Effect of Pre- and Post-Fermentation Coffee Addition on the Physicochemical, Microbiological and Sensory Properties of Kefir During Refrigerated Storage

Kefir is a self-carbonated fermented milk produced with kefir grains which is valued for its complex symbiotic microbiota and probiotic potential. Flavouring kefir is a recognised strategy to broaden consumer appeal, yet the moment at which a flavouring ingredient is introduced—before or after fermentation—may differentially affect the product. The present study evaluated the effect of the timing of instant coffee addition on the physicochemical, microbiological and sensory properties of kefir during 14 days of refrigerated storage (4 ± 1 °C). Three kefir samples were produced from UHT cow’s milk (3% fat) inoculated with 3% (w/v) kefir grains: a control without coffee (K), kefir with 0.5% (w/v) instant coffee added before fermentation (PreF_CK) and kefir with 0.5% (w/v) instant coffee added after fermentation and maturation (PostF_CK). Samples were analysed on days 1, 7 and 14. The coffee addition significantly increased the total dry matter (K = 10.72%; PreF_CK = 11.04%; PostF_CK = 11.00%; p < 0.01) and slightly modified the titratable acidity and pH (p < 0.05), whereas the ash content (expressed in terms of dry matter) did not differ among samples (p > 0.05). The titratable acidity increased and the pH decreased with storage in all samples (p < 0.05). Crucially, the coffee addition—irrespective of the timing—did not impair the viability of the kefir microbiota; the total mesophilic aerobic bacteria, Lactobacillus spp., mesophilic lactic cocci and yeast counts did not differ significantly among the samples (p > 0.05) and remained high throughout storage (lactic acid bacteria ≥8 log CFU/g; yeast ≥5.7 log CFU/g), exceeding the minimum thresholds of the standards for fermented milk products. Sensory evaluation showed that the control achieved the highest odour and taste scores (p < 0.001), while among the coffee-containing samples, the post-fermentation addition (PostF_CK) was generally scored at least as high as the pre-fermentation addition (PreF_CK); overall acceptability did not differ significantly among the samples (p > 0.05). These findings demonstrate that 0.5% instant coffee can be incorporated into kefir without compromising its characteristic microbiota or storage stability and that post-fermentation addition is a viable route for producing an acceptable coffee-flavoured kefir.