Freeze Drying of Fermented Milk Prepared from Milk and Fruit Juices

유유와 과즙으로 만든 발효유의 동결건조

Fermented milk was prepared from milk or mixture of milk and apple juice/grape juice, and it was freeze dried. pH change and growth of Lactobacillus acidophilus (KCTC 2182) during freeze drying were studied. The effects of freeze drying on sensory evaluation and volatile aroma compounds in freeze dried sample or reconstituted sample were also studied. Freezing and freeze drying did not affect pH of fermented milk. Number of viable cells of L. acidophilus was markedly reduced during freezing or freeze drying. When number of viable cells in original fermented milk was considered as 100%, survival ratio of viable cells after freezing was $64.5{\sim}85.2%$ and that after freeze drying was $10.0{\sim}21.1%$. When sensory properties of original fermented milk prepared from juice-milk (ratio 15:35) were compared with those of freeze dried/reconstituted sample, sensory properties of original sample were better than those of freeze dried/reconstituted sample. Ethanol, diacetyl, butanol and acetoin were detected in all of original samples and freeze dried/reconstituted samples while acetone was detected in samples containing high amount of grape juice. Volatile aroma compounds in original fermented milk were reduced during freeze drying. L. acidophilus produced ethanol, diacetyl and acetoin during fermentation.

본 연구에서는 과즙-우유 혼합 기질로 만들어진 발효유를 동결건조하고, 동결건조 전과 후의 생균수, pH의 변화, 관능성 및 휘발성 향기 성분의 변화를 조사하였다. 동결 또는 동결건조에 의하여 발효유의 pH는 거의 변화가 없었으나, 생균수는 동결, 특히 동결건조 도중에 급격히 감소하였는데, 동결 전의 균수를 100%로 했을 때, 동결 후의 생존율은 $64.5{\sim}85.2%$, 동결건조 후의 생존율은 $10.0{\sim}21.1%$였다. 사과쥬스-우유의 혼합 비율이 15:35인 기질로 만든 발효유와 동일한 발효유를 동결건조-환원한 시료의 관능성을 비교했을 때, 동결건조 전의 시료가 후의 시료보다 관능성이 우수하며, 이와 같은 경향은 포도쥬스-우유 혼합 시료의 경우에 보다 현저하였다. 동결건조 전과 후의 모든 시료에서 ethanol, diacetyl, butanol, acetoin이 검출되었고, 포도쥬스 함량이 높은 시료에서는 acetone도 검출되었으며, 동결건조에 의하여 모든 시료의 휘발성 향기 성분의 함량이 감소하는 경향을 보였다. 본 실험에서 검출된 향기 성분 가운데 젖산균 발효에 의하여 생성된 향기 성분은 ethanol, diacetyl, acetoin 이었다.