bulgaricus and L. acidophilus. J. Food Sei. 43, 1109-1112. Civille, G.V. and Szcesniak, A.S. (1973) Guidelines to training a texture profile panel. J. Text. Stud. 4, 204-223. Kanda, H., Wang, H.L., Hesseltine, C.W. and Warner, K. (1976) Yoghurt production by Lactobacillus fermentation of soybean milk. Process Biochem. 11(5), 23-25. Lees, G.J. and Jago, G.R. (1976a) Acetaldehyde: an intermediate in the formation of ethanol from glucose by lactic acid bacteria. J. Dairy Res. 43, 63-73. Lees, G.J. and Jago, G.R. (1976b) Formation of acetaldehyde from threonine by lactic acid bacteria. J. Dairy Res. 43, 75-83. Lees, G.J. and Jago, G.R. (1978a) Role of acetaldehyde in metabolism: a review. 1. Enzymes catalyzing reactions involving acetaldehyde. J. Dairy Sci. 61, 1205-1215. Lees, G.J. and Jago, G.R. (1978b) Role of acetaldehyde in metabolism: a review. 2. The metabolism of acetaldehyde in cultured dairy products. J. Dairy Sci. 61, 1216-1224. Meilgaard, M., Civille, G.V. and Cart, B.T. (1987) Sensory evaluation techniques. Vol. I1. CRC Press, Boca Raton, FL. Mital, B.K. and Steinkraus, K.H. (1975) Utilization of oligosaccarides by lactic acid bacteria during fermentation of soy milk. J. Food Sci. 40, 114-118. Mital, B.K. and Steinkraus, K.H. (1976) Flavor acceptability of unfermented and lactic fermented soy milks. J. Milk Food Technol. 39, 342-344. Mital, B.K. and Steinkraus, K.H. (1979) Fermentation of soy milk by lactic acid bacteria, A review. J. Food Protect. 42, 895-899. Mital, B.K., Shallenberger, R.S. and Steinkraus, K.H. (1973) a-Galactosidase activity of lactobacilli. Appl. Microbiol. 26, 783-788. Mital, B.K., Steinkraus, K.H. and Naylor, H.B. (1974) Growth of lactic acid bacteria in soy milks. J. Food Sci. 39, 1018-1022. Pette, J.W. and Lolkema, H. (1950) Acid production and aroma formation in yogurt. Neth. Milk Dairy J. 4, 261-273. Pinthong, R., Macrae, R. and Rothwell, J. (1980a) The development of a soya-based yogurt. I. Acid production by lactic acid bacteria. J. Food Technol. 15,647-652. Pinthong, R., Macrae, R. and Rothwell, J. (1980b) The development of a soya-based yogurt. I1. Sensory evaluation and analysis of volatiles. J. Food Technol. 15, 653-659. Pinthong, R., Macrae, R. and Dick, J. (1980c) The development of a soya-based yogurt. IlI. Analysis of oligosaccharides. J. Food Technol. 15, 661 667. Rasic, J. and Kurmann, J.A. (1978) Yoghurt - - Scientific Grounds, Technology, Manufacture and Preparation, Vol. 1, Technical Dairy Publishing House, Copenhagen, 39 pp.
283 Rubico, S.M., Resurreccion, A.V.A., Frank, J.F. and Beuchat, L,R. (1987) Suspension stability, texture and color of high temperature treated peanut beverage. J. Food Sci. 52, 1676-1679. SAS (1985) SAS User's Guide: Statistics. Version 5 ed. SAS Institute Inc., Cary, NC. Schaffner, D.W. and Beuchat, L.R. (1986) Fermentation of aqueous plant seed extracts by lactic acid bacteria. Appl. Environ. Microbiol. 51, 1072-1076. Shankar, P.A. (1977) Amino acid and peptide utilization by Streptococcus thermophilus in relation to yogurt manufacture. J. Appl. Baeteriol. 43, viii. Stern, N.J., Hesseltine, C.W,, Wang, H.L. and Konishi, F. (1977) Lactobacillus acidophilus utilization of sugars and production of a fermented soybean product. J. Inst. Can. Sci. Technol. Aliment. 10, 197-200. Wang, H.L., Kraidej, L. and Hesseltine, C.W. (1974) Lactic acid fermentation of soybean milk. J. Milk Food Technol. 37, 71-73. Young, C.T. and Hovis, A.R. (1990) A method for the rapid analysis of headspace volatiles of raw and roasted peanuts. J. Food Sci. 55, 279-280.
273
© 1991 Elsevier Science Publishers B.V. 0168-1605/91/$03.50 F O O D 00418
Changes in chemical composition and sensory qualities of peanut milk fermented with lactic acid bacteria Chan Lee and Larry R. Beuchat Department of Food Science and Technology, Unicersity of Georgia Agricultural Experiment Station, Griffin, GA, U.S.A. (Received 16 January 1991; accepted 10 April 1991)
The effects of fermentation of aqueous extracts of peanuts (peanut milk) with Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus, separately and in combination, on selected chemical and sensory qualities were investigated. Changes in pH, titratable acidity and viable cell populations indicated that there was a synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salicarius ssp. thermophilus during fermentation. Analysis of headspace volatiles revealed that hexanal, which is one of the c o m p o u n d s responsible for undesirable g r e e n / b e a n y flavor in peanut milk, completely disappeared as a result of fermentation. S. saliL'arius ssp. thermophilus was more effective than L. delbrueckii ssp. bulgaricus in reducing the hexanal content. The acetaldehyde content of peanut milk increased during fermentation. C h a n g e s in concentrations of these volatile c o m p o u n d s were correlated with sensory evaluation scores which showed that a significant (P_< 0.05) decrease in g r e e n / b e a n y flavor and a significant increase in creamy flavor occurred as a result of fermentation. Key words: Peanut; Lactic acid fermentation; Fermentation
Introduction There have been several studies to determine the suitability of soybean milk as a substrate for lactic acid bacterial fermentation. Some of these studies have been directed toward identifying bacterial strains capable of producing high amounts of acid (Angles and Marth, 1971a,b,c,d; Mital et al., 1974; Stern et al., 1977). The development of yogurt-like products with acceptable sensory qualities (Mital et al., 1973; Wang et al., 1974; Mital and Steinkraus, 1975, 1976, 1979; Kanda et al., 1976; Pinthong et al., 1980a,b,c) and shelf-life (Kanda et al., 1976) has been reported. Some strains of Lactobacillus acidophilus (Kanda et al., 1976) have been demonstrated to utilize stachyose and raffinose in soybean milk which are responsible, in part, for flatulence in humans. Correspondence address." L.R. Beuchat, D e p a r t m e n t of Food Science and Technology, University of Georgia Agricultural Experiment Station, Griffin, G A 30223-1797, U.S.A.
274 The g r e e n / b e a n y flavor of soybean milk is greatly reduced after fermentation with lactic acid bacteria (Wang et al., 1974). Kanda et al. (1976) reported that fermentation of soybean milk with L. acidophilus followed by the addition of lemon flavor yielded an acceptable product which could be stored at 5°C for up to 19 days without significant change. Pinthong et al. (1980b) reported that the g r e e n / b e a n y flavor of soybean milk and hexanal content in headspace above the milk were reduced after fermentation. This resulted in a general improvement of sensory qualities. Since bacteria behave differently on different substrates, growth patterns of lactic acid bacteria in peanut milk may not be the same as in soybean milk. Beuchat and Nail (1978) reported that a custard-like product resulted from fermentation of peanut milk with Lactobacillus delbrueckii ssp. bulgaricus and L. acidophilus. Schaffner and Beuchat (1986) concluded that L. delbrueckii ssp. bulgaricus and Streptococcus salicarius ssp. thermophilus may exhibit a synergistic relationship during fermentation of peanut milk. These observations give promise to the prospect of making a yogurt-type product for direct consumption. An objective of the investigation reported here was to determine the effects of fermentation of peanut milk with lactic bacterial starter cultures commercially used in the dairy industry on chemical, microbiological and sensory qualities. A second objective was to determine if a correlation existed between headspace gas volatile compound content and sensory qualities as determined by trained sensory evaluation panelists.
Materials and Methods
Seed and source Florunner cultivar peanuts (Arachis hypogaea L.) were purchased from the University of Georgia College of Agriculture, Southwest Branch Experiment Station, Plains, GA. Upon receipt, seeds were stored at 7°C and 60% relative humidity until used. Preparation of peanut milk A flow diagram for preparing peanut milk is shown in Fig. 1. Peanut kernels were dry blanched using a roller blancher (Ashton Food Machinery Co., Newark, N J) and then submerged in tap water (2: 1, w a t e r : p e a n u t ) containing 0.5% (w:v) N a H C O 3 and soaked for 18 h at 21-23°C. The soak water was then drained, and seeds were washed with fresh tap water, combined with water (2 : 1, water : peanut) and cooked at 100°C for 10 rain in a steam-jacketted kettle. After draining and washing with tap water, peanuts were mixed with distilled water ( 5 : 1 , water:peanut), coarsely ground using a Morehouse Mill (Electra Motors, Anaheim, CA, Model M-MS-3) and filtered through unbleached muslin cloth. The filtered peanut milk was passed through a laboratory homogenizer (APV Gaulin, Inc., Everett, MA, Model 15MR-8TBA) at a pressure of 4000 psi. Peanut milk was then sterilized at 121°C for 10 min and stored at 5°C until used for fermentation.
275 Peanuts
I I Soak
Dry blanch
(0.5% NaHC03, 18 hr) Discard
~
liquid
I
Drain I
I t
I
Wash in water, add water (2:1, water:peanut)
Cook
(lOOOC, 10 min) I q Grind Discard solids
i
Drain, wash, add water (5:1, water:peanut)
I
Filter I Homogenize
(4,000 psi)
I
Sterilize (121°C, tO min)
1
Peanut milk
Fig. 1. Flow diagram for preparation of peanut milk.
In tests designed to evaluate the effect of glucose on fermentation, dry-heatpasteurized (120°C, 16 h) glucose was added to sterilized peanut milk to give a concentration of 2% (w/v).
Bacterial strains and culture methods Mixed cultures of L. delbrueckii ssp. bulgaricus and S. saliuarius ssp. thermophilus were provided by Miles, Inc. (Culture 9085; Biotechnology Products Division, Madison, WI) and Chr. Hansen's Laboratory (Culture 14128; Milwaukee, WI). Cultures which had been activated in A P T (All Purpose plus Tween) broth (Difco, Detroit, MI) at 37°C were serially diluted and surface-plated on A P T agar and incubated at 37°C for 48 h to facilitate isolation of colonies of L. delbrueckii ssp. bulgaricus and S. salicarius ssp. thermophilus. For the purpose of discussion in this report, respective culture numbers have been assigned as strain numbers (9085 and 1 4 1 2 8 ) t o L. delbrueckii ssp. bulgaricus and S. saliuarius ssp. thermophilus isolates from each mixed culture. Colonies of each bacterium were picked from A P T agar and cultured in A P T broth. Pure cultures were then adapted to grow in a medium consisting of sterile A P T broth and peanut milk (3 : 2, v : v) by transferring three times at 48-h intervals. Stock cultures were held in the A P T : p e a n u t milk (1:1) at 5°C. Activation at 37015'. consisted of four successive transfers (1%
276
inoculum) in A P T : peanut milk (1:1, v :v) at 48-h intervals and one transfer (5% inoculum) after 24 h of incubation. These cultures served as inocula (2% for single strains, 1% each for two mixed strains) which were added to 200-ml quantities of sterile peanut milk containing 0 and 2% glucose and adjusted to 43°C.
Fermentation procedure Sterile peanut milk (200 ml) in 227-g commercial yogurt containers adjusted to 43°C was inoculated with 4 ml of a 24-h culture of L. delbrueckii ssp. bulgaricus or S. saliL~arius ssp. thermophilus (or 2 ml of each bacterium) grown at 37°C in the 1 : 1 mixture of A P T broth and peanut milk. Fermentation was carried out at 43°C. Analyses were performed after various periods of fermentation during a 24-h period.
Chemical analyses The p H was determined using an Accumet p H meter (Model 805MP; Fisher Scientific Co., Pittsburgh, PA). The titratable acidity was measured by titrating fermented milk with 0.1 N N a O H using 1% phenolphthalein as an indicator. Titratable acidity was calculated and expressed as percent lactic acid. Quantitation of hexanal and acetaldehyde in the headspace gas in sealed containers of unfermented and fermented peanut milk was done using a modification of a method described by Young and Hovis (1990). Peanut milk (1.5 ml) was deposited in a 5-ml vial and tightly sealed with a Teflon-lined silicone disc using a screw cap. After heating the vial at 120°C in a block heater for 15 min, headspace gas (1 ml) was withdrawn using a syringe and injected into a gas chromatograph (Hewlett-Packard, Avondale, PA, model 5890A) fitted with a flame ionization detector and a 1 m × 2 mm (i.d.) glass column packed with 80-100 mesh Propak P (Waters, Millipore Corp., Milford, MA). The carrier gas (nitrogen) flow rate was adjusted to approximately 40 m l / m i n so that acetaldehyde eluted at 0.95 _+ 0.03 min and hexanal eluted at 5.00 _+ 0.03 rain. The initial column temperature (120°C) was p r o g r a m m e d to increase at a rate of 20°C/min to 200°C where it was held for 3 min. The injector and detector t e m p e r a t u r e was 220°C. Peaks were integrated using a Hewlett-Packard HP3390A integrator. Hexanal and acetaldehyde standards were quantified using diffusion oil (Dow-Corning, Midland, MI) as a carrier.
Microbiological analyses Populations of L. delbrueckii ssp. bulgaricus and S. saliearius ssp. thermophilus were determined by serially diluting fermented peanut milk in 0.1 M potassium phosphate buffer (pH 7.0) and plating on L-S differential medium (Oxoid Inc., Columbia, MD). Colonies were counted after incubating plates for 48 h at 43°C.
Sensory et)aluation Color was measured using a G a r d n e r Colorimeter (Pacific Scientific Co., model X L 800, G a r d n e r Laboratory Division, Bethesda, MD) equipped with an X L 845 circumferential sensor. A white plate (XL-845 125D; L = 94.11, a = - 0 . 9 9 and b = 0.89) was used as a standard.
277 TABLE I Standards and intensity ratings used for descriptive analysis of flavor characteristics of unfermented and fermented peanut milk. Term
Reference standard
Soda Cooked apple Orange complex
Saltines (Nabisco) Apple sauce (Mott) Frozen orange concentrate (Minute Maid, reconstituted) Concord grape juice (Welches) Big Red gum (Wrigley)
Grape Cinnamon
Intensity rating ~ 2.5 5.0 7.5 10.0 12.5
~LRelative position on a 15-cm unstructured scale Descriptive sensory analysis of unfermented and fermented peanut milk was conducted using ten trained panelists. Sensory attributes were assigned scores for intensity using a 15-cm unstructured scale. Standards representing five flavor intensities (Table I) were provided to each panelist for judgment of flavor terms (Meilgaard et al., 1987) characteristic of raw peanuts. Flavor characteristics measured were creamy, g r e e n / b e a n y , cooked peanut, sulfur, sour, sweet, bitter and astringent. Scores for mouthfeei and color were also determined using the 15-cm scale. Anchors for mouthfeel characteristics were chalky (absent/present), viscosity (thin/thick), gummy ( a b s e n t / s t r o n g ) and smoothness ( s m o o t h / l u m p y ) . Anchor points for color (whiteness) were light/dark.
Statistical analysis Data represent means of three replicate trials and were analysed using a Statistical Analysis System (SAS, 1985) program package. Analysis of variance ( A N O V A ) was done initially to determine the main and interaction effects. Significant differences among treatment means were determined by Duncan's Multiple Range test.
Results and Discussion
Chemical analyses The pH of glucose-supplemented peanut milk fermented with S. salivarius ssp. thermophilus and a mixture of L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus decreased more rapidly compared to the pH of milk inoculated only with L. delbrueckii ssp. bulgaricus (Fig. 2A). A pH of 4.5 was obtained within 8 - 9 h in peanut milk fermented with mixed cultures and within 12-13 h in peanut milk fermented with S. salivarius ssp. thermophilus only, suggesting a synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus during fermentation. The titratable acidity of fermented glucose-supplemented peanut milk prepared using mixed cultures (Fig. 2B) did not reach that of yogurt made from cow's milk,
278
~
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,
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12 16 20 24 0 4 --;me (hr)
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Fig. 2. Changes in pH (A), titratable acidity (B), hexanal (C) and acetaldehyde (D) content in headspace gas and viable cell populations (E) of peanut milk fermented with single and mixed cultures of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus saliuarius ssp. thermophilus at 43°C. Key: ©, L. delbrueckii ssp. bulgaricus 14128; o , S. salicarius ssp. thermophilus 14128; A, mixed culture of L. delbrueckii ssp. 14128 and S. salit,arius ssp. thermophilus 14128; o, L. delbrueekii ssp. bulgaricus 9085; II, S. saliuarius ssp. thermophilus 9085; A, mixed culture of L. delbrueckii ssp. bulgaricus 9085 and S. saliuarius ssp. therrnophilus 9085.
which is about 1.0% at pH 4.4-4.5. Peanut milk fermented with the mixed cultures of L. delbrueckii ssp. bulgaricus and S. saliuarius ssp. thermophilus had higher titratable acidity values compared to peanut milk fermented with single cultures of L. delbrueckii ssp. bulgaricus or S. saliuarius ssp. thermophilus. The highest acidity was obtained (0.39%) in peanut milk fermented for 24 h with mixed cultures. Strains of combined cultures produced similar amounts of acid throughout fermentation. Changes in flavor components as determined by the analysis of headspace gas
279 volatile compounds indicate that the concentration of hexanal, which is one of the compounds responsible for an undesirable g r e e n / b e a n y flavor associated with legume seed products, steadily declined during fermentation of glucose-supplemented peanut milk (Fig. 2C). Hexanal essentially disappeared from headspace gas of peanut milk fermented with S. salivarius ssp. thermophilus or mixtures of L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus within 9 h. Essentially no difference was noted in the rate of decrease of hexanal content in peanut milk inoculated with S. salivarius ssp. thermophilus and mixed cultures. This suggests that S. salivarius ssp. thermophilus is mainly responsible for the disappearance of hexanal in glucose-supplemented peanut milk. The rate of decline in hexanal concentration was slowest in peanut milk inoculated with L. delbrueckii ssp. bulgaricus, strain 9085 being more effective than strain 14128. The effect of fermentation on hexanal was of great interest in this experiment, since Pinthong et al. (1980b) reported that the concentration of hexanal was substantially reduced during fermentation of soybean milk with lactic acid bacterial cultures. An overall reduction of hexanal in headspace gas can be associated with improvement of flavor and aroma of the fermented product and could be used as an indicator to evaluate changes in sensory qualities of peanut milk during fermentation. The concentration of acetaldehyde in glucose-supplemented peanut milk increased as a result of fermentation (Fig. 2D). The concentration of acetaldehyde in peanut milk at the end of the 24-h fermentation period ranged from 12 to 21 ppm. The acetaldehyde content of peanut milk inoculated with S. salivarius ssp. thermophilus and mixed cultures increased after 3 h of fermentation. The highest acetaldehyde concentration (21 ppm) was detected in peanut milk fermented for 24 h with L. delbrueckii ssp. bulgaricus 9085. The amount of acetaldehyde produced in peanut milk was lower than in good-quality yogurt produced from cow's milk, which ranges from 23 to 4l ppm (Rasic and Kurmann, 1978). In yogurt, glucose, which is a degradation product of lactose, is an important metabolic precursor for acetaldehyde synthesis through pyruvate and acetyl-CoA intermediates of glycolysis (Lees and Jago, 1976a). Amino acids such as threonine and methionine may also be direct precursors of acetaldehyde. Lactic acid bacteria convert threonine into acetaldehyde and glycine by threonine aldolase, whereas methionine is metabolically converted to threonine (Lees and Jago, 1976b, 1978a,b; Shankar, 1977). During fermentation of peanut milk, glucose produced by hydrolysis of sucrose by S. salivarius ssp. thermophilus as well as that added to peanut milk would be the primary precursor of acetaldehyde. Lactic acid contributes to the desirable refreshing taste of yogurt, while the by-products of lactic acid fermentation, e.g., carbonyi compounds, volatile fatty acids and alcohols, contribute to its pleasant, characteristic aroma. Yogurt aroma consists of a mixture of volatile compounds which originate from thermal degradation of milk constituents and the fermentative process. Some of these compounds play a principal role in the development of aroma and flavor of yogurt, while others only contribute to the balance of aroma compounds (Rasic and Kurmann, 1978). Carbonyl compounds consisted of acetaldehyde, diacetyl, acetoin, acetone and butanone-2. Of these, acetaldehyde is recognized as the principal flavor
280 component of yogurt (Pette and Lolkema, 1950). Other carbonyl compounds were not confirmed to be present in fermented peanut milk; however, gas chromatographic analysis revealed the presence of several volatiles in the headspace gas of peanut milk fermented with all lactic acid bacteria strains tested.
Microbiological analyses Growth of S. saliearius ssp. thermophilus in peanut milk was more rapid than growth of L. delbrueckii ssp. bulgaricus (Fig. 2E). The initial population (approx. 1.8 × 105) in peanut milk inoculated with mixed cultures of L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus was less than that in milk inoculated with S. salivarius ssp. thermophilus alone, which is better able to adapt to the peanut milk medium. The synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus, resulting in more rapid reduction of pH and increase in titratable acidity, is correlated with changes in viable populations. Similar observations from experiments using different strains of the same species were reported by Schaffner and Beuchat (1986). Changes in sensory characteristics Based on observations of the synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus during fermentation of peanut milk, yogurt-like products were made from peanut milk using both mixed and single cultures. Both 2% glucose-supplemented and unsupplemented peanut milk were used as substrates. Sensory evaluations were done to determine if changes in flavor, color and mouthfeel occurred as a result of fermentation. Analysis revealed that creamy flavor significantly increased (P_<0.05) and g r e e n / b e a n y flavor significantly decreased upon fermentation of peanut milk (Table II). Fermentation was also effective in significantly reducing sulfur, sweet and bitter flavors, and increasing the sour flavor of peanut milk supplemented with 2% glucose. The results from sensory analysis correlated with those obtained from analysis of headspace gas volatiles. The decrease in hexanal (Fig. 2C) and increase in acetaldehyde (Fig. 2D) concentrations in headspace gas after fermentation of glucose-supplemented peanut milk appear to be positively correlated with improved flavor. Hexanal has been associated with g r e e n / b e a n y flavor in soybean milk and acetaldehyde is a major compound in yogurt. Lactic acid production during fermentation (Fig. 2B) resulted in significantly increased intensity scores for sourness, which in turn may also affect scores for other sensory attributes, e.g., the g r e e n / b e a n y flavor thought to be caused, in part, by hexanal. On the other hand, the significant decrease in g r e e n / b e a n y flavor scores may actually be a result of decreased hexanal content as a result of catabolic activities of lactic acid bacteria. Whiteness of peanut milk tended to increase upon fermentation. Mouthfeel of fermented peanut milk differed greatly from that of unfermented milk in terms of viscosity, gumminess and smoothness. All three attributes were significantly enhanced, presumably due to curd formation caused by reduction in pH. The significant increase in gummy mouthfeel without the addition of gelling agents is undoubtedly a reflection of rheological changes resulting from fermentation. In
281 TABLE II Intensity scores for unfermented and fermented peanut milk supplemented with 0 and 2% glucose ~ Sensory attribute
Unfermented 0% 2%
Fermented Culture 14128
Culture 9085
0%
2%
0%
2%
Flavor
Creamy Beany/green Cooked peanut Sulfur Sour Sweet Bitter Astringent Color Whiteness
1.08 b 4.38 a 2.01 a 1.52 a 0.60 b 1.09 bc 2.48 a 2.67 a
1.57 b 3.83 a 2.12 a 1.38 ab 0.52 b 2.37 a 1.72 b 2.30 ab
2.41 a 3.03 b 1.25 b 1.19 abc 5.17 a 0.56 c 1.29 bc 2.16 ab
2.48 a 2.29 b 1.62 ab 0.83 c 4.71 a 1.19 b 0.99 c 1.62 c
2.23 a 2.88 b 1.16 b 1.00 bc 5.07 a 0.85 bc 1.36 bc 2.14 ab
2.39 a 2.71 b 1.64 ab 0.68 c 5.01 a 1.28 b 0.97 c 1.96 bc
1.14 b
1.23 ab
1.35 a
1.70 a
1.60 a
1.35 ab
2.09 a 1.39 b 0.18 b 0.55 b
2.07 a 1.30 b 0.18 b 0.60 b
2.43 4.01 0.92 2.36
2.15 3.58 0.79 2.06
2.43 3.57 0.79 2.28
1.95 a 3.58 a 0.73 a 1.94 a
Mouthfeel
Chalky Viscosity Gummy Smoothness
a a a a
a a a a
a a a a
Mean values in the same row which are not followed by the same letter are significantly different ( P _< 0.05). contrast, chalkiness was unaffected. Peanut milk can exhibit chalkiness, even after homogenization and the addition of stabilizers to formulas for the purpose of i m p r o v i n g e m u l s i o n s t a b i l i t y ( R u b i c o e t al., 1987). C i v i l l e a n d S z c e s n i a k ( 1 9 7 3 ) r e p o r t e d t h a t c h a l k i n e s s o c c u r s w h e n p a r t i c l e s a r e r e l a t i v e l y l a r g e r t h a n t h o s e in t h e s u r r o u n d i n g m e d i u m . N e v e r t h e l e s s , c h a l k i n e s s is n o t c o n s i d e r e d t o b e a s e r i o u s n e g a t i v e c h a r a c t e r i s t i c in f e r m e n t e d p e a n u t m i l k . In summary, chemical and sensory qualities of peanut milk change significantly u p o n f e r m e n t a t i o n w i t h L. delbrueckii ssp. bulgaricus a n d S. saliuarius ssp. thermophilus. M i x e d c u l t u r e s e x h i b i t e d a s y n e r g i s t i c i n t e r a c t i o n d u r i n g f e r m e n t a t i o n . T h e m a j o r c h a n g e s in h e a d s p a c e g a s v o l a t i l e c o m p o u n d s in f e r m e n t e d p e a n u t m i l k i n c l u d e a d e c r e a s e in h e x a n a l c o n t e n t a n d t h e p r o d u c t i o n o f a c o n s i d e r a b l e amount of acetaldehyde. These changes are correlated with results of sensory e v a l u a t i o n , w h i c h s h o w e d a s i g n i f i c a n t d e c r e a s e in g r e e n / b e a n y flavor and a s i g n i f i c a n t i n c r e a s e in c r e a m y f l a v o r u p o n f e r m e n t a t i o n o f p e a n u t m i l k . F e r m e n t e d p e a n u t m i l k r e p r e s e n t s a p r o d u c t w i t h p o t e n t i a l u s e as a n i n g r e d i e n t in a v a r i e t y o f f o o d s , a n d its u s e f o r t h i s p u r p o s e is u n d e r i n v e s t i g a t i o n in o u r l a b o r a t o r y .
Acknowledgements
T h i s s t u d y w a s s u p p o r t e d in p a r t f r o m g r a n t s f r o m t h e U . S . D . A . O f f i c e f o r International Development (cooperative agreement number 58-319R-7-013) and
282 the U.S. Agency for International Development,
Peanut Collaborative Research
Support Program (grant number DAN-4040-G-SS-2065-00). Recommendations do not r e p r e s e n t an official p o s i t i o n or policy of U S D A or U S A I D . W e are grateful to Carlos Bandaras for technical assistance.
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