An excessively high Lactobacillus acidophilus inoculation level in yogurt lowers product quality during storage

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Abstract

The effect of manufacturing yogurt with a wide variation in Lactobacillus acidophilus inoculation level while holding the yogurt culture inoculation level constant on the properties of the resulting yogurt was determined to find out if any problems can occur if an excessively high level of L. acidophilus is used in yogurt production. Four batches of plain, set-style yogurt were manufactured with skim milk, nonfat dry milk, yogurt cultures, and with or without L. acidophilus (0, 0.0239, 0.238, or 2.33 g/100 g). After homogenization, pasteurization, and cooling, yogurt mixes were inoculated, poured into containers, incubated to pH 4.5, and cooled. Lactobacilli and L. acidophilus counts, pH, amount of syneresis, color, apparent viscosities, and sensory scores were determined during storage. The yogurt inoculated with 0.238 g/100 g L. acidophilus had the highest L. acidophilus counts from 4 to 7 wk. Yogurts inoculated with 2.33 g/100 g L. acidophilus generally had lower lactobacilli counts, L* values, apparent viscosities, and sensory scores but more syneresis and higher a* and b* values than the remaining yogurts. An excessively high inoculated level of L. acidophilus (2.33 g/100 g) resulted in an inferior quality yogurt.

Introduction

Lactobacillus acidophilus is often added to yogurt because of its probiotic effects. Possible beneficial effects of consuming yogurt containing L. acidophilus include improving lactose digestion in individuals that have this difficulty (Kim & Gilliland, 1983), lowering serum cholesterol levels (Harrison & Peat, 1975), helping to prevent certain types of cancer (Rao, Sanders, Indranie, Simi, & Reddy, 1999), stimulating the immune system (Gill, Rutherfurd, Prasad, & Gopal, 2000), controlling urogenital infections in women (Hilton, Isenberg, Alperstein, France, & Borenstein, 1992), and preventing or controlling intestinal infections (Gilliland & Speck, 1977). For L. acidophilus to maintain a proper balance among other normal inhabitants of the intestinal tract, Speck (1976) reported that 1×108 to 1×109 viable cells of L. acidophilus should be ingested daily.

The survival of L. acidophilus in yogurt has been studied extensively. Nighswonger, Brashears, and Gilliland (1996) reported that some strains of L. acidophilus lost viability during storage, while other strains maintained nearly constant levels in yogurts during storage at 7 °C for 28 d. Dave and Shah (1997b) found that the survival of L. acidophilus in yogurts after 35 d of storage was only approximately 0.1% to 5% compared to after 5 d of storage. Shah, Lankaputhra, Britz, and Kyle (1995) found that three out of five brands of yogurt contained 107 to 108 viable cells of L. acidophilus per gram of yogurt while the remaining two brands contained less than 105 L. acidophilus cells per gram when fresh. Because of the wide range of survival of L. acidophilus in yogurt reported in the literature, the viability of L. acidophilus must be kept sufficiently high to ensure that consumers receive health benefits from L. acidophilus.

Concerns about possible low viability of L. acidophilus in commercial yogurts during storage raise the question whether higher inoculation levels could increase the viability of L. acidophilus. Information on the effect of a wide range of inoculation levels of L. acidophilus (ex. 2.68, 26.8, and 262 fold above the rate recommended by the manufacturer) with a constant yogurt culture inoculation level on survival of lactobacilli and L. acidophilus in yogurt does not appear to be available. If the L. acidophilus and lactobacilli counts are altered by wide variations in the L. acidophilus inoculation level, then any possible effects of these counts on the properties of the yogurts during storage would need to be investigated. The objective was to determine the effect of a wide variation in the L. acidophilus inoculation level while holding the yogurt culture inoculation level constant on the lactobacilli and L. acidophilus counts, pH, amount of syneresis, color (L*, a*, and b* values), apparent viscosities, and sensory (flavor, body/texture, and appearance/color) scores of the resulting set-style yogurts and find out if any production and quality problems can occur if an excessively high level of L. acidophilus is used in yogurt production.

Section snippets

Set-style yogurt manufacture and storage

Plain yogurt mix was formulated to contain 16 g/100 g total solids and manufactured with skim milk (13.25 L) and nonfat dry milk (931 g) in 17 L pails. The mixes were preheated to 60 °C, homogenized at 10.3 MPa first stage, and 3.45 MPa second stage in a Gaulin homogenizer (Manton-Gaulin Manufacturing Company, Inc., Everett, MA, USA), batch pasteurized at 85 °C for 30 min, and cooled to 40 °C. An inoculum of 7.7 mL of the yogurt culture CH-3 (a Redi-Set frozen culture consisting of Streptococcus

Yogurt manufacture

The incubation time needed to reach pH 4.5 increased with increasing L. acidophilus inoculation level. The times needed to reach pH 4.5 were 7 h 50 min, 7 h 50 min, 9 h 15 min, and 10 h 15 min for the control yogurt and the yogurts inoculated with 0.0239, 0.238, and 2.33 g/100 g L. acidophilus, respectively.

Lactobacilli and L. acidophilus counts

The log10 lactobacilli counts are presented in Table 1. There were no significant differences in log10 lactobacilli counts with storage time. However, the L. acidophilus inoculation level was highly

Conclusions

This study examined how high of a level in which L. acidophilus can be inoculated into yogurt while holding the yogurt culture inoculation level constant and still obtain a desirable product throughout storage. There were only very minor differences in properties of the control yogurt and the yogurt inoculated with 0.0239 g/100 g L. acidophilus. Although the yogurt inoculated with 0.238 g/100 g L. acidophilus had slightly lower, but nonsignificant, log10 lactobacilli counts, higher pH, more

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Approved for publication by the Director of the Louisiana Agricultural Experiment Station as manuscript number 06-18-0306.

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