**Table 4 Variance analysis of the established regression equation**

Source of variance | Sum of square of deviations | Degrees of freedom | F | Prob > F |

X_{1} | 47,000.08 | 1 | 8.18 | 0.012 6 |

X_{2} | 34,347.00 | 1 | 5.98 | 0.028 3 |

X_{3} | 16,576.33 | 1 | 2.88 | 0.111 5 |

X_{4} | 444.08 | 1 | 0.08 | 0.785 1 |

X_{1}^{2} | 576.00 | 1 | 0.10 | 0.756 2 |

X_{2}^{2} | 22,350.25 | 1 | 3.89 | 0.068 7 |

X_{3}^{2} | 3,249.00 | 1 | 0.57 | 0.464 5 |

X_{4}^{2} | 9,120.25 | 1 | 1.59 | 0.228 3 |

X_{1}X_{2} | 16,002.25 | 1 | 2.79 | 0.117 3 |

X_{1}X_{3} | 4,489.00 | 1 | 0.78 | 0.391 7 |

X_{1}X_{4} | 188,000.00 | 1 | 32.77 | < 0.000 1 |

X_{2}X_{3} | 11,859.08 | 1 | 2.06 | 0.172 8 |

X_{2}X_{4} | 5,271.73 | 1 | 0.92 | 0.354 4 |

X_{3}X_{4} | 37,105.30 | 1 | 6.46 | 0.023 5 |

Model | 356,000.00 | 14 | 4.43 | 0.004 4 |

Error term | 80,440.28 | 14 | | |

Lack of fit | 71,697.08 | 10 | 3.28 | 0.131 8 |

Pure error | 8,743.20 | 4 | | |

Cor total | 437,000.00 | 28 | | |

r^{2}=0.9157 | r_{adj}^{2} =0.9315 | CV=1.03 | | |

SBH, sheep bone hydrolysate.

X_{1}, X_{2}, X_{3}, and X_{4} were 4 factors affecting SBH Lactobacillus helveticus fermentation, which represented fermenting temperature, initial pH of SBH, fermenting time and inoculation amount, respectively. X_{1}X_{2} indicated the interaction between the factor X_{1} and X_{2}, so were X_{1}X_{4}, X_{2}X_{3}, X_{2}X_{4}, and X_{3}X_{4}.