Effect Of Addition Of Soy Protein Isolate On Goat Cheese Quality

Goat milk is considered to be the one that is most close to human milk constitute,because of its higher nutritional value than that of cow milk. Goat cheese, as one of theimportant processing products to goat milk, is famous for its high fat content, small caseinmolecule, delicate structure and delicious taste. Because of its rich protein contents andunique functional characteristics, soy protein isolate (SPI) has become to be one of the mostcommon substitutes of food protein. In this study, fresh goat milk with different SPI additionproportions (2%,4%,6%,8%, and10%w/w), high concentration direct-set lactic bacterialincluding Lactococcus lactis and leuconostoc, were used as the raw material, main culture andwith or without Propionibacterium freudenreichii ssp. Shermanii as secondary culture,respectively, to make goat milk and SPI mixed Swiss or Cheddar cheese. Texture profile andsensory evaluation during a ripening time of70days, protein digestibility in vitro and thecontents of total free amino acid (TFA) after70days, as well as the water-soluble protein inmixed cheese and whey, were evaluated. Volatile compounds of the cheese were enriched bysolid-phase microextraction (SPME) and determined by GC/MS after the cheese maturation.The main results were as follows:1. The addition of SPI could enhance the water binding capacity of mixed cheeses, andwith the increase of the addition proportion of SPI, the water binding capacity would behigher and difficult to be lost, meanwhile the hardness and chewiness decreased. For Swisscheeses, SPI addition proportion had a significantly positive correlation(r=0.964, P<0.01), asignificantly negative correlation (r=0.964, P<0.01), a negative correlation (r=-0.908,P<0.05), respectively, with moisture, hardness and chewiness of mixed cheeses. For Cheddarcheeses, SPI addition proportion had a significantly positive correlation(r=0.971, P<0.01), asignificantly negative correlation (r=0.954, P<0.01), a negative correlation (r=-0.988,P<0.05), respectively, with moisture, hardness and chewiness of mixed cheeses.2. The main water soluble protein of SPI except these of17and13kDa was retained inGS. Protein of35and53kDa in the raw materials that made mixed cheeses might play a keyrole in keeping the water binding capacity and reducing hardness and chewiness of mixedcheese. For the mixed Swiss cheese, the contents of53kDa protein molecule had a positivecorrelation(r=0.913, P<0.05), a significantly negative correlation (r=-0.979, P<0.01), asignificantly negative correlation (r=-0.927, P<0.01), respectively, with moisture, hardnessand chewiness of mixed cheeses. The contents of35kDa protein molecule had a significantly positive correlation(r=0.995, P<0.01), a negative correlation (r=-0.915, P<0.05),a negative correlation (r=-0.871, P<0.05)), respectively, with moisture, hardness andchewiness of GS. For the mixed Cheddar cheese, the contents of53kDa protein moleculehad a significantly positive correlation(r=0.984, P<0.01), a negative correlation (r=-0.912,P<0.05), a significantly negative correlation (r=-0.951, P<0.01), respectively, with moisture,hardness and chewiness of GS. The contents of35kDa protein molecule had a significantlypositive correlation(r=0.992, P<0.01), a significantly negative correlation (r=-0.974, P<0.01),a significantly negative correlation (r=-0.989, P<0.05)), respectively, with moisture,hardness and chewiness of mixed cheeses.3. Total free amino acid in mixed cheeses had no obvious difference which the cheesethat SPI added with the4%ratio was significant higher than other cheeses. The in vitrodigestibility of mixed cheeses were significantly higher (P＜0.05) than pure goat cheese,which the SPI was added at the ratio of4%was the highest.4. The major volatile compounds of cheese were acid compounds, following the ketone,alcohol, aldehyde, ester compounds; SPI addition reduced the acid and ketone compounds ofcheese significantly and increased the content of alcohol compounds. The complexity of thecheese flavor was enhanced when SPI addition was4%, with a rich cream, butter aroma andless special flavor of goat milk, phenol, pyrrole compounds appeared as well.5. Sensory evaluation revealed mixed cheeses with SPI added proportion of4%, whichwere described as homogeneous golden color, hard exquisite texture with high chewiness,glossy appearance with little eyes, fuller milky flavor and slight beany flavor, could matchedpeople preference of flavor.6. Combined with all test results, cheeses with SPI added proportion of4%was suitableto be a kind of low cost dairy food but full of both animal and plant nutrition.