<PAGE>   1


                                                                    EXHIBIT 4.19

--------------------------------------------------------------------------------



                       TRANSAMERICAN REFINING CORPORATION


                       ------------------------------

                       FIRST AMENDMENT TO LOAN AGREEMENT

                         Dated as of December 30, 1997

                       ------------------------------



--------------------------------------------------------------------------------
<PAGE>   2
       This First Amendment to Loan Agreement (this "First Amendment") is made
as of December 30, 1997, by and between TransAmerican Energy Corporation, a
Delaware corporation ("TEC"), and TransAmerican Refining Corporation, a Texas
corporation ("TARC").

       WHEREAS, TEC and Firstar Bank of Minnesota, N.A., as Trustee,  have
entered into an Indenture dated as of June 13, 1997 (the "Indenture"), pursuant
to which TEC issued $475,000,000 aggregate principal amount of its 11 1/2%
Senior Secured Notes due 2002 and $1,130,000,000 aggregate principal amount of
its 13% Senior Secured Discount Notes due 2002 (collectively, the "Notes"); and

       WHEREAS, TEC and TARC have entered into a Loan Agreement dated as of
June 13, 1997 (the "TARC Intercompany Loan Agreement"), pursuant to which TEC
agreed to loan to TARC an aggregate of $675,648,920 out of the proceeds of the
issuance of the Notes; and

       WHEREAS, TEC and TARC have agreed to an amendment to the TARC
Intercompany Loan Agreement as hereinafter set forth (the "Proposed
Amendment"); and

       WHEREAS, pursuant to Section 9.2 of the Indenture, the holders of not
less than 66-2/3% in aggregate Value (as defined in the Indenture) of the Notes
have consented to the Proposed Amendment to the TARC Intercompany Loan
Agreement;

       NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties to this First
Amendment hereby agree as follows:

                                   ARTICLE I

                AMENDMENT TO THE TARC INTERCOMPANY LOAN AGREEMENT

       Section 1.01.  Amendment to Section 1.1.  The following definition in
Section 1.1 of the TARC Intercompany Loan Agreement is hereby amended to read
in its entirety as follows:

              "Indenture" shall mean that certain Indenture dated as of the
       date hereof between the Lender and the Indenture Trustee, as
       supplemented or amended from time to time.

                                   ARTICLE II

                                 MISCELLANEOUS

       Section 2.01.  Ratification and Confirmation.  As amended and modified
by this First Amendment, the terms and provisions of the TARC Intercompany Loan
Agreement are hereby ratified and confirmed and shall continue in full force
and effect.

       Section 2.02.  Reference to TARC Intercompany Loan Agreement.  The TARC
Intercompany Loan Agreement, and any and all other agreements, documents or
instruments now or hereafter executed and delivered pursuant to the terms of
the TARC Intercompany Loan Agreement, are hereby amended so that any reference
therein to the TARC Intercompany Loan Agreement shall mean a reference to the
TARC Intercompany Loan Agreement as amended hereby.





<PAGE>   3
       Section 2.03.  Counterparts.  This First Amendment may be executed in
one or more counterparts, each of which when executed shall be deemed to be an
original, but all of which when taken together shall constitute one and the
same instrument.

       Section 2.04.  Headings.  The headings, captions and arrangements used
in this First Amendment are for convenience only and shall not affect the
interpretation of this First Amendment.

       Section 2.05. Governing Law.  THIS FIRST AMENDMENT SHALL BE CONSTRUED IN
ACCORDANCE WITH AND GOVERNED BY THE LAWS OF THE STATE OF NEW YORK, WITHOUT
REGARD TO PRINCIPLES OF CONFLICTS OF LAWS.

       IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
executed as of the date of first written above.


                                   TRANSAMERICAN REFINING CORPORATION




                                   By:                                         
                                      -----------------------------------------

                                   Name:                                       
                                        ---------------------------------------

                                   Title:                                      
                                         --------------------------------------




                                   TRANSAMERICAN ENERGY CORPORATION






                                   By:                                         
                                      -----------------------------------------

                                   Name:                                       
                                        ---------------------------------------

                                   Title:                                      
                                         --------------------------------------





                                       2